---
title: "Atoms and Molecules"
book: "School Chemistry — Grades 1 to 12"
subject: chemistry
language: en
chapter: 11
exercises: 12
source: https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules
license: CC-BY-NC-SA-4.0
credit: "One Chemistry Book, One Course (one-course.com)"
---

# Chapter 11 — Atoms and Molecules

Take a drop of water and cut it in two. Each half is still water. Cut a half in two again, and again, and again. Could you go on for ever, or would you reach a piece so small that cutting it once more gives something that is no longer water at all? Around 400 BC, the Greek thinker Democritus guessed the answer: matter is made of tiny pieces that cannot be cut, which he called *[atoms](#def-g7-atoms-and-molecules-atom)*, from the Greek for “uncuttable”. It took more than two thousand years, and the work of the chemists of the nineteenth century, to turn that guess into science. This chapter describes the pieces — [atoms](#def-g7-atoms-and-molecules-atom) — and how they group into [molecules](#def-g7-atoms-and-molecules-molecule).

**You already know.**

A [pure substance](https://one-course.com/books/chemistry/1/en/chapter/8-pure-substances-and-mixtures#def-g6-pure-substances-and-mixtures-pure-substance), such as water or oxygen, is made of one single [chemical species](https://one-course.com/books/chemistry/1/en/chapter/8-pure-substances-and-mixtures#def-g6-pure-substances-and-mixtures-species); a [mixture](https://one-course.com/books/chemistry/1/en/chapter/8-pure-substances-and-mixtures#def-g6-pure-substances-and-mixtures-mixture), such as [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air), contains several ([Chapter 8](https://one-course.com/books/chemistry/1/en/chapter/8-pure-substances-and-mixtures#ch-g6-pure-substances-and-mixtures)). [Air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) is about four fifths nitrogen and one fifth oxygen ([Chapter 4](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#ch-g4-air-a-mixture-of-gases)).

## 11.1 Atoms

**Definition 11.1 (Atom).**

All matter is built from extremely small particles called *atoms*. There are only about a hundred different kinds of atoms in nature, and every substance — water, [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air), iron, sugar, our own bodies — is made of atoms of a few of these kinds.

**Proposition 11.2 (Atoms are tiny).**

The smallest [atom](#def-g7-atoms-and-molecules-atom), the hydrogen [atom](#def-g7-atoms-and-molecules-atom), measures about a ten-millionth of a millimetre across; the others are at most a few times larger. Ten million hydrogen [atoms](#def-g7-atoms-and-molecules-atom) in a row would make a line one millimetre long. To picture it: if an apple were blown up to the size of the Earth, each of its [atoms](#def-g7-atoms-and-molecules-atom) would be about the size of a small cherry.

**Remark 11.3 (Seen without being seen).**

No microscope using light can show an [atom](#def-g7-atoms-and-molecules-atom): [atoms](#def-g7-atoms-and-molecules-atom) are far smaller than the waves of light. Since the 1980s, special microscopes that feel a surface with an extremely fine tip have made images in which single [atoms](#def-g7-atoms-and-molecules-atom) appear as bumps.

**Definition 11.4 (Symbol of an atom).**

Each kind of [atom](#def-g7-atoms-and-molecules-atom) has a name and a *symbol*: one capital letter, sometimes followed by one small letter. The symbol is the same in every language.

