Question

Why the 3rd main energy level can hold a maximum number of 18 electrons?

Answer 1

Answer:

n

th

shell has n

2

orbital and each orbital has maximum of 2 electrons. Thus, 2n

2

number of electrons are present in n

th

energy level. For third shell/energy level n=3 therefore number of electrons are 2×3

2

=18

Answer 2

Reason:

The 3rd subshell consists of an s orbital , a set of (3)p orbitals and a set of (5)d orbitals.

The maximum number of electrons in the 3s sublevel is two,

the maximum number of electrons in the 3p sublevel is six,

and the maximum number of electrons in the 3d sublevel is ten

So 2+6+10=18  

Another proof :

The third energy level has a total of 32=9 orbitals, so its capacity is 18 electrons.

Note:  the orbitals must fill in a specific order. 3s fills before 3p and then you move to 4s as this is the next least energetic.