Question

Identify the outer electron configurations for the (a) alkali metals, (b) alkaline earth metals, (c) halogens, (d) noble gases.

Answer 1

a) Alkali metals

=> group 1

=> Li:  1s2 2s  => 1s
     Na: [Ne] 3s => 3s
     K: [Ar] 4s => 4s
     Rb: [Kr] 5s => 5s
     Cs: [Xe] 6s => 6s
     Fr: [Rn] 7s => 7s

=> outer electron configuration is ns, where n is the main energy level: 1, 2, 3, 4, 5, 6,7.
     
b) Alkaline earth metals

=> group 2 => you have to add 1 electron to the alkaly metal of the same row.

=> Be: [He] 2s2 => 2s2
     Mg: [Ne] 3s2 => 3s2
     Ca: [Ar] 4s2 => 4s2
     Sr: [Kr] 5s2 => 5s2
     Ba: [Xe] 6s2 => 6s2
     Ra: [Rn[ 7s2 => 7s2

=>outer electron configuration is n s2, where n is the main energy level: 1, 2, 3, 4, 5, 6, 7

c) halogens

=> group 7

=> F: [He] 2s2 2p5 => 2s2 2p5
     Cl: [Ne] 3s2 3p5 => 3s2 3p5
     Br: [Ar] 3d10 4s2 4p5 => 4s2 4p5
     I: [Kr] 4d10 5s2 5p6 => 5s2 5p5
     At: [Xe] 4f14 5d10 6s2 6p5

=> outer electron configuration is ns2 np5, where n is the main energy level 1, 2, 3, 4, 5, 6, 7

d) Noble gases

=> group 8

I will show only the outer shell which is what is requested

=> He: 1s2
     Ne: ... 2s2 2p6
     Ar: ... 3s2 3p6
     Kr: ... 4s2 4p6
     Xe: ... 5s2 5p6
     Rn: ... 6s2 6p6

=> the outer electron configuration is ns2 np6, except for He for which it is 1s2