| [atom](#def-g7-atoms-and-molecules-atom) | [symbol](#def-g7-atoms-and-molecules-symbol) |  | model colour | [atom](#def-g7-atoms-and-molecules-atom) | [symbol](#def-g7-atoms-and-molecules-symbol) |  | model colour |
| --- | --- | --- | --- | --- | --- | --- | --- |
| hydrogen | $\ce{H}$ | ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g7-atoms-and-molecules/fig-f9085e9eeda0.svg) | white | sodium | $\ce{Na}$ | ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g7-atoms-and-molecules/fig-aa6acdf1b315.svg) | purple |
| carbon | $\ce{C}$ | ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g7-atoms-and-molecules/fig-dc0afccd9ffc.svg) | black | sulfur | $\ce{S}$ | ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g7-atoms-and-molecules/fig-bda87a58dc28.svg) | yellow |
| nitrogen | $\ce{N}$ | ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g7-atoms-and-molecules/fig-aa147a4b8e2d.svg) | blue | chlorine | $\ce{Cl}$ | ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g7-atoms-and-molecules/fig-fb62d4440b79.svg) | green |
| oxygen | $\ce{O}$ | ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g7-atoms-and-molecules/fig-c6acc93faca6.svg) | red | iron, copper | $\ce{Fe}$, $\ce{Cu}$ |  | — |

**Remark 11.5 (Where symbols come from).**

Most [symbols](#def-g7-atoms-and-molecules-symbol) are the first letters of the name: $\ce{H}$ for hydrogen, $\ce{C}$ for carbon, $\ce{O}$ for oxygen. Some come from an older, Latin name: $\ce{Na}$ for sodium (*natrium*), $\ce{Fe}$ for iron (*ferrum*), $\ce{Cu}$ for copper (*cuprum*). The second letter is always small: $\ce{Co}$ is the cobalt [atom](#def-g7-atoms-and-molecules-atom), while CO, with a capital O, is something else entirely.

**History — Dalton’s atoms, 1808.**

In 1808 the chemist John Dalton published a table of the [atoms](#def-g7-atoms-and-molecules-atom) then known, each drawn as a small circle with its own sign. He was the first to say that every [atom](#def-g7-atoms-and-molecules-atom) of one kind has the same weight, and that chemists could compare these weights. But he also guessed that water was one hydrogen [atom](#def-g7-atoms-and-molecules-atom) joined to one oxygen [atom](#def-g7-atoms-and-molecules-atom); it took another fifty years for chemists to agree that a water [molecule](#def-g7-atoms-and-molecules-molecule) holds *two* hydrogen [atoms](#def-g7-atoms-and-molecules-atom).

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g7-atoms-and-molecules/img-029f0402624a.jpg)

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g7-atoms-and-molecules/img-fe32a4ca4e3d.jpg)

*John Dalton (1766–1844).*

*Dalton’s table of 1808: his water, sign 21, is one circle of hydrogen beside one of oxygen.*

## 11.2 Molecules

**Definition 11.6 (Molecule).**

A *molecule* is a group of [atoms](#def-g7-atoms-and-molecules-atom) held tightly together. A molecule is the smallest piece of a substance that is still that substance: a water molecule is the smallest possible amount of water.

**Example 11.7 (Seven molecules).**

- the *hydrogen* [molecule](#def-g7-atoms-and-molecules-molecule) : two hydrogen [atoms](#def-g7-atoms-and-molecules-atom) ;
- the *oxygen* [molecule](#def-g7-atoms-and-molecules-molecule) , which we breathe: two oxygen [atoms](#def-g7-atoms-and-molecules-atom) ;
- the *nitrogen* [molecule](#def-g7-atoms-and-molecules-molecule) , four fifths of the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) : two nitrogen [atoms](#def-g7-atoms-and-molecules-atom) ;
- the *water* [molecule](#def-g7-atoms-and-molecules-molecule) : two hydrogen [atoms](#def-g7-atoms-and-molecules-atom) and one oxygen [atom](#def-g7-atoms-and-molecules-atom) ;
- the *carbon dioxide* [molecule](#def-g7-atoms-and-molecules-molecule) , which we breathe out: one carbon [atom](#def-g7-atoms-and-molecules-atom) and two oxygen [atoms](#def-g7-atoms-and-molecules-atom) ;
- the *methane* [molecule](#def-g7-atoms-and-molecules-molecule) , the main part of the natural gas burnt in gas cookers: one carbon [atom](#def-g7-atoms-and-molecules-atom) and four hydrogen [atoms](#def-g7-atoms-and-molecules-atom) ;
- the *nitrous oxide* [molecule](#def-g7-atoms-and-molecules-molecule) , an anaesthetic gas: two nitrogen [atoms](#def-g7-atoms-and-molecules-atom) and one oxygen [atom](#def-g7-atoms-and-molecules-atom) .

