The neutron, atomic number, mass number, isotopes and isobars
The neutron (Chadwick, 1932) is neutral and sits in the nucleus. Atomic number Z counts protons, and mass number A = protons + neutrons.
Why a third particle was needed
Helium has 2 protons and hydrogen has 1. If only protons gave mass, helium would be twice as heavy as hydrogen. In fact, it is 4 times as heavy. So the nucleus must hold another particle with mass but no charge. In 1932, James Chadwick found it and named it the neutron (n). Neutrons are in the nucleus of every atom except ordinary hydrogen. Helium has 2 protons and 2 neutrons, which explains its 4 : 1 mass ratio with hydrogen.
The three particles of the atom
| Particle | Where it is | Mass (kg) | Charge (C) | Relative charge |
|---|---|---|---|---|
| Electron (e) | Outside the nucleus, in shells | 9.109 × 10⁻³¹ | −1.602 × 10⁻¹⁹ | −1 |
| Proton (p) | Nucleus | 1.673 × 10⁻²⁷ | +1.602 × 10⁻¹⁹ | +1 |
| Neutron (n) | Nucleus | 1.675 × 10⁻²⁷ | 0 | 0 |
Atomic number and mass number
- Atomic number (Z) = number of protons. Every atom of an element has the same Z. It is the element's identity card.
- In a neutral atom, number of electrons = number of protons = Z.
- Protons and neutrons together are called nucleons.
- Mass number (A) = number of protons + number of neutrons.
- So, number of neutrons = A − Z.
- An atom is written with A raised and Z lowered before the symbol: ²³₁₁Na is sodium with A = 23 and Z = 11.
Worked example: counting the particles
Find the number of protons, electrons and neutrons in (a) ²⁷₁₃Al and (b) ³⁵₁₇Cl.
- (a) Z = 13, so protons = 13 and electrons = 13.
- Neutrons = A − Z = 27 − 13 = 14.
- (b) Z = 17, so protons = 17 and electrons = 17.
- Neutrons = A − Z = 35 − 17 = 18.
Aluminium: 13 protons, 13 electrons, 14 neutrons. Chlorine-35: 17 protons, 17 electrons, 18 neutrons
Isotopes and isobars
Memory trick: isotoPes have the same Protons; isobArs have the same A.
Worked example: why chlorine's atomic mass is 35.5
Natural chlorine is about 75% chlorine-35 and 25% chlorine-37. Find its average atomic mass.
- Average atomic mass = (35 × 75 + 37 × 25) ÷ 100
- = (2625 + 925) ÷ 100
- = 3550 ÷ 100 = 35.5
35.5 u. This is why tables give chlorine a fractional atomic mass
Isotopes are at work in Indian hospitals and laboratories. Cobalt-60 treats cancer, iodine-131 treats thyroid disease, and carbon-14 tells the age of old wood and bones.
1 marks: “An atom has 12 protons and 12 neutrons. Name the element and give its mass number.” Z = 12, so it is magnesium; A = 12 + 12 = 24.
2 marks: “Define isotopes. How many neutrons are there in ³⁵₁₇Cl and in ³⁷₁₇Cl?” Isotopes are atoms of the same element with the same atomic number but different mass numbers. ³⁵Cl: 35 − 17 = 18 neutrons; ³⁷Cl: 37 − 17 = 20 neutrons.
2 marks: “What are isobars? Are ⁴⁰₁₈Ar and ⁴⁰₂₀Ca isotopes or isobars? Give a reason.” Isobars are atoms of different elements with the same mass number. These two are isobars: the same A (40), but different Z (18 and 20).
isotoPes: same Protons. isobArs: same A. Neutrons = A − Z.