---
title: "Electron Shells and the Periodic Table"
book: "School Chemistry — Grades 1 to 12"
subject: chemistry
language: en
chapter: 23
exercises: 15
source: https://one-course.com/books/chemistry/1/en/chapter/23-electron-shells-and-the-periodic-table
license: CC-BY-NC-SA-4.0
credit: "One Chemistry Book, One Course (one-course.com)"
---

# Chapter 23 — Electron Shells and the Periodic Table

Neon glows red-orange in a sign; sodium, a soft [metal](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-metal), catches fire in water; chlorine is a choking yellow-green gas. Yet their [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) differ by only one or two [protons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton): neon has 10, sodium 11, chlorine 17. Why should neighbours in the [periodic table](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-periodic-table) behave so differently, while sodium and potassium, far apart in [atomic number](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-atomic-number), behave so alike? The answer lies in how the [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) of an [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) are arranged.

**You already know.**

An [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) has $Z$ [protons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton) in its [nucleus](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) and, when neutral, $Z$ [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) around it ([Chapter 16](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#ch-g9-inside-the-atom)). The [periodic table](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-periodic-table) orders the elements by $Z$ in periods (rows) and groups (columns); elements of a group behave alike ([Chapter 20](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#ch-g9-periodic-table-first-look)). [Atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) gain or lose [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) to become [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) ([Chapter 17](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#ch-g9-ions)).

## 23.1 Electron configuration

**Definition 23.1 (Shells, subshells and electron configuration).**

The [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) of an [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) are arranged in *shells*, numbered $n = 1, 2, 3, \ldots$ from the [nucleus](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) outwards; the higher $n$, the further from the [nucleus](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) and the less tightly held the [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus). Each shell is divided into *subshells* named by a letter: shell 1 has an s subshell (1s), shell 2 an s and a p subshell (2s, 2p), shell 3 has 3s and 3p (and 3d, met later). An s subshell holds at most 2 [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus), a p subshell at most 6. The *electron configuration* of an [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) lists its occupied subshells with their numbers of [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) as exponents: $1\mathrm{s}^2\,2\mathrm{s}^2\,2\mathrm{p}^4$ for oxygen.

**Method 23.2 (Writing an electron configuration, up to Z=20Z = 20Z=20).**

1. Count the [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) : $Z$ for a neutral [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) .
2. Fill the [subshells](#def-g10-electron-shells-configuration) in this order, each up to its maximum before starting the next: $$1\mathrm{s} \to 2\mathrm{s} \to 2\mathrm{p} \to 3\mathrm{s} \to  3\mathrm{p} \to 4\mathrm{s}.$$
3. Check that the exponents add up to the number of [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) .

Sodium, $Z = 11$: $1\mathrm{s}^2\,2\mathrm{s}^2\,2\mathrm{p}^6\,3\mathrm{s}^1$. Chlorine, $Z = 17$: $1\mathrm{s}^2\,2\mathrm{s}^2\,2\mathrm{p}^6\,3\mathrm{s}^2\,3\mathrm{p}^5$. Calcium, $Z = 20$: $1\mathrm{s}^2\,2\mathrm{s}^2\,2\mathrm{p}^6\,3\mathrm{s}^2\,3\mathrm{p}^6\,4\mathrm{s}^2$.

![The order of filling, up to Z = 20. Each small box is a place for one electron: an s subshell has 2 places, a p subshell 6.](https://one-course.com/images/onecourse/chapters/chemistry-1/g10-electron-shells/fig-7ebbfa889eab.svg)

*The order of filling, up to $Z = 20$. Each small box is a place for one [electron](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus): an s [subshell](#def-g10-electron-shells-configuration) has 2 places, a p [subshell](#def-g10-electron-shells-configuration) 6.*

## 23.2 Valence electrons

**Definition 23.3 (Valence and core electrons).**

The [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) of the outermost occupied [shell](#def-g10-electron-shells-configuration) (the [shell](#def-g10-electron-shells-configuration) with the highest $n$) are the *valence electrons*; the others are the *core electrons*. Only the valence [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) take part in [chemical reactions](https://one-course.com/books/chemistry/1/en/chapter/12-chemical-reactions-reactants-and-products#def-g7-chemical-reactions-reaction).

