Atomic Structure Notes

Edexcel GCSE Chemistry: Revision notes

Key facts

  • The model of the atom changed from Dalton's solid sphere to Thomson's plum pudding, Rutherford's nuclear model and Bohr's shells.
  • An atom has a tiny nucleus of protons and neutrons, with electrons in shells.
  • Atoms are neutral because they have equal numbers of protons and electrons.
  • Atomic number = protons; mass number = protons + neutrons.
  • Isotopes have the same protons but different neutrons.

Solid sphere (Dalton)

+++++++++−−−−−−−−

Plum pudding (Thomson)

+

Nuclear model (Rutherford)

Li3p 4n

Bohr

2,1

How the model of the atom developed

How the atom model changed

As scientists made new discoveries, the model changed from a solid sphere to a nucleus with electrons in shells.

Each new model explained evidence the old one could not. Exam answers should name the scientists or experiments in the correct order.

  1. Early 1800s

    Dalton

    Atoms are tiny, solid spheres that cannot be divided.

  2. 1897

    Thomson

    Discovers the electron: plum pudding model.

  3. 1909–11

    Rutherford

    Alpha scattering shows a tiny, dense, positive nucleus.

  4. 1913

    Bohr

    Electrons occupy fixed shells (energy levels).

  5. 1932

    Chadwick

    The nucleus contains protons and neutrons.

Solid sphere (Dalton)

+++++++++−−−−−−−−

Plum pudding (Thomson)

+

Nuclear model (Rutherford)

Li3p 4n

Bohr

2,1

Dalton, plum pudding, nuclear and Bohr models

Which experiment showed that an atom has a tiny, dense, positively charged nucleus?

What is inside an atom?

An atom has a tiny nucleus of protons and neutrons surrounded by electrons in shells.

Almost all of the mass of an atom is in the nucleus, but the nucleus is very small compared with the atom, so the atom is mostly empty space. The charges of protons and electrons balance, so atoms have no overall charge.

C6p 6n

Carbon atom

2,4

A carbon atom: 6 protons, 6 neutrons, 6 electrons

Proton

Relative charge:
+1
Relative mass:
1

Neutron

Relative charge:
0
Relative mass:
1

Electron

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

Why is an atom electrically neutral?

Atomic number and mass number

The atomic number is the number of protons; the mass number is protons plus neutrons.

The atomic number is unique to each element and equals the number of electrons in a neutral atom. The mass number is the total number of protons and neutrons in the nucleus. All atoms of an element have the same number of protons.

  • Protons = electronsatomic number
  • Neutronsmass number − atomic number

Worked example

Sodium has atomic number 11 and mass number 23. How many protons, electrons and neutrons does an atom have?

An atom has atomic number 17 and mass number 35. How many neutrons does it have?

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons.

Isotopes have the same number of protons but different mass numbers. Because isotopes occur in different natural abundances, the relative atomic mass (ArA_r) of an element is a weighted average and is often not a whole number.

C6p 6n

Carbon-12

2,4

C6p 8n

Carbon-14

2,4

Two isotopes of carbon have the same protons but different neutrons
  • ArA_r (Higher tier)∑(mass×% abundance)100\dfrac{\sum(\text{mass} \times \%\text{ abundance})}{100}

Worked example

Chlorine exists as 75% X35X2235Cl\ce{^{35}Cl} and 25% X37X2237Cl\ce{^{37}Cl}. Calculate its relative atomic mass.

What is the same in two isotopes of an element?

Try an exam question

Describe how Rutherford's alpha-particle scattering experiment changed the model of the atom.

[3 marks]

That's the notes covered.

Carry on to the next subtopic.