The Periodic Table

The early periodic table

John NewlandsDmitri 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:

·       Most reactive.

·       Lowest boiling and melting point.

  • 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.