Matter & Bonding Lesson # 1 Early Models of the Atom

Introduction Chemistry is the science of “stuff.” More specifically, it is the study of matter, its properties, and its transformations. Chemists are people who analyze substances and seek to find new ways of making new substances in order to enrich our lives.

Atomic Models Scientists have tried to explain their observations by developing many models of matter. These models have been changed, combined or thrown away as new evidence was discovered. 1. About 450 B.C. A Greek scholar named Empedocles. Matter was made up of four elements, which were earth, air, fire, and water. These elements mixed together in different proportions made up different substances.

Atomic Models (continued) 2. About 400 B.C. A Greek scientist named Democritus. Matter was made of tiny particles that could not be broken down further. He called these particles atoms after the Greek word atomos, which means indivisible.

Atomic Models (continued) 3. A.D. 500 – 1600 Alchemists were a combination of philosopher, mystic, magician, and chemist. They still accepted the four element model. For centuries, they performed numerous experiments trying to change cheap metals like iron and lead into gold. They did make up chemical symbols for substances that we now know are elements and compounds. They also invented many laboratory tools that we still use today, such as beakers, filters, stirring rods, and distillation apparatus.

Atomic Models (continued) 4. A.D. 1800’s An English chemist named John Dalton. His atomic model for matter stated that: All matter is made up of atoms, which are particles too small to see. Each element has its own kind of atom, with its own particular mass. Compounds are created when atoms of different elements link to form molecules. Atoms cannot be created or destroyed, or subdivided in chemical changes.

Atomic Models (continued) Dalton’s atomic model, however, could not explain why you get a spark when you touch a metal doorknob. He soon discovered that matter is able to develop positive and negative charges, which are quantities of electricity that may build up on an object. A new model was made introducing tiny negatively charged particles that could be separated from their atoms and moved to other atoms.

Dalton’s Model of the Atom Featureless Sphere

Atomic Models (continued) A scientist named Michael Faraday. He found that electric current could cause changes in some compounds in solution. The atoms could gain electric charges and charged atoms called ions. Dalton’s model was changed to include: Matter must contain positive and negative charges. Opposite charges attract and like charges repel. Atoms combine to form molecules because of electrical attractions between atoms.

Atomic Models (continued) 5. A.D. 1900’s A scientist named J.J. Thompson. He discovered very light negative particles, called electrons. New model became known as “raisin-bun” model. Atoms contain particles called electrons. Electrons have a small mass and a negative charge. The rest of the atom is a sphere of positive charge. The electrons are “in” this sphere, so that the atom is neutral or uncharged.

Thompson’s Model of the Atom Positive sphere with negatives embedded in it

Atomic Models (continued) A scientist named Ernest Rutherford. He aimed radiation (alpha particles) at a thin sheet of gold foil. He predicted that the particles would pass straight through the gold foil. However, a small number of the particles bounced back from the foil. Rutherford explained his observations in a new model – the nuclear model. An atom has a tiny, dense, positive core called the nucleus, which contains protons. The nucleus is surrounded by mostly empty space, and rapidly moving negative electrons.

Rutherford’s Gold Foil Experiment

Rutherford’s Model of the Atom Positive core with negatives surrounding

Atomic Models (continued) A scientist named James Chadwick. He discovered the existence of neutrons which are similar to protons, but carry no charge. This led to the discovery of isotopes, which is used to distinguish between atoms of the same element that have different masses, since their neutron number can vary, but the charge will not.

Isotopes

Atomic Models (continued) A scientist named Niels Bohr. He discovered that electrons circle the nucleus in orbits of fixed energy. The energy of the electrons only changes when it is manipulated, like when it is heated, for example. At this point the electrons gain energy and can move up to higher energy levels, called “excited states” but will always fall back down to their original level, the “ground state” and releasing that energy in the form of light. A flame test is used to measure the amount of energy released when atoms are heated – the colour will vary depending on this amount.

Bohr Model of the Atom Positive and neutral core with negatives in orbitals (shells) surrounding it

Videos History of the Atom – Animation