![Ball-and-stick models of the seven molecules of . Each stick stands for a bond between two atoms; why some atoms share two or three sticks is explained in a later chapter.](https://one-course.com/images/onecourse/chapters/chemistry-1/g7-atoms-and-molecules/fig-8d4473a48a24.svg)

*Ball-and-stick models of the seven [molecules](#def-g7-atoms-and-molecules-molecule) of [Example 11.7](#ex-g7-atoms-and-molecules-seven). Each stick stands for a bond between two [atoms](#def-g7-atoms-and-molecules-atom); why some [atoms](#def-g7-atoms-and-molecules-atom) share two or three sticks is explained in a later chapter.*

**Proposition 11.8 (Molecules of a pure substance and of a mixture).**

In a [pure substance](https://one-course.com/books/chemistry/1/en/chapter/8-pure-substances-and-mixtures#def-g6-pure-substances-and-mixtures-pure-substance) made of [molecules](#def-g7-atoms-and-molecules-molecule), all the [molecules](#def-g7-atoms-and-molecules-molecule) are identical: pure water contains only water [molecules](#def-g7-atoms-and-molecules-molecule). A [mixture](https://one-course.com/books/chemistry/1/en/chapter/8-pure-substances-and-mixtures#def-g6-pure-substances-and-mixtures-mixture) contains [molecules](#def-g7-atoms-and-molecules-molecule) of several kinds: [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) is mostly nitrogen [molecules](#def-g7-atoms-and-molecules-molecule) and oxygen [molecules](#def-g7-atoms-and-molecules-molecule), about four of nitrogen for each one of oxygen, with a few argon [atoms](#def-g7-atoms-and-molecules-atom) that stay single, and very few [molecules](#def-g7-atoms-and-molecules-molecule) of carbon dioxide.

![Zooming in, enormously. Left: in air, molecules of nitrogen (blue) far outnumber those of oxygen (red), and an argon atom (pale blue) travels alone; the molecules of a gas are far apart. Right: in liquid water, the molecules are all identical and touch one another.](https://one-course.com/images/onecourse/chapters/chemistry-1/g7-atoms-and-molecules/fig-6f0066104dcf.svg)

*Zooming in, enormously. Left: 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), [molecules](#def-g7-atoms-and-molecules-molecule) of nitrogen (blue) far outnumber those of oxygen (red), and an argon [atom](#def-g7-atoms-and-molecules-atom) (pale blue) travels alone; the [molecules](#def-g7-atoms-and-molecules-molecule) of a gas are far apart. Right: in liquid water, the [molecules](#def-g7-atoms-and-molecules-molecule) are all identical and touch one another.*

## 11.3 Chemical formulas

**Definition 11.9 (Chemical formula).**

The *chemical formula* of a [molecule](#def-g7-atoms-and-molecules-molecule) lists the [symbols](#def-g7-atoms-and-molecules-symbol) of its [atoms](#def-g7-atoms-and-molecules-atom), each followed by a small number written below the line — the *subscript* — giving how many [atoms](#def-g7-atoms-and-molecules-atom) of that kind the [molecule](#def-g7-atoms-and-molecules-molecule) contains. A subscript 1 is not written.

