The term electronic configuration refers to the arrangement of the electrons around the central nucleus from the inner energy shell to the outer energy shell. 

Electronic configuration
The number of outer electrons can be obtained from the periodic table using the group number, it is always the same. This does not apply to the transition metals.


The valency of an element from the main groups can be obtained from the group number. For elements in groups 1 & 2 the valency is the same as the group number. For elements in groups 1517 the valency is equal to the 18  group number
Example: What is the valency of selenium (group 16) Groups 15 to 17 have a valency = 18  group number Valency of selenium = 18  16 = 2 
Formula
Formulae of binary (two element) compounds can be found using the valencies of the elements. If the group numbers of the elements combining are known, then the formula is easy to determine.
Hydrogen (valency 1) combines with all of the elements from Na to Cl in period 3. The formula of the hydrides formed follows a pattern.
Group 
1

2

13

14

15

16

17

Hydride 
NaH

MgH_{2}

AlH_{3}

SiH_{4}

PH_{3}

H_{2}S

HCl

Example: What is the formula of the compound formed between sodium (group 1) and selenium (group 16) sodium valency = group number = 1 Formula of sodium selenide = Na_{2}Se 
Electronic configuration
The full electronic configuration is expected to include the subshells s, p, d, up to element 54. The configuration is written from the inner (lower energy) shells outwards.
Show all the configurations from 136
Hydrogen  1s^{1}^{}  Potassium  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 
Helium  1s^{2}  Calcium  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 
Lithium  1s^{2} 2s^{1}  Scandium  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{1} 
Beryllium  1s^{2} 2s^{2}  Titanium  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{2} 
Boron  1s^{2} 2s^{2} 2p^{1}  Vanadium  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{3} 
Carbon  1s^{2} 2s^{2} 2p^{2}  Chromium  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{1} 3d^{5} 
Nitrogen  1s^{2} 2s^{2} 2p^{3}  Manganese  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{5} 
Oxygen  1s^{2} 2s^{2} 2p^{4}  Iron  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{6} 
Fluorine  1s^{2} 2s^{2} 2p^{5}  Cobalt  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{7} 
Neon  1s^{2} 2s^{2} 2p^{6}  Nickel  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{8} 
Sodium  1s^{2} 2s^{2} 2p^{6} 3s^{1}^{}  Copper  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{1} 3d^{10} 
Magnesium  1s^{2} 2s^{2} 2p^{6} 3s^{2}  Zinc  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{10} 
Aluminium  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{1}  Gallium  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{10} 4p^{1} 
Silicon  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{2}  Germanium  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{10} 4p^{2} 
Phosphorus  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{3}  Arsenic  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{10} 4p^{3} 
sulfur  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{4}  Antimony  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{10} 4p^{4} 
Chlorine  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{5}  Bromine  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{10} 4p^{5} 
Argon  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6}  Krypton  1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{10} 4p^{6} 
Each member of the same group has an identical outer electronic arrangement in terms of subshells, that can be represented by ns^{x} np^{y}.
Example: The electronic configuration of the elements in group 14 are as follows:
In all cases the outer energy shell can be expressed as ns^{2} np^{2} 