Atomic Structure Notes

Oxford AQA IGCSE Physics: Revision notes

Key facts

  • An atom has a small, positive nucleus (protons and neutrons) surrounded by electrons; its radius is about 10−1010^{-10} m.
  • The nucleus is tiny but holds almost all the atom's mass.
  • Alpha particle scattering showed a small, dense, positive nucleus.
  • Proton: mass 1, charge +1. Neutron: mass 1, charge 0. Electron: very small mass, charge −1.
  • Atomic number = protons; mass number = protons + neutrons; isotopes have the same atomic number but different mass numbers.

Inside the atom

A tiny positive nucleus of protons and neutrons holds almost all the mass, with electrons in the space around it.

An atom has a small central nucleus containing protons and neutrons, surrounded by electrons. The atomic radius is about 10−1010^{-10} m. The nucleus is much smaller than the atom but holds almost all its mass; the electrons have negligible mass.

Li3p 4n

Lithium

2,1

A lithium atom: protons and neutrons in the nucleus, electrons in shells.

Where is almost all of the mass of an atom?

Evidence for the nucleus

Firing alpha particles at thin foil showed atoms are mostly empty space with a small, dense, positive nucleus.

In alpha particle scattering, most alpha particles pass straight through, so atoms are mostly empty space. A few deflect at large angles or bounce back, so there is a small, dense, positive nucleus repelling the positive alpha particles. This replaced the plum pudding model with the nuclear model.

thin metal foilmost pass straight throughdeflectedbounces back
Alpha particles fired at thin metal foil (not to scale).
+++++++++−−−−−−−−

Plum pudding (Thomson)

+

Nuclear model (Rutherford)

Models of the atom: plum pudding, then the nuclear model.

Most alpha particles pass straight through the foil. What does this show?

Particles and charge

Protons and neutrons have the same mass, electrons almost none; a neutral atom has equal protons and electrons.

In a neutral atom the number of electrons equals the number of protons. Atoms that lose or gain electrons form ions: losing electrons gives a positive ion, gaining gives a negative ion.

Li3p 4n

Lithium atom

2,1

+Li3p 4n

Lithium ion

2

Lithium atom: 3 protons and 3 electrons. Losing one electron gives a Li⁺ ion: still 3 protons, now 2 electrons.

Proton

Relative mass:
1
Relative charge:
+1

Neutron

Relative mass:
1
Relative charge:
0

Electron

Relative mass:
Very small
Relative charge:
−1

Worked example

An ion has 11 protons and 10 electrons. What is its overall charge?

An atom gains one electron. It becomes a...

Atomic number and isotopes

Atomic number counts protons, mass number counts protons plus neutrons, and isotopes differ only in neutrons.

The atomic number is the number of protons. The mass number is protons plus neutrons. Isotopes are atoms of the same element with different numbers of neutrons. Write mass number above atomic number: for carbon-14, X614X26214C\ce{^{14}_{6}C}.

C6p 6n

Carbon-12

2,4

C6p 8n

Carbon-14

2,4

Isotopes of carbon: same protons and electrons, different neutrons.

Worked example

How many neutrons are in an atom of carbon-14, X614X26214C\ce{^{14}_{6}C}?

Sodium-23 has atomic number 11. How many neutrons does it have?

Try an exam question

A carbon-14 atom is written X614X26214C\ce{^{14}_{6}C}. State the numbers of protons, neutrons and electrons, and explain why it is an isotope of carbon-12.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.