**Example 11.10 (The seven formulas).**

The [molecules](#def-g7-atoms-and-molecules-molecule) of [Example 11.7](#ex-g7-atoms-and-molecules-seven) have these formulas:

| [molecule](#def-g7-atoms-and-molecules-molecule) | formula | [atoms](#def-g7-atoms-and-molecules-atom) |
| --- | --- | --- |
| hydrogen | $\ce{H2}$ | 2 hydrogen |
| oxygen | $\ce{O2}$ | 2 oxygen |
| nitrogen | $\ce{N2}$ | 2 nitrogen |
| water | $\ce{H2O}$ | 2 hydrogen, 1 oxygen |
| carbon dioxide | $\ce{CO2}$ | 1 carbon, 2 oxygen |
| methane | $\ce{CH4}$ | 1 carbon, 4 hydrogen |
| nitrous oxide | $\ce{N2O}$ | 2 nitrogen, 1 oxygen |

**Method 11.11 (Reading a formula).**

To read the formula of a [molecule](#def-g7-atoms-and-molecules-molecule):

1. split it into [symbols](#def-g7-atoms-and-molecules-symbol) : each one starts with a capital letter;
2. after each [symbol](#def-g7-atoms-and-molecules-symbol) , read the [subscript](#def-g7-atoms-and-molecules-formula) (no [subscript](#def-g7-atoms-and-molecules-formula) means 1);
3. add up the [subscripts](#def-g7-atoms-and-molecules-formula) to get the total number of [atoms](#def-g7-atoms-and-molecules-atom) .

For $\ce{C2H6O}$ (ethanol, the alcohol of drinks): 2 carbon, 6 hydrogen, 1 oxygen, 9 [atoms](#def-g7-atoms-and-molecules-atom) in all.

**Remark 11.12 (A number in front).**

A number written in front of a formula counts [molecules](#def-g7-atoms-and-molecules-molecule): $\ce{3H2O}$ means three water [molecules](#def-g7-atoms-and-molecules-molecule), which together hold 6 hydrogen [atoms](#def-g7-atoms-and-molecules-atom) and 3 oxygen [atoms](#def-g7-atoms-and-molecules-atom). Do not confuse $\ce{O2}$, one [molecule](#def-g7-atoms-and-molecules-molecule) made of two oxygen [atoms](#def-g7-atoms-and-molecules-atom), with $\ce{2O}$, two oxygen [atoms](#def-g7-atoms-and-molecules-atom) that are not joined.

**Example 11.13 (Methane in the kitchen).**

The natural gas of a gas cooker is mostly methane, $\ce{CH4}$. Each of its [molecules](#def-g7-atoms-and-molecules-molecule) is one carbon [atom](#def-g7-atoms-and-molecules-atom) with four hydrogen [atoms](#def-g7-atoms-and-molecules-atom) around it. The gas is invisible and has no smell: the smell of “gas” is a substance added on purpose, so that a leak is noticed.

![The blue flame of a gas cooker: the methane of natural gas is burning.](https://one-course.com/images/onecourse/chapters/chemistry-1/g7-atoms-and-molecules/img-f79dfd750e3e.jpg)

*The blue flame of a gas cooker: the methane of natural gas is [burning](https://one-course.com/books/chemistry/1/en/chapter/5-burning-what-a-fire-needs#def-g4-what-a-fire-needs-burning).*

## 11.4 Molecular models

**Remark 11.14 (Models, and their colours).**

Chemists build *molecular models* with balls for the [atoms](#def-g7-atoms-and-molecules-atom) and sticks for the bonds between them. The colours are a convention used all over the world — carbon black, hydrogen white, oxygen red, nitrogen blue — and not the real colour of [atoms](#def-g7-atoms-and-molecules-atom): a single [atom](#def-g7-atoms-and-molecules-atom) has no colour at all. A model also shows the shape of a [molecule](#def-g7-atoms-and-molecules-molecule), which a formula does not: the water [molecule](#def-g7-atoms-and-molecules-molecule) is bent, the carbon dioxide [molecule](#def-g7-atoms-and-molecules-molecule) is straight.

