La Chimie: The periodic table

The periodic table

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The periodic table

There are over a hundred elements. All substances are made from one or more of these elements. The modern periodic table is a way of showing how these elements can be ordered. (

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The alkali metals, halogens and noble gases are three important groups in the periodic table.

  • The alkali metals are soft, reactive metals. They react vigorously with water and become more reactive as you go down the group.
  • The halogens are reactive non-metals. They become darker as you go down the group. Their reactivity decreases as you go down the group and their boiling points increase. A more reactive halogen will displace a less reactive halogen from solutions of its salts.
  • The noble gases are unreactive non-metals, which exist as single atoms.
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The alkali metals

The alkali metals have the following properties in common:

  • they have low melting and boiling points compared to most other metals
  • they are very soft and can be cut easily with a knife
  • they have low densities (lithium, sodium and potassium will float on water)
  • they react quickly with water, producing hydroxides and hydrogen gas
  • their hydroxides and oxides dissolve in water to form alkaline solutions

As you go down the group, the atomic number of the alkali metals increases, and their properties change:

  • their melting points decrease
  • their densities increase
  • they become softer
  • they become more reactive

You will probably see lithium, sodium and potassium at school, but rubidium and caesium are considered to be too reactive to use in the classroom. Francium is radioactive and very rare.

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The halogens

Melting point and boiling point

The halogens have low melting points and boiling points. This is a typical property of non-metals. You can see from the graph that fluorine, at the top of Group 7, has the lowest melting point and boiling point in the Group. The melting points and boiling points then increase as you go down the Group.

State at room temperature (20°C)

Fluorine and chlorine are gases, bromine is a liquid, and iodine and astatine are solids. You should remember this trend down the periodic table - the top two elements are gases, the bottom two are solids and the middle element is liquid.


The halogens become darker as you go down the group. Fluorine is very pale yellow, chlorine is yellow-green and bromine is red-brown. Iodine crystals are shinygrey, but easily turn into a dark purple vapour when they are warmed up.

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Melting and boiling points of Group 7 elements


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The noble gases

The noble gases have the following properties in common:

  • they are non-metals
  • they are very unreactive gases
  • they are colourless
  • they exist as single atoms (they are monatomic)


Used in balloons and airships. It is much less dense than air, so balloons filled with it float upwards.

Neon: Used in advertising signs. It glows when electricity is passed through it, and different coloured 'neon lights' can be made by coating the inside of the glass tubing with other chemicals


Used in light bulbs. The very thin metal filament inside the bulb would react with oxygen and burn away if the bulb were filled with air instead of argon. As argon is unreactive, it stops the filament burning away.

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