Protons, neutrons and electrons

Three subatomic particles

Atoms are built from three kinds of subatomic particles. Protons and neutrons are packed together in the nucleus; electrons move in shells around it.

Protons and neutrons have almost the same mass, and each is about 1840 times heavier than an electron. So more than 99.9% of an atom's mass is in its nucleus, even though the nucleus takes up a tiny fraction of its volume. Masses this small are measured in atomic mass units (amu), where 1 amu is one-twelfth of the mass of a carbon-12 atom. A proton or a neutron is close to 1 amu.

Why atoms are neutral

A proton's positive charge is exactly as strong as an electron's negative charge. An atom has the same number of electrons as protons, so the charges cancel and the atom as a whole has no charge. The attraction between the positive nucleus and the negative electrons is what holds an atom together.

That raises a puzzle: protons all repel each other, so what stops a nucleus flying apart? The answer is the strong nuclear force, an attraction between protons and neutrons that is far stronger than electric repulsion but only works over the tiny distances inside a nucleus. Neutrons add to this "glue" without adding any repulsion, which is why bigger nuclei need more neutrons than protons to stay together.

Atomic number and mass number

Two numbers describe the nucleus of an atom:

• The atomic number (symbol Z) is the number of protons. It decides the element. Every atom with 11 protons is sodium, wherever it is in the Universe. The periodic table is arranged in order of atomic number.
• The mass number (symbol A) is the number of protons + neutrons. Because electrons have so little mass, it gives the mass of the atom in amu, to the nearest whole number.

Chemists write these as nuclide notation, with the mass number at the top left of the symbol and the atomic number at the bottom left. You can also write the name and mass number, as in sodium-23 or Na-23.

Working out the particles

For any neutral atom:

• protons = atomic number
• electrons = protons (= atomic number)
• neutrons = mass number − atomic number

Worked example: iron-56 has atomic number 26. Protons = 26, electrons = 26, neutrons = 56 − 26 = 30.

Worked example: uranium-238 has atomic number 92. Protons = 92, electrons = 92, neutrons = 238 − 92 = 146. Notice how many more neutrons than protons this heavy nucleus has.

Ions: when electrons don't match protons

Protons are locked in the nucleus and never change in a chemical reaction, but electrons in the outer shell can be gained or lost. An atom that has gained or lost electrons is called an ion.

• Losing electrons leaves more protons than electrons, so the ion is positive. A sodium atom (11 p, 11 e) loses one electron to become Na⁺ (11 p, 10 e).
• Gaining electrons makes a negative ion. An oxygen atom (8 p, 8 e) gains two to become O²⁻ (8 p, 10 e).

Charge = protons − electrons. The element stays the same because the protons haven't changed. You'll see why atoms gain or lose electrons in the periodic table chapter.