--- title: "Elements in the Universe and on Earth" book: "School Chemistry — Grades 1 to 12" subject: chemistry language: en chapter: 21 exercises: 12 source: https://one-course.com/books/chemistry/1/en/chapter/21-elements-in-the-universe-and-on-earth license: CC-BY-NC-SA-4.0 credit: "One Chemistry Book, One Course (one-course.com)" --- # Chapter 21 — Elements in the Universe and on Earth The calcium in your bones, the iron in your blood and the oxygen you breathe were not made on Earth. They were made inside stars that lived and died before the Sun was born, and they have been passed from rock to sea, from sea to plant, from plant to animal, for billions of years. Some elements are everywhere, others rare. Which elements make up the stars, the Earth, the oceans — and us? **You already know.** A [chemical element](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-element) is the set of [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) with the same [atomic number](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-atomic-number) $Z$; the mass of an [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) is close to $A \times 1.67 \times 10^{-27}\,\mathrm{kg}$, where $A$ is its [mass number](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-atomic-number) ([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) lists the 118 elements by [atomic number](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-atomic-number) ([Chapter 20](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#ch-g9-periodic-table-first-look)). [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)). ## 21.1 Elements of the stars **Definition 21.1 (Abundance).** The *abundance* of an element in a body of matter — a star, the Earth’s crust, the ocean, a living being — is its share of the total mass, usually given in percent. **Proposition 21.2 (The Sun is hydrogen and helium).** The surface of the Sun is, by mass, about $74\,\%$ hydrogen and $25\,\%$ helium; all the other elements together make up only about $1.3\,\%$. Most of the visible matter of the Universe has a similar composition: hydrogen and helium, with a pinch of everything else. **Proposition 21.3 (Elements made in stars).** Hydrogen and most of the helium were formed in the first minutes of the Universe. Nearly all the heavier elements — carbon, oxygen, iron, gold — were made later, inside stars, and scattered into space when stars exploded. New stars and planets formed from that enriched dust. How the stars do it is told in the physics book. ![The Crab Nebula, the cloud left by a star that exploded nearly a thousand years ago, spreading new elements into space (NASA, ESA, J. Hester and A. Loll).](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-elements-universe-earth/img-8fcb8009e37d.jpg) *The Crab Nebula, the cloud left by a star that exploded nearly a thousand years ago, spreading new elements into space (NASA, ESA, J. Hester and A. Loll).* ## 21.2 The Earth **Proposition 21.4 (The Earth’s crust).** The thin rocky skin of the Earth, the crust, is almost half oxygen and more than a quarter silicon by mass: rocks and sand are mostly oxides of silicon and of a few [metals](https://one-course.com/books/chemistry/1/en/chapter/20-the-periodic-table-a-first-look#def-g9-periodic-table-first-look-metal). Eight elements — oxygen, silicon, aluminium, iron, calcium, sodium, potassium, magnesium — make up about $99\,\%$ of it. Hydrogen and helium, so common in the stars, are rare: the light gases escaped from the young Earth. **Remark 21.5 (Inside the Earth).** The crust is not the whole Earth. Iron, heavy, sank towards the centre when the young Earth was molten: the core of the Earth is mostly iron, with some nickel. **Proposition 21.6 (The oceans and the air).** Sea water is mostly water — hydrogen and oxygen — with about $3.5\,\%$ of dissolved salts by mass. Among the dissolved [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion), chloride and sodium make up about $85\,\%$, magnesium and sulfate another $10\,\%$. 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) is mostly nitrogen and oxygen. ![Abundances by mass. Left: the eight main elements of the Earth’s crust. Right: the six main elements of a lean adult human body (computed from estimated numbers of atoms).](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-elements-universe-earth/fig-9a8b820bd3ef.svg) ![Abundances by mass. Left: the eight main elements of the Earth’s crust. Right: the six main elements of a lean adult human body (computed from estimated numbers of atoms).](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-elements-universe-earth/fig-87605c4e2f8d.svg) *[Abundances](#def-g9-elements-universe-earth-abundance) by mass. Left: the eight main elements of the Earth’s crust. Right: the six main elements of a lean adult human body (computed from estimated numbers of [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom)).