Atomic Structure Revision Cards

A brief history of the atom...

This is a brief history of the atomic structure:


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The Plum Pudding Model

In 1897, when testing how certain gasses conduct electricity, J. J. Thompson discovered the electron.

This meant that a new model of the atom had to be put forward. So, Thompson put forward the idea of the Plum Pudding Model.

Plum Pudding Model:

rings of negatively charged electrons embedded in a positive sphere.


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The Rutherford experiment

In 1913, Ernest Rutherford and his team used alpha particles to probe inside atoms. The results of his research led to the discovery of the nucleus.

Rutherford's results showed that:

  • The atom consisted of electrons revolving a positvely charged nucleus
  • the positive particles (protons) were 1840x heavier than electrons
  • atoms have equal number of protons (+) and electrons (-), so overall, they are neutral
  • virtually all of the atoms mass is concentrated in the nucleus
  • electrons are contained in a much larger region around the nucleus


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Inside the nucleus

Rutherford's experiments didn't explain the actual mass of the atom though. This was solved when, in 1932, James Chadwick discovered the neutron. The neutron has a mass equal to that of a proton but it has no charge. All atoms have neutrons, except hydrogen.


The modern day view of the atom is that:

An atom is made up of a very small, dense, positively charged nucleus composed of protons (+) and neutrons (0) with negativley charged electron in shells orbitting the nuceus.

The radius of an atom is about 10x-8 cm. Thats 0.0000001 cm.

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Atomic Number and Mass Number

The Atomic number of an element is the number of protons inside the nuleus of an atom.

Since the amount of electrons in the atom is equal to the number of protons in the atom, the atomic number can also show the number of electrons in the atom of an element

The Mass number is the total number of protons + neutrons in the nucleus of an atom.

N.B. elements can be identified by their atomic numbers

In the periodic table, you see elements beside their atomic number and their mass number:


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Electronic configuration

There are rules that are to be followed when arranging electrons into shells:

  • Electrons are filled from shells starting from shell 1, the closest shell to the nucleus
  • Electons are the filled into the 2nd, 3rd, 4th ... shells each shell being further away from the nucleus than the last.
  • The firt shell can hold a maximum of two electrons
  • The next sets of shells can hold a maximum of eight electrons
  • You have to fill the current shell before moving on to the next shell.

In the Periodic Table

  • The group number of an element = the number of electrons in the outer shell
  • The Period tells you how many shells the atom has


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Electronic configuration of first 20 elements


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Isotopes are atoms of the same element that have the same atomic number but different mass numbers, resulting in a different number of neutrons

  • All atoms of the same element will have the same atomic number
  • Many atoms though, have different mass numbers
    • These are called isotopes

Isotopes only affect mass - not chemical properties.

Chlorine has two isotopes:


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