**Example 23.4 (Counting valence electrons).**

Oxygen, $1\mathrm{s}^2\,2\mathrm{s}^2\,2\mathrm{p}^4$: outer [shell](#def-g10-electron-shells-configuration) $n =
2$, $2 + 4 = 6$ [valence electrons](#def-g10-electron-shells-valence). Sodium, $\ldots 3\mathrm{s}^1$: 1 [valence electron](#def-g10-electron-shells-valence). Chlorine, $\ldots 3\mathrm{s}^2\,3\mathrm{p}^5$: 7. Neon, $1\mathrm{s}^2\,2\mathrm{s}^2\,2\mathrm{p}^6$: 8, a full [shell](#def-g10-electron-shells-configuration).

## 23.3 The table rebuilt from configurations

**Proposition 23.5 (Period and group from the configuration).**

For the elements up to $Z = 20$:

- the period of an element is the number $n$ of its outer [shell](#def-g10-electron-shells-configuration) ;
- elements of a same group have the same number of [valence electrons](#def-g10-electron-shells-valence) — one in [group](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-period) 1, two in [group](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-period) 2, three to eight in [groups](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-period) 13 to 18 (helium, with 2, is an exception).

The left two columns, where the last [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) enter an s [subshell](#def-g10-electron-shells-configuration), form the s block; [groups](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-period) 13 to 18 form the p block.

![The first twenty elements, with the number of electrons in each shell under the symbol (2.8.1 for sodium: 2 in shell 1, 8 in shell 2, 1 in shell 3). The colours mark the s and p blocks.](https://one-course.com/images/onecourse/chapters/chemistry-1/g10-electron-shells/fig-5f0216c4d5e5.svg)

*The first twenty elements, with the number of [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) in each [shell](#def-g10-electron-shells-configuration) under the symbol ($2.8.1$ for sodium: 2 in [shell](#def-g10-electron-shells-configuration) 1, 8 in [shell](#def-g10-electron-shells-configuration) 2, 1 in [shell](#def-g10-electron-shells-configuration) 3). The colours mark the s and p blocks.*

## 23.4 Families

**Definition 23.6 (Chemical families).**

A *chemical family* is a group of the [periodic table](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-periodic-table) whose elements share their main chemical properties. Three families are named: the *alkali metals* ([group](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-period) 1 except hydrogen: lithium, sodium, potassium…), soft [metals](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-metal) that react strongly with water; the *halogens* ([group](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-period) 17: fluorine, chlorine, bromine, iodine), coloured, reactive [non-metals](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-metal); and the *noble gases* ([group](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-period) 18: helium, neon, argon…), which hardly react with anything.

**Proposition 23.7 (Why a family behaves alike).**

The elements of a family have the same number of [valence electrons](#def-g10-electron-shells-valence), and chemistry is the business of the [valence electrons](#def-g10-electron-shells-valence): lithium, sodium and potassium all have one, the [halogens](#def-g10-electron-shells-family) seven, the [noble gases](#def-g10-electron-shells-family) a full outer [shell](#def-g10-electron-shells-configuration).

![Families of the periodic table: alkali metals, alkaline-earth metals (group 2), halogens and noble gases.](https://one-course.com/images/onecourse/chapters/chemistry-1/g10-electron-shells/fig-36288aba7521.svg)

*Families of the [periodic table](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-periodic-table): [alkali metals](#def-g10-electron-shells-family), alkaline-earth [metals](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-metal) ([group](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-period) 2), [halogens](#def-g10-electron-shells-family) and [noble gases](#def-g10-electron-shells-family).*

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g10-electron-shells/img-028868107598.jpg)

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g10-electron-shells/img-496d12f1f4f0.jpg)

*Sodium, an [alkali metal](#def-g10-electron-shells-family), cut with a knife: soft and silvery when fresh, it dulls at once in [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air). Photo: Dnn87, CC BY-SA 3.0.*

*Chlorine, a [halogen](#def-g10-electron-shells-family): a yellow-green gas, here sealed in a glass ampoule. Photo: W. Oelen, CC BY-SA 3.0.*

**In the lab — Sodium in water.**

Behind a screen, the teacher drops a piece of sodium the size of a grain of rice into a large bowl of water. It floats, melts into a shiny ball and whizzes about, fizzing, until it has gone; the gas given off is hydrogen, and the water left is basic.

**Safety.**

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g10-electron-shells/fig-f133bcfe28d0.svg)

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g10-electron-shells/fig-2ebfb929f83d.svg)

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g10-electron-shells/fig-16febdb222da.svg)

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g10-electron-shells/fig-11944cae5055.svg)

Sodium: releases flammable hydrogen with water (GHS02), causes severe burns (GHS05). Chlorine: an oxidising gas (GHS03), toxic if inhaled (GHS06), also an irritant and very toxic to aquatic life. Both are handled only by the teacher.