* ## 21.3 The human body **Proposition 21.7 (What we are made of).** By mass, a lean adult body is about $61\,\%$ oxygen, $23\,\%$ carbon and $10\,\%$ hydrogen — mostly in water and in the [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) of life — followed by nitrogen, calcium (in bones and teeth) and phosphorus. Counted in [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) rather than in mass, the order changes: there are more hydrogen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) in a body than all other [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) together, because hydrogen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) are so light. A body of $70\,\mathrm{kg}$ holds about $7 \times 10^{27}$ [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom). **Method 21.8 (Reading an abundance chart).** 1. Check what is measured: mass (most charts) or number of [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) . 2. Read the share of each element; add those of the main ones to see how much they cover. 3. To compare two bodies, look for the elements common to both and for those found in one only. Oxygen tops both charts of this chapter: it is the commonest element of the crust and of the human body by mass. ## 21.4 The same atoms, recycled **Example 21.9 (The journey of a carbon atom).** A carbon [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) made in a star long ago can sit for millions of years in limestone, be released as carbon dioxide by a volcano, be taken up by a leaf, become part of a sugar in a fruit, pass into an animal that eats the fruit, and be breathed out again as carbon dioxide. [Atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) are not destroyed by [chemical changes](https://one-course.com/books/chemistry/1/en/chapter/6-changes-that-cannot-be-undone#def-g5-irreversible-changes-chemical-change): they are passed on. ![A carbon atom passes from rock to air to leaf to animal and back to the air: the atom itself is never destroyed.](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-elements-universe-earth/fig-ba97f17a420f.svg) *A carbon [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) passes from rock to [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) to leaf to animal and back to 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): the [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) itself is never destroyed.* ![Rock, sea and air: three very different mixtures of the same elements.](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-elements-universe-earth/img-8ae99578c545.jpg) *Rock, sea and [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air): three very different [mixtures](https://one-course.com/books/chemistry/1/en/chapter/8-pure-substances-and-mixtures#def-g6-pure-substances-and-mixtures-mixture) of the same elements.* ## 21.5 Exercises **Exercise 21.1 ★.** Which two elements make up almost all of the Sun? **Solution of Exercise 21.1.** Hydrogen (about $74\,\%$) and helium (about $25\,\%$). **Exercise 21.2 ★.** Which element is the most abundant in the Earth’s crust? In the human body (by mass)? **Solution of Exercise 21.2.** Oxygen in both. **Exercise 21.3 ★.** Where were most of the elements heavier than helium made? **Solution of Exercise 21.3.** Inside stars, and in their explosions. **Exercise 21.4 ★.** What percentage of the mass of sea water is dissolved salt? **Solution of Exercise 21.4.** About $3.5\,\%$. **Exercise 21.5 ★★.** Using the crust chart, add the shares of oxygen and silicon. What fraction of the crust is that, roughly? **Solution of Exercise 21.5.** $46.6 + 27.7 = 74.3\,\%$: about three quarters. **Exercise 21.6 ★★.** Why is there so little hydrogen and helium in the Earth’s crust, when the Sun is made almost entirely of them? **Solution of Exercise 21.6.** They are very light gases: they escaped from the young Earth into space (and helium forms no compounds that could have kept it in the rocks). **Exercise 21.7 ★★.** What mass of carbon is there in a $70\,\mathrm{kg}$ body that is $23\,\%$ carbon? **Solution of Exercise 21.7.** $0.23 \times 70 = 16.1\,\mathrm{kg}$. **Exercise 21.8 ★★.** Why is oxygen the commonest element of the body by mass, while hydrogen is the commonest by number of [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom)? **Solution of Exercise 21.8.** Oxygen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) are 16 times heavier than hydrogen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom): fewer oxygen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) still weigh more. **Exercise 21.9 ★★.** A $1000\,\mathrm{kg}$ block of granite is a typical piece of the crust. Using the chart, what mass of aluminium does it hold, about? **Solution of Exercise 21.9.** About $0.081 \times 1000 = 81\,\mathrm{kg}$ of aluminium. **Exercise 21.10 ★★.