1.3 Early Models of the Atompp. 13 - 15

Ancient Models of the Atom

•Democritus: he stated that there must be a smallest particle, which he called an atom.

•Empedocles: proposed that all matter is composed of four essential substances: earth, air, water, and fire.

Dalton’s Atomic Theory:

•Dalton (1808): he suggested that atoms are solid spheres like billiard balls.

• all matter is made up of tiny, indivisible particles called atoms• all atoms of an element are identical• atoms of different elements have different properties• atoms are rearranged to form new substances in chemical

reactions, but they are never created or destroyed (Law of conservation of mass)

•Problem: Dalton’s theory did not address how things acquire electrical charge.

J. J. Thomsom (1897) gave a new model of the atom which included electrons spaced evenly in a positively charged sphere: the “plum pudding model.” The fruit represents the electrons

J. J. Thomson used a device known as a cathode ray tube to perform his experiments

Thomson’s plum pudding model of the atom could be called the blueberry muffin model.

Thomson’s Atomic Theory

Ernest Rutherford 1909: discovery of the nucleus

•Rutherford tested Thomson’s theory via his famous gold foil experiment.

•He concluded that an atom contains a positively charged nucleus (composed of protons) surrounded by mostly empty space. Some of the empty space is occupied by electrons.

James Chadwick, 1932:

•His experiments confirmed that nuclei contain neutral particles as well as protons. They were called neutrons. •A neutron is similar in mass to a proton but has no charge; it is neither positive nor negative.•The term isotope is used to distinguish between atoms that contain the same number of protons (same element) but different numbers of neutrons.

• e.g. chlorine-35 vs. chlorine-37

Niels Bohr (1962):

•Rutherford’s model could not explain why electrons do not collapse into the nucleus since they are attracted to the protons.•Bohr proposed an atomic model that does explain this behaviour: •The phenomenon that led to this model will be discussed in section 1.4.

Homework:

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