## 23.5 Stable ions and the noble-gas rule

**Definition 23.8 (Duet and octet rules).**

The [noble gases](#def-g10-electron-shells-family), with a full outer [shell](#def-g10-electron-shells-configuration), hardly react: their configuration is especially stable. When [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) of the first elements form [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion), they gain or lose [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) so as to reach the configuration of the nearest [noble gas](#def-g10-electron-shells-family): two [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) in [shell](#def-g10-electron-shells-configuration) 1, like helium, for the lightest [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) — the *duet rule* — and eight [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) in the outer [shell](#def-g10-electron-shells-configuration), like neon or argon, for the others — the *octet rule*.

**Example 23.9 (Ions predicted by the rule).**

Sodium, $2.8.1$, loses its single [valence electron](#def-g10-electron-shells-valence): $\ce{Na+}$, $2.8$, like neon. Magnesium, $2.8.2$, loses two: $\ce{Mg^{2+}}$. Aluminium, $2.8.3$, loses three: $\ce{Al^{3+}}$. Chlorine, $2.8.7$, gains one: $\ce{Cl-}$, $2.8.8$, like argon. Oxygen, $2.6$, gains two: $\ce{O^{2-}}$. Sulfur, $2.8.6$: $\ce{S^{2-}}$. Lithium, $2.1$, loses one and keeps $2$, like helium: $\ce{Li+}$.

![The sodium ion has the configuration of neon, the nearest noble gas.](https://one-course.com/images/onecourse/chapters/chemistry-1/g10-electron-shells/fig-fc3bf74fd63a.svg)

*The sodium [ion](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) has the configuration of neon, the nearest [noble gas](#def-g10-electron-shells-family).*

**Remark 23.10 (Limits of the rule).**

The rule works well for the first twenty elements and their simple [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion). Many elements, such as iron ($\ce{Fe^{2+}}$ and $\ce{Fe^{3+}}$), do not follow it; the reasons, and a better description of [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus), are given in the Year 1 volume.

## 23.6 Exercises

**Exercise 23.1 ★.**

Write the [electron configurations](#def-g10-electron-shells-configuration) of carbon ($Z = 6$), nitrogen ($Z = 7$) and magnesium ($Z = 12$).

**Solution of Exercise 23.1.**

C: $1\mathrm{s}^2\,2\mathrm{s}^2\,2\mathrm{p}^2$. N: $1\mathrm{s}^2\,2\mathrm{s}^2\,2\mathrm{p}^3$. Mg: $1\mathrm{s}^2\,2\mathrm{s}^2\,2\mathrm{p}^6\,3\mathrm{s}^2$.

**Exercise 23.2 ★.**

How many [valence electrons](#def-g10-electron-shells-valence) do carbon, nitrogen and magnesium have?

**Solution of Exercise 23.2.**

Carbon 4, nitrogen 5, magnesium 2.

**Exercise 23.3 ★.**

In which period and which group is the element of configuration $1\mathrm{s}^2\,2\mathrm{s}^2\,2\mathrm{p}^6\,3\mathrm{s}^2\,3\mathrm{p}^3$?

**Solution of Exercise 23.3.**

Outer [shell](#def-g10-electron-shells-configuration) $n = 3$: period 3. $2 + 3 = 5$ [valence electrons](#def-g10-electron-shells-valence): [group](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-period) 15 (phosphorus).

**Exercise 23.4 ★.**

Name the three families of this chapter and give two elements of each.

**Solution of Exercise 23.4.**

[Alkali metals](#def-g10-electron-shells-family) (lithium, sodium, potassium); [halogens](#def-g10-electron-shells-family) (fluorine, chlorine, bromine, iodine); [noble gases](#def-g10-electron-shells-family) (helium, neon, argon).

**Exercise 23.5 ★.**

State the [octet rule](#def-g10-electron-shells-octet).

**Solution of Exercise 23.5.**

[Atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) tend to gain or lose [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) so as to have eight [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) in their outer [shell](#def-g10-electron-shells-configuration), like the nearest [noble gas](#def-g10-electron-shells-family).

**Exercise 23.6 ★★.**

Using the noble-gas rule, give the [ion](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) formed by potassium ($Z = 19$) and by fluorine ($Z = 9$), with their configurations.

**Solution of Exercise 23.6.**

K, $2.8.8.1$, loses one: $\ce{K+}$, $1\mathrm{s}^2\,2\mathrm{s}^2\,2\mathrm{p}^6\,3\mathrm{s}^2\,3\mathrm{p}^6$ (argon). F, $2.7$, gains one: $\ce{F-}$, $1\mathrm{s}^2\,2\mathrm{s}^2\,2\mathrm{p}^6$ (neon).