![A molecular model kit: balls for atoms, with holes for the sticks that stand for bonds.](https://one-course.com/images/onecourse/chapters/chemistry-1/g7-atoms-and-molecules/img-35f13914d8de.jpg)

*A molecular model kit: balls for [atoms](#def-g7-atoms-and-molecules-atom), with holes for the sticks that stand for bonds.*

**Method 11.15 (From a model to a formula).**

To write the formula of a [molecule](#def-g7-atoms-and-molecules-molecule) from its model:

1. name the [atoms](#def-g7-atoms-and-molecules-atom) from their colours;
2. count the balls of each colour;
3. write the [symbols](#def-g7-atoms-and-molecules-symbol) with the counts as [subscripts](#def-g7-atoms-and-molecules-formula) , carbon first, then hydrogen, then the other [atoms](#def-g7-atoms-and-molecules-atom) in alphabetical order of their [symbols](#def-g7-atoms-and-molecules-symbol) .

A black ball joined to four white ones: one carbon, four hydrogen, $\ce{CH4}$.

## 11.5 Exercises

**Exercise 11.1 ★.**

What is an [atom](#def-g7-atoms-and-molecules-atom)? What is a [molecule](#def-g7-atoms-and-molecules-molecule)?

**Solution of Exercise 11.1.**

An [atom](#def-g7-atoms-and-molecules-atom) is one of the extremely small particles from which all matter is built (about a hundred kinds exist). A [molecule](#def-g7-atoms-and-molecules-molecule) is a group of [atoms](#def-g7-atoms-and-molecules-atom) held tightly together, the smallest piece of a substance that is still that substance.

**Exercise 11.2 ★.**

Give the [symbols](#def-g7-atoms-and-molecules-symbol) of hydrogen, carbon, nitrogen, oxygen, sodium and iron.

**Solution of Exercise 11.2.**

$\ce{H}$, $\ce{C}$, $\ce{N}$, $\ce{O}$, $\ce{Na}$, $\ce{Fe}$.

**Exercise 11.3 ★.**

How many [atoms](#def-g7-atoms-and-molecules-atom) of each kind are there in one [molecule](#def-g7-atoms-and-molecules-molecule) of carbon dioxide, $\ce{CO2}$? How many [atoms](#def-g7-atoms-and-molecules-atom) in all?

**Solution of Exercise 11.3.**

1 carbon [atom](#def-g7-atoms-and-molecules-atom) and 2 oxygen [atoms](#def-g7-atoms-and-molecules-atom): 3 [atoms](#def-g7-atoms-and-molecules-atom) in all.

**Exercise 11.4 ★.**

Name the [molecules](#def-g7-atoms-and-molecules-molecule) $\ce{H2O}$, $\ce{O2}$, $\ce{CH4}$ and $\ce{N2}$.

**Solution of Exercise 11.4.**

Water, oxygen, methane, nitrogen.

**Exercise 11.5 ★.**

A [molecule](#def-g7-atoms-and-molecules-molecule) of nitrogen dioxide, a brown gas found in polluted [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air), is made of one nitrogen [atom](#def-g7-atoms-and-molecules-atom) and two oxygen [atoms](#def-g7-atoms-and-molecules-atom). Write its formula.

**Solution of Exercise 11.5.**

$\ce{NO2}$.

**Exercise 11.6 ★★.**

Explain the difference between $\ce{O2}$ and $\ce{2O}$, and between $\ce{CO}$ and $\ce{Co}$.

**Solution of Exercise 11.6.**

$\ce{O2}$ is one [molecule](#def-g7-atoms-and-molecules-molecule) made of two joined oxygen [atoms](#def-g7-atoms-and-molecules-atom); $\ce{2O}$ is two separate oxygen [atoms](#def-g7-atoms-and-molecules-atom). $\ce{CO}$ is a [molecule](#def-g7-atoms-and-molecules-molecule) of one carbon and one oxygen [atom](#def-g7-atoms-and-molecules-atom) (two capital letters, two [symbols](#def-g7-atoms-and-molecules-symbol)); $\ce{Co}$ is one [atom](#def-g7-atoms-and-molecules-atom) of cobalt (one [symbol](#def-g7-atoms-and-molecules-symbol), capital then small letter).

