Atomic Theory

What is an Atom? An ATOM is the smallest part of an element that has all of the element’s properties. Atoms of different elements are different from each other.

Atomic Theory This is the study of the nature of atoms and how they combine to form all types of matter. It explains how atoms make up their elements and how they make up all other forms of matter, including compounds and mixtures.

Ancient Atomic Theories The idea that matter is made up of combinations of simpler forms of matter is very ancient. Ancient Greeks – thought that matter was all made up of 4 elements – Fire, Water, Earth and Air Ancient China – thought there were 5 elements – Fire, Water, Wood, Metal and Earth

John Dalton In the early 1800’s a scientist and teacher, John Dalton, suggested that matter is made up of atoms. He believed that atoms could vary in size, mass or colour.

Dalton’s Atomic Theory All matter is made up of small indivisible particles called atoms All atoms of an element are identical to each other in size and mass Atoms of different elements have different properties Atoms of different elements combine in specific ways to form new substances

Dalton’s Atoms

J.J. Thomson In the late 1800’s an English physicist researched atoms and decided that he thought atoms might be made of a combination of particles. He experimented with electric currents in glass tubes called cathode ray tubes. Using this tube he was able to cause non- radioactive atoms to produce streams of negatively charge particles that were later found to be electrons.

J.J. Thomson In 1897, Thomson proposed a revolutionary new model for atoms, in which each atom was composed of smaller particles He discovered that atoms were composed of both positive and negatively charged particles.

Thomson’s Model of the Atom This can be compared to as a blueberry muffin. The blueberries represent negative charges, and the batter of the muffin represents positive charges.

New Zealand born scientist tested Thompson’s model of the atom while working in England Rutherford’s famous “Gold Foil” experiment led him to include a nucleus in his atomic model

Rutherford shot positively charged particles at a very thin foil of pure gold

In the experiment most high speed positive particles went right through the gold foil, however some bounced back WHY? …. Because of the gold’s NUCLEUS!

Rutherford discovered the nucleus. The nucleus is the centre of the atom He also discovered that the nucleus contains most of the atom’s mass. The nucleus is far larger than the rest of the atom. He calculated that comparing the size of the nucleus to other atomic sub-particles was like comparing a football field to a green pea. The nucleus

Based on the “Gold Foil” experiment Rutherford revised the existing atomic model to include a nucleus Similar to Thomson’s model except that all of the atom’s positive charge and mass were concentrated at a tiny point in the centre of the atom and the electrons surrounded this point

Chadwick was Rutherford’s student and he refined the concept of the nucleus He discovered that the nucleus contains neutral (no charge) particles called neutrons and positive charge particles called protons Each neutron has almost the same mass as each proton in the atom

Danish physicist that helped transform Rutherford’s model to a model that we use today Bohr suggested that electrons surround the nucleus in specific energy levels called energy shells He found that electrons jump between these energy shells by gaining or losing energy

The max number of electrons in the first three energy shells are; 2, 8, and 8 respectively

Most advanced and accurate model of the atom Electrons exist in specific energy levels that surround the nucleus in a form resembling a cloud