Thomsn’s Plum Pudding Thomsn’s Plum Pudding Model of an ATomModel of an ATom

The History of Atomic TheoryThe History of Atomic Theory

Thomson’s Plum Pudding Thomson’s Plum Pudding ModelModel

In In 18971897, the , the English scientist English scientist J.J. Thomson J.J. Thomson provided the first provided the first hint that an atom hint that an atom is made of even is made of even smallersmaller particles. particles.

Thomson ModelThomson Model He proposed a He proposed a

model of the atom model of the atom that is sometimes that is sometimes called the “called the “PlumPlum PuddingPudding” model. ” model.

Atoms were made Atoms were made from a positively from a positively chargedcharged substancesubstance with negatively with negatively charged electrons charged electrons scatteredscattered about, like about, like raisins in a pudding.raisins in a pudding.

Mini-activityMini-activity

Distribute balls of modelling clay and sets Distribute balls of modelling clay and sets of six small ball bearings.of six small ball bearings.

Students to create their own model of an Students to create their own model of an atom from the ball of modelling clay using atom from the ball of modelling clay using the ball bearings to represent the the ball bearings to represent the electrons.electrons.

Students share their design of an atomic Students share their design of an atomic model such as it may have been following model such as it may have been following Thomson’s discovery.Thomson’s discovery.

Thomson ModelThomson Model

Thomson studied Thomson studied the the passage passage of an of an electric current electric current through a gas.through a gas.

As the current As the current passed through passed through the gas, it gave off the gas, it gave off rays of rays of negatively negatively charged particles.charged particles.

Thomson ModelThomson Model

This surprised This surprised Thomson, Thomson, because the because the atoms of the gas atoms of the gas were uncharged. were uncharged. Where had the Where had the negative charges negative charges come from?come from?

Where did they come from?

Thomson concluded that the negative charges came from within the atom.

A particle smaller than an atom had to exist.

The atom was divisible!

Thomson called the negatively charged “corpuscles,” today known as electrons.

Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom.

But he could never find them.

Focus QuestionsFocus Questions

What would the atom’s electric charge be if it only had What would the atom’s electric charge be if it only had electrons in it?electrons in it?

How did Thomson’s work change scientific How did Thomson’s work change scientific understanding of atomic structure?understanding of atomic structure?

What knowledge about atoms did the ‘plum pudding’ What knowledge about atoms did the ‘plum pudding’ atomic model signify?atomic model signify?

If the electrons each held a negative charge in a neutral If the electrons each held a negative charge in a neutral atom, where in the atom were the positive charges held?atom, where in the atom were the positive charges held?