Electronic structure and trends in the periodic table.


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.

Group 1
Group 2
Group 3
Group 4
Group 5
Group 6
Group 7
Group 8
Period 1





Period 2


 1s2, 2s1


 1s2, 2s2


 1s2, 2s2, 2p1


1s2, 2s2, 2p2


1s2, 2s2, 2p3


1s2, 2s2, 2p4


1s2, 2s2, 2p5


1s2, 2s2, 2p6

Period 3


 1s2, 2s2, 2p6, 3s1


 1s2, 2s2, 2p6, 3s2


 1s2, 2s2, 2p6, 3s2, 3p1


 1s2, 2s2, 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.

Down a group
Across a period

- Atomic size increases

- Atomic size decreases
-Click to see the trend in period 2
-Click to see the trend in period 3
-Click to see the trend in period 4

-Metallic character increases - Metallic character decreases
- 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
- First ionisation energy decreases
Click for more information on core charge and first ionisation energy.
- First ionisation energy increases


Click to see a more detailed explanation of the above trends.

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)

Aluminium oxide(Al2O3) is amphoteric. It reacts with both acids and bases according to the reactions below.
+ 6H+(aq) => 2Al3+(aq) + 3H2O(l) (Shows aluminium oxide acting as a base)
Al2O3(s) + 2OH-(aq) + 3H2O(l) => 2Al(OH)4-(aq) (Shows aluminium oxide acting as an acid)

While sulfur trioxide reacts with water to produce a very strong acid according to the reaction below.
+ 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 group?
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 across a period?
vi) change in atomic radius down a group?
vii) change in electronegativity down a group?
viii) change in electronegativity across a period?