**Exercise 11.7 ★★.**

How many hydrogen [atoms](#def-g7-atoms-and-molecules-atom) and how many oxygen [atoms](#def-g7-atoms-and-molecules-atom) are there in $\ce{3H2O}$? In $\ce{5O2}$?

**Solution of Exercise 11.7.**

$\ce{3H2O}$: $3\times 2 = 6$ hydrogen [atoms](#def-g7-atoms-and-molecules-atom) and $3\times 1 = 3$ oxygen [atoms](#def-g7-atoms-and-molecules-atom). $\ce{5O2}$: no hydrogen, $5\times 2 = 10$ oxygen [atoms](#def-g7-atoms-and-molecules-atom).

**Exercise 11.8 ★★.**

A model is made of one black ball joined to two red balls. Name the [atoms](#def-g7-atoms-and-molecules-atom), write the formula, and name the [molecule](#def-g7-atoms-and-molecules-molecule).

**Solution of Exercise 11.8.**

Black is carbon, red is oxygen: one carbon, two oxygen, $\ce{CO2}$, carbon dioxide.

**Exercise 11.9 ★★.**

In Dalton’s table, water is made of one hydrogen [atom](#def-g7-atoms-and-molecules-atom) and one oxygen [atom](#def-g7-atoms-and-molecules-atom). Write the formula Dalton had in mind, and say how many [atoms](#def-g7-atoms-and-molecules-atom) his water [molecule](#def-g7-atoms-and-molecules-molecule) is missing.

**Solution of Exercise 11.9.**

Dalton’s water was $\ce{HO}$. The real [molecule](#def-g7-atoms-and-molecules-molecule), $\ce{H2O}$, has one more hydrogen [atom](#def-g7-atoms-and-molecules-atom): Dalton’s [molecule](#def-g7-atoms-and-molecules-molecule) was missing one [atom](#def-g7-atoms-and-molecules-atom).

**Exercise 11.10 ★★.**

Is pure water a [mixture](https://one-course.com/books/chemistry/1/en/chapter/8-pure-substances-and-mixtures#def-g6-pure-substances-and-mixtures-mixture)? Is [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air)? Answer by describing the [molecules](#def-g7-atoms-and-molecules-molecule) each one contains.

**Solution of Exercise 11.10.**

Pure water is not a [mixture](https://one-course.com/books/chemistry/1/en/chapter/8-pure-substances-and-mixtures#def-g6-pure-substances-and-mixtures-mixture): all its [molecules](#def-g7-atoms-and-molecules-molecule) are identical $\ce{H2O}$ [molecules](#def-g7-atoms-and-molecules-molecule). [Air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) is a [mixture](https://one-course.com/books/chemistry/1/en/chapter/8-pure-substances-and-mixtures#def-g6-pure-substances-and-mixtures-mixture): it contains nitrogen [molecules](#def-g7-atoms-and-molecules-molecule), oxygen [molecules](#def-g7-atoms-and-molecules-molecule), argon [atoms](#def-g7-atoms-and-molecules-atom) and a few carbon dioxide [molecules](#def-g7-atoms-and-molecules-molecule).

**Exercise 11.11 ★★★.**

A hydrogen [atom](#def-g7-atoms-and-molecules-atom) measures about a ten-millionth of a millimetre across. How many hydrogen [atoms](#def-g7-atoms-and-molecules-atom), side by side, would make a line $1\,\mathrm{cm}$ long? And a line as long as a football pitch, $100\,\mathrm{m}$?

**Solution of Exercise 11.11.**

Ten million [atoms](#def-g7-atoms-and-molecules-atom) make $1\,\mathrm{mm}$, so $1\,\mathrm{cm}$ $= 10\,\mathrm{mm}$ takes $10 \times$ ten million $=$ one hundred million [atoms](#def-g7-atoms-and-molecules-atom). $100\,\mathrm{m} = 100\,000$ mm takes $100\,000\times$ ten million $=$ one million million [atoms](#def-g7-atoms-and-molecules-atom).

