2.1.1Foundations in Chemistry
Atomic Structure and Isotopes
5 min read 5 parts Lesson 2 of 18
Section 1 of 5Inside the Atom
Atoms are mostly empty space, with a small nucleus surrounded by electrons in energy levels.
Nucleus
- A tiny, dense, positively charged centre containing protons and neutrons, which holds almost all of the mass of the atom.
Proton
- Relative mass 1 and relative charge +1; the number of protons defines which element an atom belongs to.
Neutron
- Relative mass 1 and no charge; neutrons add mass and help to hold the nucleus together.
Electron
- Relative mass of about 1/1836 and relative charge −1;
- electrons move around the nucleus in shells and are lost or gained to form ions.
Section 2 of 5Atomic Number, Mass Number and Ions
1
The atomic number Z is the number of protons in the nucleus, and in a neutral atom it equals the number of electrons.
2
The mass number A is the total number of protons plus neutrons, so the number of neutrons is A − Z.
3
A positive ion forms when an atom loses electrons, so it has more protons than electrons and the charge is the difference.
4
A negative ion forms when an atom gains electrons, so it has more electrons than protons.
5
Protons and neutrons stay the same when an ion forms, because only electrons are transferred in chemical reactions.
Section 3 of 5Isotopes
Isotopes
Definition
- Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Chemical properties
- They have the same electron configuration, so they react in the same way and have identical chemical properties.
Physical properties
- Different mass leads to differences in properties such as density, melting point and rate of diffusion.
Example
- Chlorine-35 has 18 neutrons and chlorine-37 has 20 neutrons, but both have 17 protons and 17 electrons.
| Particle | Relative mass | Relative charge |
|---|---|---|
| Proton | 1 | +1 |
| Neutron | 1 | 0 |
| Electron | 1/1836 | −1 |
Section 4 of 5Worked Example
Question State the numbers of protons, neutrons and electrons in the ion ²⁷Al³⁺ (atomic number of aluminium = 13).
- Protons = atomic number = 13.
- Neutrons = mass number − atomic number = 27 − 13 = 14.
- A 3+ charge means three electrons are lost from the neutral atom, so electrons = 13 − 3 = 10.
Answer 13 protons, 14 neutrons and 10 electrons.
Section 5 of 5Exam Checklist & Tips
2.1.1 – Key Points for the Exam
- Atomic number The number of protons in the nucleus of an atom, shown by the symbol Z.
- Mass number The total number of protons and neutrons in the nucleus of an atom.
- Isotopes Atoms of the same element with different numbers of neutrons and therefore different masses.
- Ion An atom or group of atoms that has gained or lost electrons and so carries an overall charge.
Common Mistakes
- Stating that isotopes have different numbers of protons or electrons; only the neutron number differs.
- Saying that isotopes have different chemical properties, when their electron arrangements are identical.
- Calculating neutrons as A + Z rather than A − Z.
- Changing the proton number when working out the particles in an ion, instead of changing the electron number.