Gold and oxygen could hardly be more different: one is a shiny yellow metal, the other an invisible gas. Yet their atoms are built from exactly the same three parts: protons, neutrons and electrons. The only difference is how many of each they have. Like building with the same set of blocks, change the count and you get a different element. Once you can count these particles, you can draw a picture of any of the first 20 atoms and start to predict how it will behave.
| Particle | Symbol | Charge | Relative mass | Location |
|---|---|---|---|---|
| Proton | p⁺ | +1 | 1 | In the nucleus |
| Neutron | n⁰ | 0 (neutral) | 1 | In the nucleus |
| Electron | e⁻ | −1 | about 1/1836 (almost 0) | In shells (energy levels) around the nucleus |
The atomic number is the number of protons in the nucleus. It is what makes an element that element: every atom with 6 protons is carbon, and every atom with 79 protons is gold. In a neutral atom, the number of electrons equals the number of protons, so the positive and negative charges cancel. The mass number is the total number of protons plus neutrons, the heavy particles in the nucleus. The mass number is not the same as the atomic mass on the periodic table, which is an average with decimals. For the first 20 elements, rounding the atomic mass to the nearest whole number gives the mass number of the most common form of the element (this shortcut fails for a few heavier elements, such as copper and bromine).
Drawing a Bohr-Rutherford diagram
- Find the element on the periodic table. Note its atomic number and round its atomic mass to get the mass number.
- Calculate the protons (atomic number), neutrons (mass number minus atomic number) and electrons (equal to protons for a neutral atom).
- Draw a small circle for the nucleus and write the number of protons and neutrons inside it, for example 13 p and 14 n.
- Draw circles (shells) around the nucleus. Fill the first shell with up to 2 electrons, the second with up to 8 and the third with up to 8.
- Any electrons left over go in the fourth shell (only potassium and calcium need it among the first 20 elements).
- Check that the total number of electrons drawn equals the number of protons.
Isotopes
Atoms of the same element always have the same number of protons, but they can have different numbers of neutrons. These different forms are called isotopes. Isotopes of an element react the same way chemically, because chemistry depends mostly on electrons, but they have different masses. An isotope is named with its mass number, such as carbon-14. The atomic mass on the periodic table is an average of all the natural isotopes. Chlorine's atomic mass is 35.45 because about three quarters of chlorine atoms are chlorine-35 and about one quarter are chlorine-37.
| Isotope | Protons | Neutrons | Notes |
|---|---|---|---|
| Hydrogen-1 | 1 | 0 | About 99.98% of hydrogen atoms |
| Hydrogen-2 (deuterium) | 1 | 1 | Makes heavy water, used in Canadian CANDU nuclear reactors |
| Carbon-12 | 6 | 6 | About 99% of carbon atoms |
| Carbon-14 | 6 | 8 | Radioactive; used to find the age of ancient bones and wood (radiocarbon dating) |
Ions
Atoms are most stable when their outer shell is full, like the noble gases. To get there, an atom may gain or lose electrons in a chemical change (its protons stay the same). The result is an ion, a charged atom. Losing electrons leaves more protons than electrons, so the ion is positive. Gaining electrons gives more electrons than protons, so the ion is negative. Metals usually lose electrons and non-metals usually gain them. The charge is protons minus electrons.
- Sodium atom: 11 p, 11 e, shells 2, 8, 1
- Loses its 1 outer electron
- Sodium ion: 11 p, 10 e, shells 2, 8
- Charge: 11 − 10 = +1, written Na⁺
- Chlorine atom: 17 p, 17 e, shells 2, 8, 7
- Gains 1 electron to fill its outer shell
- Chloride ion: 17 p, 18 e, shells 2, 8, 8
- Charge: 17 − 18 = −1, written Cl⁻