# AQA MODULE 1 - Atomic structure

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The Atom:

Atoms form all elements. They are made from three types of particles - protons,neutrons and electrons. The protons and neutrons are found in the nucleus, whilst the electrons are found in shells orbiting around the nucleus.

THE HISTORY OF THE ATOMIC STRUCTURE

In the 19th century, Dalton believed atoms were solid spheres and that different spheres formed different elements.

In 1897, J.J Thomson realised that the atom contains even smaller, negatively charged electrons. His theory suggests that the rest of the atom must have been positively charged, resembling a plum pudding.

In 1909, Rutherford fired alpha particles (positive) at a thin sheet of gold expecting most of the particles to be deflected by the 'positive' plum pudding, however only a few did. This proved that only a small part of the atom was positive. He concluded the presence of a tiny positively charged nucleus, which was surrounded by a cloud of negative electrons and the majority of the atom was actually empty space.

Later, Bohr realised a cloud of electrons would cause the atom to collapse. He came up with the principles of electrons in fixed orbitals. He also thought that each shell had a fixed energy which is incorrect.

Now, the model of the atom is based on quantum mechanics.

MASS NUMBER AND ATOMIC NUMBER

The mass number - The total number of protons and neutrons in the nucleus of the atom.

The atomic number - The amount of protons in the nucleus of the atom, all atoms of the same element have the same amount of protons in their nucleus.

RELATIVE MASS

Relative atomic mass (Ar) - The average mass of an atom of an element on a scale where an atom of carbon 12 is exactly 12, because it is an average it is not usually a whole number.

Relative isotopic mass - The mass of an atom of an isotope of an element on a scale where carbon 12 is exactly 12, usually a whole number.

Relative molecular mass (Mr) - The average mass of a molecule on a scale where an atom of carbon 12 is exactly 12. To find the Mr, you add up all the relative atomic masses of all the atoms in the molecule

Relative formula mass - The average mass of a formula unit on a scale where an atom of carbon 12 is exactly 12. To find the relative formula mass, you just add the relative formula masses of all the ions in the formula unit.

IONS

Ions are formed when atoms lose or gain electrons in order to fulfill a full outer electron shell. When an atom donates electrons, it becomes a positively charged ion are named cations. When an atom gains electrons, they become negatively charged and are named anions.

ISOTOPES

Isotopes of an element are atoms with the same number of protons but different numbers of neutrons, for example -

The arangement of electrons determine the chemical properties of…