**Exercise 23.7 ★★.**

An element has 2 [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) in its third [shell](#def-g10-electron-shells-configuration) and its third [shell](#def-g10-electron-shells-configuration) is its outer [shell](#def-g10-electron-shells-configuration). Give its configuration, its [atomic number](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-atomic-number) and its name.

**Solution of Exercise 23.7.**

$1\mathrm{s}^2\,2\mathrm{s}^2\,2\mathrm{p}^6\,3\mathrm{s}^2$: $Z = 12$, magnesium.

**Exercise 23.8 ★★.**

Using stable [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion), write the formula of the [ionic compound](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ionic-compound) formed by magnesium and chlorine, then by aluminium and oxygen.

**Solution of Exercise 23.8.**

$\ce{Mg^{2+}}$ and $\ce{Cl-}$: $\ce{MgCl2}$. $\ce{Al^{3+}}$ and $\ce{O^{2-}}$: $\ce{Al2O3}$.

**Exercise 23.9 ★★.**

Look at the table of the first twenty elements. Which elements have the same number of [valence electrons](#def-g10-electron-shells-valence) as oxygen?

**Solution of Exercise 23.9.**

Sulfur (6 [valence electrons](#def-g10-electron-shells-valence), $2.8.6$), in the same group, 16.

**Exercise 23.10 ★★.**

Why does neon, unlike sodium and chlorine, form no [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) in [chemical reactions](https://one-course.com/books/chemistry/1/en/chapter/12-chemical-reactions-reactants-and-products#def-g7-chemical-reactions-reaction)?

**Solution of Exercise 23.10.**

Its outer [shell](#def-g10-electron-shells-configuration) is already full (eight [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus)): it has nothing to gain by losing or gaining [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus).

**Exercise 23.11 ★★.**

The [ion](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) $\ce{X^{2+}}$ has the configuration of argon. Find $X$.

**Solution of Exercise 23.11.**

Argon has 18 [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus); $\ce{X^{2+}}$ has lost 2, so X has 20: calcium.

**Exercise 23.12 ★★★.**

Helium has 2 [valence electrons](#def-g10-electron-shells-valence) but sits in [group](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-period) 18, not [group](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-period) 2. Explain using its configuration.

**Solution of Exercise 23.12.**

Helium, $1\mathrm{s}^2$, has a full outer [shell](#def-g10-electron-shells-configuration) ([shell](#def-g10-electron-shells-configuration) 1 holds only two [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus)), like the other [noble gases](#def-g10-electron-shells-family); it does not behave like the [group](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-period) 2 [metals](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-metal).

**Exercise 23.13 ★★★.**

Potassium reacts with water even more violently than sodium. Using the distance of the [valence electron](#def-g10-electron-shells-valence) from the [nucleus](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus), suggest why.

**Solution of Exercise 23.13.**

Its [valence electron](#def-g10-electron-shells-valence) is in [shell](#def-g10-electron-shells-configuration) 4, further from the [nucleus](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) than sodium’s in [shell](#def-g10-electron-shells-configuration) 3: less tightly held, it is lost more easily.

**Exercise 23.14 ★★★.**

Give the configurations of $\ce{S^{2-}}$, $\ce{Cl-}$, $\ce{K+}$ and $\ce{Ca^{2+}}$. What do they have in common?

**Solution of Exercise 23.14.**

All four have 18 [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) and the configuration of argon:

$$
1\mathrm{s}^2\,2\mathrm{s}^2\,2\mathrm{p}^6\,3\mathrm{s}^2\,3\mathrm{p}^6.
$$

**Exercise 23.15 ★★★.**

An element of period 3 forms the [ion](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) $\ce{Y^{3-}}$. Find its group, its configuration and its name.

**Solution of Exercise 23.15.**

It gains 3 [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) to reach an octet: it has 5 [valence electrons](#def-g10-electron-shells-valence), [group](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-period) 15. In period 3, it is phosphorus:

$$
1\mathrm{s}^2\,2\mathrm{s}^2\,2\mathrm{p}^6\,3\mathrm{s}^2\,3\mathrm{p}^3.
$$

## 23.7 Problem: Ruby, Sapphire and Corundum

**Problem 23.1.**

Weekend problem — the electrons behind a gemstone: from two configurations to the number of electrons moved in one gram

Rubies and sapphires are gem forms of corundum, an [ionic compound](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ionic-compound) of aluminium and oxygen; a trace of other [metals](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-metal) gives them their colours. Corundum is hard enough to scratch almost anything, and powdered corundum is used to polish lenses. Take the mass of a [nucleon](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton) as $1.67 \times 10^{-27}\,\mathrm{kg}$; aluminium-27 has 27 [nucleons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton), oxygen-16 has 16.

