Atomic Structure

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What are atoms made of?

Atoms are made up of Three small particles:

Protons                  Neutrons                  Electrons

The protons and neutrons exist in a dense core at the centre of the atom. This is called the Nucleus.

The electrons are spread out aroung the edge of the atom. They orbit the nucleus in layers called shells (energy levels).

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Mass and Electrical Charge

There are two properties of protons, neutrons and electrons that are especially important:

  • Mass
  • Electrical Charge

Particle             Mass           Charge 

Proton                  1                +1

Neutron                1                 0

Electron               1                 -1

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The Atom So Far

The nucleus is:

  • made up of protons and neutrons
  • positively charges because of the protons
  • dense because it contains all the amss of the atom in a tiny space

Electrons are:

  • very small and ligh, and negtively charges
  • able to be lost or gained in chemical reactions
  • found thinly orbiting in layers called shells (energy levels)
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Discovering atomic structure

450 BC

The greek philosopher Democritus speculates that everything is made up of tiny particles. He calls them atoms, from the Greek atomos meaning indivisible. His theory is ignored for over 2,000 years


John Dalton re-introduces the idea of tiny particles, and calls the atoms, like Democritus. He says that they are like hard balls that cannot be split, but can join together and make all materials in the universe.


JJ Thomson discovers the electron. He believes that they are distributed equally throughout the atom. This becomes known as the 'plum pudding' model of the atom.

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Discovering atomic structure Cont.


Ernest Rutherford discovers the nucleus and protons by firing alpha particles at gold foil in the Geiger-Marsden experiment. Most pass straight through but a few bounce back. From these unexpected results, he deduces that atoms are mostly empty, but contain a small, dense nucleus.


Niels Bohr says that electrons orbit the nucleus in fixed layers which are called electron shells.


James Chadwick discovers neutrons. He finds that they weigh the same as protons but have no electrical charge.

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How many protons?

Each element has a definite and fixed number of protons. All the atoms of a particular element always contain the same number of protons. For example:

  • hydrogen atoms always contain 1 proton
  • carbon atoms always contain 6 protons.

If the number of protons changes, then the atom becomes a different element. The number of protons in an atom is known as the atomic number or proton number. It is the smaller of the two numbers shown in most periodic tables.

What are the atomic numbers of these elements?

Sodium        Na        11

Iron              Fe        26

Tin               Sn        50

Fluroine        F          9

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What is mass number

Electrons have a mass of almost zero, which means that the mass of each atom results almost entirely from the number of protons and neutrons in the nucleus.

The sum of the protons and neutrons in an atom's nucleus is the mass number.

It is the larger of the two numbers shown in most periodic tables.

Atoms         Protons       Neutrons         Mass Number

Hydrogen          1                  0                        1

Lithium             3                  4                        7

Aluminium       13                14                       27

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Glossary Of Key Words

Atom - The smallest particle that can exist on its own.

Atomic number - The number of protons in the nucleus of an atom, also known as the proton number.

Electron - Negatively-charged particle that orbits the nucleus of an atom.

Electron Shell - Layers around the nucleus of an atom. Electrons orbit the nucleus in the shells.

Element - A substance made up of only one type of atom.

Mass Number - The number of protons and neutrons in the nucleus of an atom.

Neutron - A neutral particle, woth a mass of 1. It is found in the nuceus of an atom

Nucleus - The dense, postively charged centre of an atom made up of protons and neutrons.

Periodic Table - The table that lists all the elements in order of increasing atomic number arranged into groups and periods.

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Glossary of Key Words Cont.

Proton - A positively charged particle, with a mass of 1. It is found in the nucleus of an atom.

Proton Number - The number of protons in the nucleus of an atom, also known as the atomic number.

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