Electronic structure and trends in the
Elements with the same number
of valence(outer shell) electrons are placed in the same group. While
elements with electrons in the same valence shell(outer shell) are placed
in the same period.
2p6, 3s2, 3p1
2p6, 3s2, 3p5
Notice how the elements in
group 1 have 1 valence electron while all the elements in period two have
energy level 2 as the valence(outer) energy level.
Now lets have a look at some trends. The following trends are obvious
when elements are placed in the periodic table and can be explained using the notion of core charge. Spend a moment to review core charge.
Atomic size increases
-Click to see the trend in period 2
-Click to see the trend in period 3
-Click to see the trend in period 4
- Electronegativity(ability to attract electrons into the valence shell) decreases. As a result reducing(electron giving)
strength increases, while oxidising(electron taking) strength decreases.
- Electronegativity increases. As a result reducing(electron giving)
strength decreases, while oxidising(electron taking) strength increases.
-Click to see trend across period 3
-Click to see trend across period 4
ionisation energy decreases Click for more information
on core charge and first ionisation energy.
The oxides of
elements tend to be more acidic as we move across the period.
Sodium oxide reacts with water according to the reaction below to produce
a highly alkaline solution.
2Na(s) + 2H2O(l) => 2NaOH(aq) + H2(g)
oxide(Al2O3) is amphoteric.
It reacts with both acids and bases according to the reactions below.
Al2O3(s) + 6H+(aq) => 2Al3+(aq) + 3H2O(l) (Shows
aluminium oxide acting as a base)
Al2O3(s) + 2OH-(aq) + 3H2O(l) =>
aluminium oxide acting as an acid)
While sulfur trioxide reacts with water to produce a very strong acid
according to the reaction below.
SO3(g) + H2O(l) => H2SO4(aq)
Above are the graphs showing the change in a particular characteristic of an atom.
Which graph best represents the:
i) change in core charge as we move down a gorup?
ii) change in core charge as we move across a period?
iii) change in first ionisation energy across a period?
iv) change in first ionisation energy down a group?
v) change in
atomic radius acrossa period?
vi) change in atomic radius down a group?
vii) change in electronegativity down a group?
viii) change in electronegativity across a period?