Chemistry 30 Bob Yake Room 250

Introduction to the atom and valence electrons The idea of the atom comes from the ancient Greeks, who had an idea of particles that made up matter. They named these particles atomos – meaning small, indivisible particle. This Greek idea of atoms was accepted up to 1803 when John Dalton built on the idea and formulated his theory. The theory stated that atoms are indivisible, those of a given element are identical, and compounds are combinations of different atoms. This theory held until 1897 when J.J. Thomson discovered electrons (he named them corpuscles). Thomson won the Nobel Prize for this discovery. Thomson went on to develop the Plum Pudding model of the atom.

This model showed the atom composed of a cloud of electrons scattered throughout a spherical cloud of positive charge. In 1911, Ernest Rutherford did an experiment where he fired positively charged particles at sheets of gold leaf. Most passed through with little deflection, but some deflected with large angles. This would only be possible if atoms were mainly empty space with the positive charge in the centre (the nucleus).

2 years later, in 1913, Neils Bohr modified Rutherford’s model by saying that electrons circled the nucleus in discrete orbits (each orbit having its own fixed energy). Electron energies were quantised; electrons could not occupy values of energy between the fixed energy levels.

In 1926, Erwin Schrödinger refined the model further to what is now the accepted concept. He stated the electrons do not move around the nucleus in set paths but rather in waves. It is impossible to know the exact location of any electron; instead electrons exist in ‘clouds of probability’ called orbitals. Orbitals are where we are most likely to find the electrons. This model is called the quantum mechanical model of the atom.

Other contributors to modern atomic theory Max Planck – Max Planck is considered to be the discoverer of energy quanta (the framework of quantum theory). He won a Nobel Prize for his work in 1918. Werner Heisenberg – Heisenberg is best known for his uncertainty principle: (you can determine the velocity of an electron but not its position or you determine where the electron is, but not its velocity.) Albert Einstein – Einstein contributed to modern atomic theory by proving, experimentally, the underlying mechanisms of quantum theory using nothing more than a microscope and Brownian motion. Louis de Broglie – contributed to the understanding of quantum mechanics by postulated that all matter, including electrons, exhibit wave-like properties. This was later proven experimentally in 1927.

Ways that atoms are represented in chemistry Atoms play a central role in the study of Chemistry. They are represented in a variety of ways that are universally accepted: The Periodic Table – this table, developed by Mendeleev, lists all the known elements by increasing atomic numbers (1 is Hydrogen). Atomic numbers are derived from the number of protons in the nucleus of an atom. Each element on the table has a unique name, symbol, atomic mass, orbital structure, and number. Molecular formulas – a molecular formulas is the ‘short-hand’ method of showing the chemical composition of a substance. An example is CO2 (carbon dioxide). It is composed of 1 Carbon atom (C) and 2 Oxygen (O) Atoms bonded together.

Ball & Stick - Atoms are also represented by “ball & stick” models Ball & Stick - Atoms are also represented by “ball & stick” models. These models are a very simple way of showing the approximate structure of common molecules. This is an example of a water molecule.

Space-filling model – this type of model is not common but does show the various atoms that make up molecules. This example shows water (again). Note that atoms in these models are assigned specific colors. Common ones include: Red – Oxygen White – Hydrogen Black – Carbon Blue – Nitrogen Yellow - Sulphur Purple – Phosphorus Silver – Metals (Co, Fe, Ni, Cu) Various shades of Green - Halogens