Topic C1 Atomic Mystery

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Atoms are very small, but made up of even smaller particles – the proton, the electron and the neutron.

The protons and neutrons are in the nucleus but the electrons orbit the nucleus in shells or energy levels.

The number of protons is called the ATOMIC NUMBER and this number is used to place the element in the Periodic Table.

The number of protons + the number of neutrons is called the MASS NUMBER. This used to be was used to place elements in the PT but it meant that occasionally elements did not fit in with those around them. 

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Atomic number

Each proton has a mass of 1 and an electric charge of +1.

Each neutron has a mass of 1 but has zero electric charge.

The electron has virtually no mass but an electric charge of -1.

The number of electrons = the number of protons so atoms are electrically neutral. 

Have a special notation butcare needed because not the same as that in PT.

A= mass number    Z = atomic number    N = no of neutrons

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Isotopes are atoms of the same element but the atoms have different numbers of neutrons. Isotopes have the same number of protons and hence the same atomic number but because they have different numbers of neutrons they will have different mass numbers.

eg C-12 and C-14 are isotopes of carbon 

 Because isotopes have the same electron arrangement they have the same chemical properties.


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Electrons are arranged in shells around the nucleus but each shell can only hold a certain number before it becomes full

Shell 1 = 2  Shell 2 = 8  Shell 3 = 18 (but at KS4 you will only ever put 8 in)  Shell 4 = 32 (but at KS4 you will only ever put 2 in)

 Electron arrangement – this is the arrangement of electrons in shells for a given element

eg Na is 11 so it has 11 electrons. Electron arrangement = 2.8.1

eg Cl is 17 so it has 17 electrons. Electron arrangement = 2.8.7

 No. of electrons in the outer shell is the same as the Gp no and the number of figures is the period no. So for Na, 2.8.1 it must be in Gp and period 3 of the PT.

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When the elements are arranged in order of their you get the Periodic table.

Elements end up in vertical groups of elements which have similar properties eg the Group 1 metals

  • They are all stored under oil.
  • All three metals could be cut with a knife indicating that they were soft. Potassium was the softest and lithium the hardest.
  • They were all shiny when freshly cut but they tarnished quickly. Lithium did so the slowest while potassium was the fastest.
  • They all floated on the water showing them to be less dense than water.
  • They all fizzed meaning a gas was given off.
  • Potassium burst into a purple flame.
  • In all three the water turned blue/purple showing that an alkali had formed.
  • Potassium is the most reactive and lithium is the least reactive.
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Chemical Reactivity

The chemical reactivity of an element is down to its outermost electrons and it is also related to its electron structure. The reactivity depends on how easily/difficult it is to lose/gain electrons. In group 1 the reactivity increases down the group because the outer electron is further away from the nucleus and so more easily lost.

There will be a trend in the properties.


In early PT gaps were left for elements yet to be discovered.

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