The early periodic table
|John Newlands||Dmitri Mendeleev|
|· Ordered his table in order of atomic mass|
· Realised similar properties occurred every eighth element – ‘law of octaves’ but broke down after calcium.
|· Ordered his table in order of atomic mass, but not always strictly.|
· Left gaps for undiscovered elements.
- The table is called a periodic table because similar properties occur at regular intervals.
- Elements with similar properties are found in the same column (groups)
The modern periodic table
- When electrons, protons and neutrons were discovered in the early 20th century, elements were ordered in atomic (proton) number.
- When this was done, all elements were placed in appropriate groups.
- Elements in the same group have the same number of electrons in their highest occupied energy level (outer shell)
Trends within the periodic table
Group 1 – Alkali metals
- Low density (first 3 are less dense than water)
- React with non-metals to form an ionic compound, in which the metal ion carries a charge of +1. These compounds are white solids which dissolve to form colourless solutions.
- They react with water to release hydrogen
- They form hydroxides which dissolve in water to give alkaline solutions (hence the name alkali metals)
As you go further down:
- The elements get more reactive because the
electrons are further away from the positive nucleus, so
are lost more easily.
- They have lower melting and boiling points
Group 7 – The halogens
- They react with metals to form ionic compounds in which the halide ion carries a -1 charge.
- A more reactive halogen can displace a less reactive in an aqueous solution of its salt.
E.g. Chlorine will displace bromine if we bubble the gas through a solution of potassium bromide:
Chlorine + Potassium Bromide à Potassium Chloride + Bromine
As you go further down:
- The element is less reactive because the higher the energy level of the outer electrons, the less easily electrons are gained (attracted to the positive nucleus.)
- The higher its melting and boiling point
The transition elements
Compared to group 1, the transition elements:
- Are harder and stronger
- Have higher melting points (except for mercury) and higher densities
- Much less reactive and don’t react as vigorously with oxygen or water
- Conduct heat and electricity due to their delocalised electrons
- They have ions with many different charges, so form coloured compounds, and are useful as catalysts.