- Name the particles in an atom and their charges
- Work out the numbers of protons, neutrons and electrons from the atomic number and mass number
- Explain isotopes and calculate an average relative atomic mass
The ancient Greek philosopher Democritus believed that matter cannot be divided forever: in the end an indivisible grain remains. He called it an atom, meaning “uncuttable”. Atoms really are tiny — a few million of them side by side would make a line only about one millimetre long. But in the 20th century scientists found that even the atom has parts: it contains still smaller particles.
What is an atom made of?
At the centre of an atom is a positively charged nucleus. The nucleus is made of protons and neutrons. Negatively charged electrons move around the nucleus. The nucleus is tens of thousands of times smaller than the atom, yet it holds almost all of the atom's mass.
| Particle | Charge | Relative mass | Where it is |
|---|---|---|---|
| Proton (p⁺) | +1 | 1 | in the nucleus |
| Neutron (n⁰) | 0 | 1 | in the nucleus |
| Electron (e⁻) | −1 | 1/1836 | around the nucleus |
An atom is electrically neutral: it has as many electrons as protons, so the positive and negative charges cancel out.
Atomic number and mass number
The number of protons in the nucleus. It gives the element's place in the periodic table and the charge of the nucleus. In a neutral atom the number of electrons also equals Z.
- Amass number — the total number of protons and neutrons
- Zatomic number — the number of protons
- Nthe number of neutrons
The mass number is written at the top left of the symbol: ²³Na, ³⁵Cl.
Sodium has atomic number 11 and mass number 23 (²³Na). How many protons, neutrons and electrons does its atom have?
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Electrons (neutral atom): e = p = 11.
Neutrons: N = A − Z = 23 − 11 = 12.
Isotopes
Atoms of the same element always have the same number of protons, but the number of neutrons can differ. Such atoms are called isotopes. For example, hydrogen has three isotopes: protium ¹H (no neutrons), deuterium ²H (1 neutron) and tritium ³H (2 neutrons). Natural carbon consists of ¹²C, ¹³C and ¹⁴C.
The relative atomic mass (Ar) shown in the periodic table is the average mass of all the natural isotopes of an element. That is why it is often not a whole number.
Natural chlorine is 75% ³⁵Cl and 25% ³⁷Cl. Calculate the average relative atomic mass of chlorine.
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Ar = 0.75 · 35 + 0.25 · 37 = 26.25 + 9.25 = 35.5.
This is exactly the value given for chlorine in the periodic table.
Electron shells
Electrons are arranged around the nucleus in electron shells (energy levels). The first shell holds at most 2 electrons, the second at most 8. In the first 20 elements, the fourth shell starts filling once the third has 8 electrons. For example: H — 1; C — 2, 4; O — 2, 6; Na — 2, 8, 1; Cl — 2, 8, 7; Ca — 2, 8, 8, 2.
- 1897J. J. Thomson discovers the electron
- 1911Ernest Rutherford discovers the atomic nucleus
- 1913Niels Bohr proposes his model of the atom with electron orbits
- 1932James Chadwick discovers the neutron
Key points
- An atom consists of a positive nucleus and electrons moving around it.
- The nucleus contains protons (+1) and neutrons (0); an electron has a charge of −1.
- The atomic number Z is the number of protons; a neutral atom has Z electrons too.
- Mass number A = Z + N, so the number of neutrons is N = A − Z.
- Isotopes are atoms of the same element with different numbers of neutrons.
Check yourself
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