Electrons in a positive pudding

The idea

Thomson's model pictures the atom as a sphere of positive charge with electrons distributed throughout it, balancing the charges; it is called the plum pudding model.

A puzzle for Thomson

Electrons carry negative charge. Yet an atom as a whole is neutral. Thomson had to decide where the positive charge could be.

Which picture did Thomson choose for the atom?

• A sphere of positive charge with electrons spread through it
• A tiny positive centre with electrons far away
• All the electrons together in one lump

Pudding, melon, atom

Open each tab.

The plum pudding model

Thomson pictured the atom as a positive sphere with electrons scattered through it, so the charges balance.

Thomson's model pictures the atom as a sphere of positive charge with electrons distributed throughout it, balancing the charges. It is called the plum pudding model.

Notes

Thomson's model: a sphere of positive charge with electrons spread through it, balancing the charges. It is the plum pudding model.

Check yourself

In Thomson's model, where is the positive charge?

In the watermelon picture, what do the seeds stand for?

Why did Thomson's atom need positive charge at all?

Which name belongs to Thomson's model?

• Spread through a sphere, with electrons inside it — correct. Yes!
• Collected in a tiny nucleus at the centre. That is Rutherford's later model. Thomson's model has no nucleus.
• Carried by the electrons. Electrons carry negative charge. The positive charge belongs to the sphere.
• The positively charged matter. The red pulp stands for the positively charged matter.
• Electrons — correct. Yes!
• Neutrons. Neutrons were discovered much later, in 1932, and are not part of this picture.
• Electrons are heavier than atoms. Electrons have a much smaller mass than atoms.
• Cathode rays travel from the anode. Cathode rays travel from the cathode to the anode, and this is not the reason.
• Atoms are neutral, so the negative charge of electrons must be balanced — correct. Yes!
• Plum pudding model — correct. Yes!
• Planetary model. The planetary model is Rutherford's, with electrons revolving around a nucleus.
• Shell model. Bohr's model has electrons in fixed shells.