The History of the Periodic Table
In 1869 Dimitri Mendeleev created a Table of Elements in which he ordered the known elements at the time according to their atomic mass.
He created it to try to find patterns between similar elements and so arranged all of the chemicals with similar properties in the same vertical groups.
Mendeleev had to leave gaps in his table to make the patterns work, and he told other scientists that these gaps would be filled when new elements were discovered in the future. He was also able to predict the properties of these new elements from the position in his table.
His Periodic Table of Elements became widely accepted and developed as the one we know today when new elements discovered were found to match his predictions.
We now know that to make the patterns work properly elements must be ordered by their atomic number,
not mass as Mendeleev originally did.
The Modern Periodic Table
The Periodic Table is laid out in columns and rows, called groups and periods. The elements in each of these are related to each other in some way.
The vertical columns, called groups, go from 1, on the left side, to group 0, on the right hand side. Elements in the same group all have the same number of outer shell electrons. E.g. Oxygen and Sulfur both have 6 outer shell electrons. Types of elements e.g. halogens are all found in the same group.
The horizontal rows, called periods, go from 1 to seven and are labelled down the left hand side of the table. As the periods increase the atoms have more full electron shells, meaning as you move down the periodic table the atoms of elements become heavier and bigger. E.g. Sodium (2, 8, 1) and Aluminium (2, 8, 3) are both in period 3, and they both have electrons in three of their shells. An element's period can sometimes help us to predict its reactivity.
Group 1 - Alkali Metals
Found in the very left hand column of the table, group 1, including lithium, sodium and so on...
These metals all have one outer shell electron and so they react by losing it. They become more reactive as you go down the group as the atoms are bigger so electrostatic forces between the nucleus and outer electron are less, meaning the outer electron is easily lost.
These metals ALWAYS bond ioncially, NEVER covalently.
They have relatively low melting and boiling points, they're low density and very soft.
All of the alkali metals react with cold water vigorously. They whizz, float and fizz furiously as hydrogen is produced. They form their hydroxide solution in water, which is alkaline hence the name 'alkali metals'. Sodium and Potassium even melt during the reaction, and Potassium often ignites, so you can imagine what the more reactive ones do...
Found right in the centre of the periodic table, starting in period 4, including Titanium and Silver.
They are good conductors of heat and electricity. They're dense, strong and shiny. They have relatively high melting points compared with other metals and are much less reactive.
They often have more than one ion.. E.G. copper is Cu2+. Their compounds are very colourful and are responsible for hair colours. They are also known to be very useful catalysts in industry.
Most importantly - Transition Metals have a very strange electron arrangement. The 3rd energy level/shell can actaully hold 18 electrons not 8 as is originally taught and you have to think of the extra ten as a reserve. The transition metals' electrons fill these reserve spaces hence their place in the table. So after Calcium with electron arrangement (2, 8, 8, 2) comes Scandium with electron arrangement (2, 8, 9, 2) and so on until Gallium in group three where electron arrangement returns to normal (2, 8, 18, 3).
Group 7 - Halogens
Found in column 7, second to the right hand side, including Chlorine and Iodine.
They all have 7 electrons in their outer shell and so react by gaining one or sharing one, hence they can bond ionically or covalently. They also exist diatomically naturally, (meaning they're found in pairs).
The halogens will react vigorously with alkali metals to form salts called 'metal halides'. Also more reactive halogens will displace less reactive halogens in competition reactions.
Oppositely to the alkali metals, the halogens' reactivity decreases as you move down the group for a similar reason. As they need to attract another electron to react and the atoms become bigger as we move down the group, it means that the electrostatic forces between the nucleus and outer shell electrons are quite small so attracting another electron is quite hard, hence the larger the atom the less reactive it is...
Group 0 - The Noble Gases
Found in the last column, on the very right hand side of the table, including Helium and Argon.
They were only discovered recently in relation to the other elements as their properties are hard to notice... They are all colourless gases, that don't react with anything, this is because they have a full outer shell of electrons so are stable.
They all have a slightly strange property, that they all give out a different colour of light if you pass an electric current through them, giving them useful everyday duties...
- Neon lights are used in shop signs, the type you'd see in Las Vegas. They give out a bright red light.
- Helium is used in baloons to make them float as it is less dense than air. It can also be used with neon in lasers.
- Argon is used in lamp bulbs as a temperature control system, it can also be used as a more powerful laser.