A TOMIC T HEORY N OTES

A TOMIC T HEORY First attempts of defining atoms happened 2500 years ago. Philosophers theorized if matter could be divided indefinitely into smaller and smaller pieces, or whether there was a piece so small that it could not be divided. They called this smallest piece atomos, which means indivisible. From this, we get the word atom.

A RISTOTLE – 322 BC ( ABOUT 2400 YEARS AGO ) Aristotle was a prominent Greek philosopher who described matter as composed by different combinations of water, fire, earth and air. As he was very influential during his time, his views went undisputed for the next 2000 years, which delayed the current atomic model’s development.

A FTER A RISTOTLE In Europe and the Middle East, a new brand of secretive researcher appeared; they were called Alchemists, and were trying to understand and experiment with matter while using mystical thinking. They based their ideas on Aristotle’s view of matter. Their main purpose was finding the philosopher's stone, a special material which would be able to transform other matter such as lead into gold. They tried for more than a thousand years, but were unsuccessful. By the 19 th century, many people were doing experiments that led them to question Aristotle’s four element theory.

J OHN D ALTON Suggested that particles which make up matter are actually like small, hard spheres, different for each different element. He brought back the idea of an atom being the smallest particle of an element. All matter are made up of small particles called atoms Atoms cannot be created, destroyed or divided into smaller particles All atoms of the same element are equal in mass and size, but they are different in mass and size from atoms of another element Compounds are created when atoms of different elements link together in definite proportions D ALTON ’ S A TOMIC T HEORY

J.J. T HOMSON In 1897, J.J. determined that currents were streams of negatively charged particles, later called electrons. He found that every substance he tested produced these particles. Based on his experiments, he theorized that all atoms must have these particles; he proposed that atoms were actually formed by even smaller particles. Thomson proposed the raisin bun model, where an atom would be a positively charged ball with negative particles embedded in it like raisins.

E RNEST R UTHERFORD Designed an experiment to probe inside atoms. He exposed a very thin sheet of gold to a stream of high speed, heavy, positively charged particles, called alpha particles. He noticed that when they were shot through the gold, the particles mostly went through, but some bounced in unexpected directions. This was the discovery of the nucleus. A decade later, he also established that the nucleus must have at least two kinds of particles: a positive proton, and a neutral neutron.

N IELS B OHR Bohr worked under Ernest Rutherford, and focused on the regions around the nucleus, which was known to contain electrons. He proposed that the electrons were organized into specific energy levels, or shells. The placement of each electron would depend on how much energy they have. The more energy an electron has, the closest they will be to the outer shell.

W HAT IS AN ATOM ? An atom is the smallest particle of an element that retains the properties of the element. An atom is composed of subatomic particles: protons, electrons and neutrons. The nucleus is tiny, dense and composed of protons and neutrons. They have much more mass than electrons, about 1800 times more. Electrons are arranged around the nucleus in specific levels. A neutral atom has the same number of protons and electrons.

E LECTRIC C HARGE Electric charge comes in two types: positive and negative. Protons are positive, and electrons are negative; since positive and negative attract each other, protons and electrons are attracted together. Each proton counts as +1 and each electron counts as -1. All neutral atoms have the same amount of protons and electrons; this means that the charge will add up to zero. Example: Oxygen has 8 electrons and 8 protons; the overall charge is zero.

N UCLEUS The nucleus has a positive charge because of the protons. For any atom more complicated than Hydrogen, the nucleus must contain neutrons. Neutrons keep the protons separated from each other. The number of neutrons varies depending on how many are necessary to keep the nucleus stable. E LECTRONS Electrons occupy special regions called energy levels, or shells, which surround the nucleus. The region occupied by electrons account for way over 99.99% of the volume of the atom. Each electron occupies the whole energy level at a time.

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