chemistry unit one

everything on unit one of AS chemistry AQA

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1.1 Fundamental particles.

Nucleus: The tiny, positively charged centre of an atom composed of protons and neutrons.

Nucleons: Protons and neutrons – the sub-atomic particles found in the nuclei of atoms.

Strong nuclear force: The force that holds protons and neutrons together within the nucleus of the atom.

Electrostatic forces: The forces of attraction and repulsion between electrically charged particles.

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Sub-atomic particles:


Atoms are made of three fundamental particels – protons, neutrons and electrons

Protons and neutrons form the nucleus.

Protons and neutrons are collectively called nucleons.

Electrons surround the nucleus.

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The arrangement of the sub-atomic particles:


Protons and neutrons are held in the centre by the strong nuclear force.

The electrons and protons are held together by electrostatic forces.

The strong nuclear force is stronger than the electrostatic forces, so it is able to overcome the repulsion between the protons in the nulcues.

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the arangement of the electrons

Electron shells:

The shell closest to the nucleus fills first.

First shell holds up to 2 electrons

Second shell holds up to 8 electrons

Third shell holds up to 18 electrons

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Electron diagrams:

We can write electron diagrams in short hand.

Step 1: Write the number of electrons in each shell, starting with the inner shell and working outwards.

Step 2: Separate each number by a comma.

Step 3: For carbon we write 2,4, for sulphur 2,8,6 and for Na+ 2,8.

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Mass number, atomic number and isotopes

Proton number: The number of protons in the nucleus of an atom, the same as the atomic number.

Mass number and atomic number:

The atomic number is the number of protons. As the number of protons is the same as the number of electrons in a neutral atom, the atomic number also represents the number of electrons.

The mass number is the total number of protons plus neutrons. This is because the nucleons are responsible for almost all of the weight of the atom.

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Carbon Dating:

Sometimes isotopes are unstable and the nucleus of the atom breaks down. This break down gives off bits of the nucleus or energetic rays.

This causes radioactivity.

Carbon-14 is a well-known radioactive isotope. It is used to date organic matter.

Always a tiny fixed proportion of carbon-14 in living matter.

When the living material dies the carbon breaks down and the reactivity levels fall slowly.

As the half-life of carbon-14 is known, scientists can work backwards and work out how long it took for the level of radioactivity to fall from the level it was when the organism was living to the level it is now.

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