5.1 Developing the Atomic Theory pp

Learning Goals: Know who the key atomic theorists are & what their contribution was Know the model of the atom that each proposed (be able to draw them and explain their meaning)

Ancient Theories Ancient Greek philosophers theorised that all substances were made from the combination of four fundamental elements: earth, air, fire, water. Today, we still use the term element. We have many more elements from which most substances are built up.

About 440 B.C.E., the Greek philosopher Democritus hypothesized that breaking down rock into powder and then breaking that into smaller and smaller pieces until a single invisible particle was reached. He called this particle an atom which means uncutable or invisible in Greek Ancient Theories

John Dalton (1766–1844), A scientist and teacher in England, reconsidered the ancient idea that each different kind of element is composed of a different kind of atom. Dalton thought that the atom would be like a featureless sphere. Like a billiard ball

John Dalton (1766–1844), He explained the nature of matter as follows: All matter is made of small, indivisible particles called atoms. All the atoms of an element are identical in properties such as size and mass. Atoms of different elements have different properties. Atoms of different elements can combine in specific ways to form new substances. Atoms cannot be created, destroyed, or subdivided in a chemical change.

Atoms Are Composed of Smaller Particles J. J. Thomson (1856–1940), an English physicist, researched the idea that atoms might be made from a combination of particles. He experimented with electric currents in glass tubes called cathode ray tubes.

J. J. Thomson (1856–1940), In 1897 he proposed a revolutionary new model for atoms. It is known as the raisin bun model. The dough would be the positively charged sphere and the raisins would be the negative charges. It is also known as the plum pudding model

This would leave the atom with a neutral charge: the positive charges are balanced by the negative charges. The negative charges are known as electrons. Electron – subatomic particle with a negative charge

Ernest Rutherford (1891–1937) Tested Thomson’s model of the atom. He conducted an experiment in which he shot positively charged particles at a very thin foil of pure gold

Ernest Rutherford (1891–1937) In the experiment, most of the high speed positive particles went right through the foil. However, about 1 in positive particles bounced back from the foil as if it had been deflected by something very massive and positively charged.

Ernest Rutherford (1891–1937) Rutherford had discovered the nucleus, the centre of the atom. This tiny positively charged part of the atom also contains most of the atom’s mass.

Ernest Rutherford (1891–1937) Based on his gold foil experiment, Rutherford revised the atomic model using his prediction that all atoms everywhere contain a nucleus. His model was like Thomson’s except that all of the atom’s positive charge and most of the atom’s mass were concentrated at a tiny point in the centre. The electrons surrounded the nucleus and occupied most of the atom’s volume, but they contained only a small fraction of the atom’s total mass.

Ernest Rutherford (1891–1937)

Inside the Nucleus James Chadwick (1891–1974), Rutherford’s student, refined the concept of the nucleus. Discovered that the nucleus contains neutral particles as well as positively charged particles. The neutral particles in the nucleus of the atom are called neutrons. The positively charged particles in the atom are called protons. Each neutron in an atom has about the same mass as each proton in the same atom, but the neutron carries no electrical charge

Niels Bohr (1885–1962), Bohr suggested that electrons surround the nucleus in specific energy levels, called shells. He discovered that electrons jump between these shells by gaining or losing energy. Each shell can contain only a specific number of electrons. The maximum number of electrons that can exist in each of the first three shells is two, eight, and eight.

Niels Bohr (1885–1962),

The Quantum Mechanical Model The most advanced and accurate model of the atom, and the one in use today by physicists and chemists, is called the quantum mechanical model. In this model, electrons do not exist as tiny points inside an atom. Electrons exist in specific energy levels, but they surround the positively charged nucleus in a form resembling a cloud.

The Quantum Mechanical Model

A Summary of the Atom All elements are composed of atoms, and one atom is the smallest unit of any element. The atoms themselves are made of different kinds of smaller particles, called subatomic particles. The three subatomic particles are protons, neutrons, and electrons, and they have different properties.

Homework Read pp. 168 – 175 Answer the following questions: p. 175 # 1, 2, 4, 5 p. 177 # 1, 3 – 9, 11, 12