History of the Periodic Table
The two ways that elements used to be organised are :
¬By their physical and chemical properties
¬By their relative atomic mass
This is because back then they didn’t know about protons, electrons, or atomic structures.
The known elements were organised in order of relative atomic mass, and when this was done they realised that there was a periodic pattern in the properties of the elements, so they named it the periodic table.
Newlands came up with the Law of Octaves in 1864.
He found that when the elements were arranged in order of relative atomic mass, every eighth element had similar properties. He organised the elements into rows of seven because of this.
His pattern (of every eighth element having similar properties), broke down as the table went on. He hadn’t left any gaps for undiscovered elements , which caused problems in his table.
His work has been critiqued because:
¬His groups contained elements with properties that weren’t similar
¬He mixed the metals with non-metals
¬He didn’t leave any gaps for undiscovered elements
In 1869 (5 years after Newlands), Dmitri Mendeleev came up with his own table of elements.
He too put the elements in order of their relative atomic mass.
He noticed that the properties of the elements were related to their atomic mass (in a periodic way), and arranged his table so that elements with similar properties fell into the same vertical columns.
He found that he had to leave gaps in his table, so that elements with similar properties stayed in the same vertical columns.
Mendeleev thought that the gaps left in his table would be filled with elements that hadn’t been discovered at the time.
He was able to predict the properties and work out the atomic masses of the missing elements, and when the elements were discovered people realised that his predictions were right.
Modern Periodic Table
Some people weren’t fond of the periodic table. When it was first released there wasn’t a lot of evidence to show that the elements really fit into the table the way that they did. When newly discovered elements fit into Mendeleev's table it provided some proof that the table was correct.
When even more evidence surfaced scientists found out that the table was useful for predicting the properties of elements.
In the 19th century, protons, neutrons, and electrons were discovered. Scientists now accept that the periodic table is useful for summarising that structure of atoms.
Modern Periodic Table pt2
¬Elements in the same group have the same number of elections in their outer shell (except for transition metals)
¬Group numbers are equal to the number of electrons in the highest energy level
¬The nucleus is positively charged, and attracts the negative charge of the electrons, holding them into place. The further away from the nucleus an electron is, the less attraction.
¬When there are more electron shells, the attraction of the nucleus to the outer shell is less. The other shells get in the way and reduce the attraction. This is called shielding.
¬Electrons in the highest energy level are more likely to get lost because they have less attraction to the nucleus. This is why group one metals get more reactive as you go down the group.
¬The outer shell is also less likely to gain an electron when they’re far away from the nucleus. This is because there's less attraction from the nucleus that pulls electrons to the atom. This is why group 7 elements get less reactive as you go down the group.
Group 1 - The Alkali Metals
As you go down the group the metals become:
¬More reactive – because they’re further from the nucleus and are more easily lost.
¬Have lower melting and boiling points.
¬Alkali metals have low densities.
¬They only have one outer electron, making them very reactive.
¬They’re keen to lose their outer electron.
¬Covalent bonding is out of the question, they’d rather just give the electron away.
¬They form 1⁺ ions by making ionic compounds with non-metals.
¬They produce white compounds, that make colourless solutions when they dissolve in water.
¬They react aggressively with water (tend to fizz). Produce hydrogen when this happens. Form hydroxides that dissolve in water to give alkaline solutions.
Group 7 - The Halogens
As you go down the group:
¬They become less reactive – its harder for them to gain the electron that they need because their outer shell is far from the nucleus.
¬Have higher melting and boiling points
¬Halogens are non-metals with coloured vapours:
¬Fluorine is a poisonous yellow gas
¬Chlorine is a poisonous green gas
¬Bromine is a red-brown volatile liquid
¬Iodine is a purple vapour
¬Halogens form ionic compounds with metals and Create 1⁻ ions (these are called halides). They make ionic compounds with metals. In aqueous solutions, a more reactive halogen will displace a less reactive one
The Transition Elements
The transition elements (Sometimes called transition metals), are in the middle of the periodic table. Common properties include:
¬Good conductors of heat and electricity
¬Dense, strong, and shiny
¬Less reactive than group 1 elements
¬Denser, stronger, and harder than group 1 elements
¬High melting points (except mercury)
¬The compounds that they form are colourful. The colours in gemstones and pottery glazers are down to transition metals.
¬They make good catalysts (e.g. iron is used in the Haber process for making ammonia)