Atomic Structure Notes

Edexcel GCSE Physics: Revision notes

Key facts

  • An atom has a tiny positive nucleus (protons and neutrons) surrounded by negative electrons.
  • A neutral atom has equal numbers of protons and electrons.
  • Atomic number = protons; mass number = protons + neutrons. Isotopes have the same protons but different neutrons.
  • Electrons change orbit when they absorb or emit radiation; losing an outer electron makes a positive ion.
  • Rutherford's scattering experiment replaced the plum pudding model with a nuclear atom.

Inside the atom

An atom has a tiny, dense, positive nucleus surrounded by negative electrons.

The nucleus holds the protons and neutrons. Its radius is much smaller than the atom's, yet almost all the mass is in it.

Typical atoms have a diameter of order 10−1010^{-10} m.

In a neutral atom the number of protons equals the number of electrons, so the charges balance.

Li3p 4n

Lithium atom

2,1

A lithium atom: 3 protons, 4 neutrons, 3 electrons.
  • Neutral atomprotons = electrons

A neutral atom has 11 protons. How many electrons does it have?

Atomic number and isotopes

The atomic number counts protons; the mass number counts protons plus neutrons.

Write a nucleus as X613X26213C\ce{^{13}_{6}C}: the top number is the mass number, the bottom is the atomic number.

Isotopes are atoms of the same element (same atomic number) with different numbers of neutrons, so different mass numbers.

Carbon-12 and carbon-14 both have 6 protons, but 6 and 8 neutrons.

C6p 6n

Carbon-12

2,4

C6p 7n

Carbon-13

2,4

Carbon-12 and carbon-13 are isotopes: the same protons, different neutrons.
  • Atomic numbernumber of protons
  • Mass numberprotons + neutrons

Worked example

How many protons, neutrons and electrons are in a neutral atom of X614X26214C\ce{^{14}_{6}C}?

What is the same in all isotopes of an element?

Particle masses and charges

Protons and neutrons are about 1900 times heavier than electrons, and a positron is a positive electron.

A positron has the same mass as an electron but the opposite (positive) charge. It is the antimatter partner of the electron.

−1−0.500.51ProtonNeutronElectronPositronParticleRelative charge
Relative charge of the four particles (a positron is a positive electron).

Proton

Relative mass:
1
Relative charge:
+1

Neutron

Relative mass:
1
Relative charge:
0

Electron

Relative mass:
Very small (about 1/1836)
Relative charge:
−1

Positron

Relative mass:
Very small (about 1/1836)
Relative charge:
+1

Which particle has the same mass as an electron but a positive charge?

Electrons and ions

Electrons jump between orbits by absorbing or emitting radiation, and losing an outer electron makes a positive ion.

Electrons orbit the nucleus at set distances (energy levels). They move to a higher orbit when they absorb electromagnetic radiation and drop to a lower orbit when they emit it.

An atom forms a positive ion by losing an outer electron. There are then more protons than electrons.

Na11p 12n

Sodium atom

2,8,1

+Na11p 12n

Sodium ion

2,8

A sodium atom loses an electron to form a positive ion.

Absorbs radiation

  • Electron moves to a higher orbit

Emits radiation

  • Electron drops to a lower orbit

Loses an outer electron

  • Atom becomes a positive ion

Why does an atom that loses an electron become positively charged?

How the atom model changed

Evidence from experiments replaced the plum pudding model with a nuclear atom and then electron shells.

The plum pudding model had electrons embedded in a ball of positive charge.

In Rutherford's experiment, alpha particles were fired at thin gold foil. Most went straight through, but a few were deflected strongly or bounced back.

The Bohr model put electrons in orbits at fixed distances.

Solid sphere (Dalton)

+++++++++−−−−−−−−

Plum pudding (Thomson)

+

Nuclear model (Rutherford)

Li3p 4n

Bohr

2,1

How the model of the atom developed.

  1. 1904

    Plum pudding

    Electrons in a ball of positive charge.

  2. 1909–11

    Rutherford scattering

    A tiny, dense, positive nucleus.

  3. 1913

    Bohr model

    Electrons orbit at fixed energy levels.

Most alpha particles pass straight through

  • Most of the atom is empty space

Some deflect or bounce back

  • A small, dense, positive nucleus

What did the plum pudding model suggest?

Try an exam question

Describe Rutherford's alpha particle scattering experiment and explain what it showed about the structure of the atom.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.