6 min readConcept sheetSection A MCQ (1 mark) and objective (2 marks, two parts); Section B very short answer (2 marks): the model and one limitation

Thomson's plum pudding model and its limits

Thomson pictured the atom as a sphere of positive charge with electrons dotted through it, like seeds in a watermelon. It could not explain alpha particle scattering.

Thomson's modelThe atom is a sphere of evenly spread positive charge with electrons embedded in it; the total charge is zero

Other namesPlum pudding model; also compared to a watermelon (red pulp = positive charge, seeds = electrons)

Main limitationIt could not explain why a few alpha particles bounced back from gold foil, because it had no nucleus

Thomson's idea

Thomson knew that atoms are neutral but contain negative electrons. So, he reasoned, an atom must also hold an equal amount of positive charge. In his model, the atom is a sphere of positive charge spread evenly throughout. The electrons are embedded in it here and there. The positive and negative charges are equal, so the atom as a whole is neutral. It was called the plum pudding model: the electrons are the plums in a pudding of positive charge.

Two ways to picture it

Plum puddingThe pudding is the positive charge. The plums (dry fruit) dotted through it are the electrons.

WatermelonThe red pulp is the positive charge. The black seeds are the electrons. But a real atom has far fewer electrons than a watermelon has seeds.

Limits of Thomson's model

It could not explain the alpha scattering results: a few alpha particles turned back from a thin gold foil.
Positive charge spread thinly through the atom is too weak to turn back a fast alpha particle. The model predicted that nearly all would pass straight through, with tiny deflections.
It had no nucleus, so it said nothing about where the mass of the atom is concentrated.
It was still a step forward: it was the first model to fit the facts that atoms contain electrons and are neutral.

How to draw Thomson's model

Draw a large circle and shade it lightly. Label it sphere of positive charge.
Write + signs spread evenly across the circle.
Draw small dots with − signs scattered inside it. Label one electron.
Below it, write: total positive charge = total negative charge, so the atom is neutral.

In real life

Think of a slice of the watermelons sold on Indian roadsides in summer. The red pulp is the positive charge, and the black seeds are the electrons.

Exam angle

2 marks: “Explain, with an everyday fruit as an example, how Thomson pictured the atom.” Thomson's atom is a sphere of evenly spread positive charge with electrons embedded in it, the two charges being equal. It is like a watermelon: the red pulp is the positive charge and the seeds are the electrons.

2 marks: “Which result of the alpha scattering experiment could Thomson's model not explain, and why?” A few alpha particles turned through large angles or bounced back. Thomson's spread-out positive charge is too weak to do this; it predicts that nearly all would pass straight through. A small, dense, positive centre was needed.

1 marks: “In the watermelon picture of Thomson's model, what do the seeds stand for?” The electrons.

Remember it

Thomson's pudding: positive pudding, electron plums, no nucleus.