** Which elements appear among the main elements of both the crust and the human body? **Solution of Exercise 21.10.** Oxygen and calcium (and, beyond the eight main elements of the crust, hydrogen, carbon and phosphorus are present in the crust in small amounts). **Exercise 21.11 ★★★.** A $70\,\mathrm{kg}$ body holds about $1.06\,\mathrm{kg}$ of calcium. Compute the mass of one calcium [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) ($A = 40$), then the number of calcium [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom). Write it in scientific notation. **Solution of Exercise 21.11.** $40 \times 1.67 \times 10^{-27} = 6.68 \times 10^{-26}\,\mathrm{kg}$; $1.06 / 6.68 \times 10^{-26} \approx 1.6 \times 10^{25}$ calcium [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom). **Exercise 21.12 ★★★.** The [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) of your body have all been part of other things before. Use the journey of a carbon [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) to explain why the total number of carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) on Earth hardly changes. **Solution of Exercise 21.12.** [Atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) are neither created nor destroyed by [chemical changes](https://one-course.com/books/chemistry/1/en/chapter/6-changes-that-cannot-be-undone#def-g5-irreversible-changes-chemical-change): they only pass from one compound to another (rock, gas, sugar, living body). The carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) of the Earth are used again and again. ## 21.6 Problem: How Many Atoms Am I? **Problem 21.1.** Weekend problem — from an abundance chart to the number of oxygen atoms in a human body A lean adult of $70\,\mathrm{kg}$ is, by mass, about $61\,\%$ oxygen, $23\,\%$ carbon and $10\,\%$ hydrogen. 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 $1.67 \times 10^{-27}\,\mathrm{kg}$; an oxygen [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) has 16 [nucleons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton), a carbon [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) 12, a hydrogen [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) 1. [Electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) are neglected. **Part I — The masses.** 1. Compute the mass of oxygen in the body. 2. Compute the mass of carbon and of hydrogen. 3. What percentage of the mass do these three elements make up together? 4. Most of this oxygen and hydrogen is in one compound. Which? **Part II — One [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom).** 5. Compute the mass of one oxygen [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) . 6. Compute the mass of one carbon [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) and of one hydrogen [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) . 7. How many hydrogen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) weigh as much as one oxygen [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) ? 8. Why can [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) be neglected? **Part III — The count.** 9. Compute the number of hydrogen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) in the body. 10. Compute the number of carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) . 11. Compute the number of oxygen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) in the body. 12. A careful estimate gives $1.61 \times 10^{27}$ oxygen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) in such a body. Compare with your count, and state the number of oxygen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) in a $70\,\mathrm{kg}$ body to two significant figures. **Solution of Problem 21.1.** **1.** $0.61 \times 70 = 42.7\,\mathrm{kg}$. **2.** Carbon $0.23 \times 70 = 16.1\,\mathrm{kg}$; hydrogen $0.10 \times 70 = 7.0\,\mathrm{kg}$. **3.** $61 + 23 + 10 = 94\,\%$. **4.** Water, $\ce{H2O}$. **5.** $16 \times 1.67 \times 10^{-27} = 2.67 \times 10^{-26}\,\mathrm{kg}$. **6.** Carbon: $12 \times 1.67 \times 10^{-27} = 2.00 \times 10^{-26}\,\mathrm{kg}$; hydrogen: $1.67 \times 10^{-27}\,\mathrm{kg}$. **7.** 16. **8.** 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 [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) are less than a thousandth of the mass. **9.** $7.0 / 1.67 \times 10^{-27} \approx 4.2 \times 10^{27}$ hydrogen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom). **10.** $16.1 / 2.00 \times 10^{-26} \approx 8.0 \times 10^{26}$ carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom). **11.** $42.7 / 2.67 \times 10^{-26} \approx 1.60 \times 10^{27}$ oxygen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom). **12.** Our count, $1.60 \times 10^{27}$, agrees with the estimate $1.61 \times 10^{27}$: a $70\,\mathrm{kg}$ body holds about $1.6 \times 10^{27}$ oxygen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom).