chemistry c3 the periodic table

•Newlands:

–Arranged elements according to atomic weights

–‘Law of Octaves’ similar properties repeated every 8th element

–Some elements placed in inappropriate groups, due to strictly following order of weight

–Sometimes more than one element in each ‘position

Mendeleev:

–Also arranged elements according to atomic weights

–Elements repeat properties periodically (regularly), also placed in columns, or groups

–Gaps left for undiscovered elements

–Three new elements discovered in his lifetime that fitted these gaps

•Electrons, protons and neutrons were discovered early in the 20th century

•Elements now arranged by atomic (proton)numbers

•All elements now in appropriate groups

•Elements in the same group have the same number of electrons in their highest occupied energy level (outer shell)

Physical properties:

–Low density (the first three elements in the group are less dense than water)

–The further down the group an element is:

•the more reactive the element •the lower its melting point and boiling point

•Chemical properties:

–All react with non-metals to form ionic compounds in which the metal ion carries a charge of +1

–The compounds are white solids that dissolve in water to form colourless solutions

–All react with water, releasing hydrogen

–All form hydroxides that dissolve in water to give alkaline solutions

Compared with the elements in Group 1, transition elements:

–have higher melting points (except for mercury) and higher densities

–are stronger and harder

–are much less reactive and so do not react as vigorously with water or oxygen

•Many transition elements have ions with different charges, form coloured compounds and are useful as catalysts

• The elements in Group 7 of the periodic table react with metals to form ionic compounds in which the halide ion carries a charge of –1
• In Group 7, the further down the group an element is:

–the less reactive the element

–the higher its melting point and boiling point

• A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt.

•The trends in reactivity within groups in the periodic table can be explained because the higher the energy level of the outer electrons:

–the more easily electrons are lost

–the less easily electrons are gained

•Group 1 elements react by losing an electron

–Therefore Li is less reactive than K, because it its outer electron is nearer to the attractive nucleus

•Group 7 elements react by gaining an electron

–So F2 is more reactive than I2, because its outer electrons are nearer the positive nucleus