Development of the periodic table
- He arranged the known elements in order of their atomic mass and found similar properties amongst every eighth element.
- He noticed periodicity (repeated patterns).
- By strictly following the order of atomic mass some of the elements had been placed in the wrong group.
- He hadn't realised that there were still undiscovered elements so the table wasn't accuarate.
- He realised that some elements were yet to be discovered.
- He left gaps to allow fo the discovery of new elements.
- He could use his table to predict the existence of other elements and to work out what properties they would have.
The modern periodic table
- The elements are arranged in periods (rows) according to the number of electrons in their outer energy level (shell).
- The atomic structure determines what group they're in. Elements with the same number of electrons in their outer shell are in the same group.
- Elements with the same number of electrons in their outer shells have similar properties. This is why elements of the same groups show similar trends.
- Elements are also ordered by their proton number.
Trends in reactivity
A Trend is a pattern in reactivity.
Alkali metals (G1) become more reactive as you go down the group because the outer energy level (shell) gets further away from the influence of the nucleus, and so an electron is lost more easily. Group 1 elements form ionic bonds.
The Halogens (G7) become less reactive as you go down the groups because the outer shell gets further away from the influence of the nucleus, and so an electron is gained less easily. Group 7 elements form covalent bonds.
The more energy levels an atom has....
- the more easiy electrons are lost
- the less easily electrons are gained
The Alkali Metals
Group 1 (known as the alkali metals) contains 6 elements.
Alkali metals have low melting and boiling points, have a low density (lithium, sodium and potassium are less dense than water) and become more reactive as you go down the group.
The alkali metals are stored under oil because they react vigourously with oxygen and water. When alkali metals react with water, a metal hydroxide is formed and hydrogen gas is given off. E.g.
Potassium + Water --------> Potassium Hydroxide + Hydrogen
If a metal hydroxide (e.g. potassium hydroxide) is dissolved in water, an alkaline solution is formed.
Alkali metals will react with non-metals to form ionic compunds. When this happens the metal atom loses one electron to form a metal ion with a positive charge (1+). The products are white solids that dissolve in water to form colourless solutions.
To summarise: Going down the group, reactivity increases whilst melting and boiling points decrease.
You can find the transition elements in the centre of the periodic table.
Many transition metals...
- form coloured compounds
- have ions with different charges
- can be used as catalysts to speed up reactions
Like all other metals, transition metals...
- are good conductors of heat and electricity
- can be easily bent or hammmered into shape
In comparision to group 1 metals, transition metals...
- have higher densities and higher melting points (except mercury --> liquid at room temperature)
- are harder and more mechanically strong (except mercury)
- are much less reacitve and don't need to be stored in oil
There are 5 elements in group 7 (known as the halogens). They're non-metals.
- have melting and boiling points that increase as you go down the group (at room temperature, fluorine and chlorine are gases, and bromine is a liquid)
- have coloured vapours (poisonous)
- exist as molecules made up of pairs of atoms (strong covalent)
- become less reactive as you go down the group
Halogens react with metals to produce ionic salts. The halogen atom gains one electron to form a halide ion (e.g. chloride, bromide or iodide ion) that carries a negative charge (-1). E.g.
Lithium + Chlorine --------> Lithium Chloride
Halogens react with other non-metallic elements to form molecular compounds. E.g.
Hydrogen + Chlorine -------> Hydrogen Chloride
If one atom is more reactive than another than a displacement reaction will occur.
A more reactive halogen will displace a less reactive halogen from an aqueous solution of its salt. E.g.
- chlorine will displace both bromine and iodine
- bromine will displace iodine
chlorine + sodium bromide → sodium chloride + bromine