**Exercise 11.12 ★★★.**

Glucose, the sugar that feeds our cells, has the formula $\ce{C6H12O6}$. How many [atoms](#def-g7-atoms-and-molecules-atom) does one glucose [molecule](#def-g7-atoms-and-molecules-molecule) contain? What percentage of them are hydrogen [atoms](#def-g7-atoms-and-molecules-atom)? What percentage are carbon [atoms](#def-g7-atoms-and-molecules-atom)?

**Solution of Exercise 11.12.**

$6 + 12 + 6 = 24$ [atoms](#def-g7-atoms-and-molecules-atom). Hydrogen: $\frac{12}{24} = 50\,\%$. Carbon: $\frac{6}{24} = 25\,\%$ (and oxygen the other $25\,\%$).

## 11.6 Problem: The Atoms of a Breath

**Problem 11.1.**

Weekend problem — counting the atoms in a breath of air, before and after it has been in our lungs

Nitrogen passes through the lungs unchanged, so it makes a good yardstick. In dry [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air), for every 78 [molecules](#def-g7-atoms-and-molecules-molecule) of nitrogen there are about 21 [molecules](#def-g7-atoms-and-molecules-molecule) of oxygen and 1 [atom](#def-g7-atoms-and-molecules-atom) of argon: 100 particles ([molecules](#def-g7-atoms-and-molecules-molecule) or single [atoms](#def-g7-atoms-and-molecules-atom)) in all. 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) breathed out, once its water vapour is removed, the same 78 nitrogen [molecules](#def-g7-atoms-and-molecules-molecule) come with about 16 [molecules](#def-g7-atoms-and-molecules-molecule) of oxygen, 4 [molecules](#def-g7-atoms-and-molecules-molecule) of carbon dioxide and 1 [atom](#def-g7-atoms-and-molecules-atom) of argon.

**Part I — The [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) we breathe in.**

1. Write the formulas of the nitrogen, oxygen and carbon dioxide [molecules](#def-g7-atoms-and-molecules-molecule) , and give the number of [atoms](#def-g7-atoms-and-molecules-atom) in each.
2. Argon is counted in [atoms](#def-g7-atoms-and-molecules-atom) , not in [molecules](#def-g7-atoms-and-molecules-molecule) . What does this tell us about argon?
3. In these 100 particles of [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) breathed in, how many nitrogen [atoms](#def-g7-atoms-and-molecules-atom) are there? How many oxygen [atoms](#def-g7-atoms-and-molecules-atom) ?
4. How many [atoms](#def-g7-atoms-and-molecules-atom) are there in all in these 100 particles?
5. What percentage of these [atoms](#def-g7-atoms-and-molecules-atom) are nitrogen [atoms](#def-g7-atoms-and-molecules-atom) ? Round to the nearest whole number.

**Part II — The [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) we breathe out.**

6. In the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) breathed out that goes with the same 78 nitrogen [molecules](#def-g7-atoms-and-molecules-molecule) , how many oxygen [atoms](#def-g7-atoms-and-molecules-atom) are there inside oxygen [molecules](#def-g7-atoms-and-molecules-molecule) ? Inside carbon dioxide [molecules](#def-g7-atoms-and-molecules-molecule) ? In all?
7. Compare with question 3. How many oxygen [atoms](#def-g7-atoms-and-molecules-atom) are missing? Into which [molecule](#def-g7-atoms-and-molecules-molecule) , made in the body from the hydrogen of food and removed here with the water vapour, have they gone?
8. How many carbon [atoms](#def-g7-atoms-and-molecules-atom) are there in the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) breathed in? In the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) breathed out?
9. [Air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) brings no carbon [atoms](#def-g7-atoms-and-molecules-atom) into the lungs. Where can the carbon [atoms](#def-g7-atoms-and-molecules-atom) of the breath come from? (Think of what the body uses to live.)
10. How many oxygen [molecules](#def-g7-atoms-and-molecules-molecule) have disappeared, and how many carbon dioxide [molecules](#def-g7-atoms-and-molecules-molecule) have appeared?

