1. Thomsn’s Plum Pudding Model of an ATom The History of Atomic Theory

3. Thomson’s Plum Pudding Model In 1897, the English scientist J.J. Thomson provided the first hint that an atom is made of even smaller particles. In 1897, the English scientist J.J. Thomson provided the first hint that an atom is made of even smaller particles.

4. Thomson Model He proposed a model of the atom that is sometimes called the “Plum Pudding” model. He proposed a model of the atom that is sometimes called the “Plum Pudding” model. Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding. Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding.

5. Mini-activity Distribute balls of modelling clay and sets of six small ball bearings. Distribute balls of modelling clay and sets of six small ball bearings. Students to create their own model of an atom from the ball of modelling clay using the ball bearings to represent the electrons. Students to create their own model of an atom from the ball of modelling clay using the ball bearings to represent the electrons. Students share their design of an atomic model such as it may have been following Thomson’s discovery. Students share their design of an atomic model such as it may have been following Thomson’s discovery.

6. Thomson Model Thomson studied the passage of an electric current through a gas. Thomson studied the passage of an electric current through a gas. As the current passed through the gas, it gave off rays of negatively charged particles. As the current passed through the gas, it gave off rays of negatively charged particles.

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

8. 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.

9. Focus Questions What would the atom’s electric charge be if it only had electrons in it? What would the atom’s electric charge be if it only had electrons in it? How did Thomson’s work change scientific understanding of atomic structure? How did Thomson’s work change scientific understanding of atomic structure? What knowledge about atoms did the ‘plum pudding’ atomic model signify? What knowledge about atoms did the ‘plum pudding’ atomic model signify? If the electrons each held a negative charge in a neutral atom, where in the atom were the positive charges held? If the electrons each held a negative charge in a neutral atom, where in the atom were the positive charges held?