**Part I — The [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom).**

1. Give the numbers of [protons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton) and [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) of an aluminium [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) ( $Z = 13$ ) and of an oxygen [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) ( $Z = 8$ ).
2. Write their [electron configurations](#def-g10-electron-shells-configuration) .
3. How many [valence electrons](#def-g10-electron-shells-valence) does each have? In which period and group is each?
4. Is aluminium a [metal](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-metal) or a [non-metal](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-metal) ? And oxygen?

**Part II — The [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion).**

5. Using the [octet rule](#def-g10-electron-shells-octet) , which [ion](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) does aluminium form? Write its configuration.
6. Which [ion](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) does oxygen form? Write its configuration.
7. Which [noble gas](#def-g10-electron-shells-family) has the same configuration as both [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) ?
8. How many [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) does an aluminium [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) lose, and an oxygen [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) gain?

**Part III — The formula.**

9. Find the formula of corundum, the neutral compound of these [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) .
10. In one formula unit, how many [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) are lost by the aluminium [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) ? Gained by the oxygen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) ?
11. Why must these two numbers be equal?
12. A ruby contains a trace of chromium [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) $\ce{Cr^{3+}}$ in place of some $\ce{Al^{3+}}$ . Why can a $\ce{Cr^{3+}}$ [ion](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) take the place of an $\ce{Al^{3+}}$ [ion](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) without upsetting the charges?

**Part IV — One gram of corundum.**

13. How many [nucleons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton) are there in one formula unit of corundum?
14. Compute the mass of one formula unit ( [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) neglected).
15. How many formula units are there in $1\,\mathrm{g}$ of corundum?
16. How many $\ce{Al^{3+}}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) and $\ce{O^{2-}}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) is that?
17. Why can the [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) be neglected in question 14?
18. How many [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) were transferred from aluminium to oxygen to make $1\,\mathrm{g}$ of corundum?

**Solution of Problem 23.1.**

**1.** Aluminium: 13 [protons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton), 13 [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus). Oxygen: 8 [protons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton), 8 [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus).

**2.** Al: $1\mathrm{s}^2\,2\mathrm{s}^2\,2\mathrm{p}^6\,3\mathrm{s}^2\,3\mathrm{p}^1$. O: $1\mathrm{s}^2\,2\mathrm{s}^2\,2\mathrm{p}^4$.

**3.** Al: 3 [valence electrons](#def-g10-electron-shells-valence), period 3, [group](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-period) 13. O: 6, period 2, [group](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-period) 16.

**4.** Aluminium is a [metal](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-metal), oxygen a [non-metal](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-metal).

**5.** $\ce{Al^{3+}}$: $1\mathrm{s}^2\,2\mathrm{s}^2\,2\mathrm{p}^6$.

**6.** $\ce{O^{2-}}$: $1\mathrm{s}^2\,2\mathrm{s}^2\,2\mathrm{p}^6$.

**7.** Neon.

**8.** Aluminium loses 3, oxygen gains 2.

**9.** $2 \times (+3) + 3 \times (-2) = 0$: $\ce{Al2O3}$.

**10.** Lost: $2 \times 3 = 6$. Gained: $3 \times 2 = 6$.

**11.** [Electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) are not created or destroyed: those lost by aluminium are exactly those gained by oxygen, and the compound is neutral.

**12.** It carries the same charge, $+3$: the charges of the crystal still balance.

**13.** $2 \times 27 + 3 \times 16 = 102$ [nucleons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton).

**14.** $102 \times 1.67 \times 10^{-27} = 1.70 \times 10^{-25}\,\mathrm{kg}$.

**15.** $10^{-3} / 1.70 \times 10^{-25} \approx 5.87 \times 10^{21}$ formula units.

**16.** $2 \times 5.87 \times 10^{21} = 1.17 \times 10^{22}$ $\ce{Al^{3+}}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion); $3 \times 5.87 \times 10^{21} = 1.76 \times 10^{22}$ $\ce{O^{2-}}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion).

**17.** An [electron](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) is about 1836 times lighter than a [nucleon](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton); the 50 [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) of a formula unit weigh less than three hundredths of one [nucleon](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton).

**18.** $6 \times 5.87 \times 10^{21} \approx 3.5 \times 10^{22}$ [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) transferred for $1\,\mathrm{g}$ of corundum.