**Part III — The count.**

11. Describe the ball-and-stick model of each of the four kinds of particle in breathed-out [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) , with the colours of the balls (argon is shown as a single pale blue ball).
12. How many [atoms](#def-g7-atoms-and-molecules-atom) are there in all in the dried [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) breathed out? By how many [atoms](#def-g7-atoms-and-molecules-atom) does it exceed the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) breathed in? Explain the difference with the carbon [atoms](#def-g7-atoms-and-molecules-atom) gained and the oxygen [atoms](#def-g7-atoms-and-molecules-atom) missing.

**Solution of Problem 11.1.**

**1.** Nitrogen $\ce{N2}$, 2 [atoms](#def-g7-atoms-and-molecules-atom); oxygen $\ce{O2}$, 2 [atoms](#def-g7-atoms-and-molecules-atom); carbon dioxide $\ce{CO2}$, 3 [atoms](#def-g7-atoms-and-molecules-atom).

**2.** Argon [atoms](#def-g7-atoms-and-molecules-atom) do not join into [molecules](#def-g7-atoms-and-molecules-molecule): argon is made of single [atoms](#def-g7-atoms-and-molecules-atom).

**3.** Nitrogen: $78 \times 2 = 156$ [atoms](#def-g7-atoms-and-molecules-atom). Oxygen: $21\times 2 =
42$ [atoms](#def-g7-atoms-and-molecules-atom).

**4.** $156 + 42 + 1 = 199$ [atoms](#def-g7-atoms-and-molecules-atom).

**5.** $\frac{156}{199} \approx 0.78$, about $78\,\%$.

**6.** In oxygen [molecules](#def-g7-atoms-and-molecules-molecule): $16\times 2 = 32$; in carbon dioxide [molecules](#def-g7-atoms-and-molecules-molecule): $4 \times 2 = 8$; in all $32 + 8 = 40$.

**7.** $42 - 40 = 2$ oxygen [atoms](#def-g7-atoms-and-molecules-atom) are missing: one oxygen [molecule](#def-g7-atoms-and-molecules-molecule)’s worth. They have gone into water [molecules](#def-g7-atoms-and-molecules-molecule), $\ce{H2O}$, which the body makes with the hydrogen of its food; that water leaves partly as water vapour in the breath (removed in these figures).

**8.** Breathed in: 0. Breathed out: 4 (one in each $\ce{CO2}$).

**9.** From the food the body uses: sugars and fats contain carbon [atoms](#def-g7-atoms-and-molecules-atom), and the body releases them in the carbon dioxide it breathes out.

**10.** $21 - 16 = 5$ oxygen [molecules](#def-g7-atoms-and-molecules-molecule) have disappeared and 4 carbon dioxide [molecules](#def-g7-atoms-and-molecules-molecule) have appeared.

**11.** Nitrogen: two blue balls joined by three sticks. Oxygen: two red balls joined by two sticks. Carbon dioxide: a black ball between two red balls, each joined to it by two sticks, in a straight line. Argon: a single pale blue ball.

**12.** $156 + 32 + 12 + 1 = 201$ [atoms](#def-g7-atoms-and-molecules-atom) (the 4 carbon dioxide [molecules](#def-g7-atoms-and-molecules-molecule) hold 12 [atoms](#def-g7-atoms-and-molecules-atom)). That is $201 - 199 = 2$ [atoms](#def-g7-atoms-and-molecules-atom) more than the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) breathed in: 4 carbon [atoms](#def-g7-atoms-and-molecules-atom) gained from food, minus the 2 oxygen [atoms](#def-g7-atoms-and-molecules-atom) that left in water.