Question

Why can Hydrogen and helium only hold two electrons in there valence shell, but Neon can hold 8?(You will receive extra credit if you can explain this with quantum numbers)

Answer 1

Lets explain it with quantum numbers.

There are 4 quantum numbers.

n , l , ml , ms

n is principle quantum number, l is azimuthal quantum number, ml is magnetic quantum number. ms is spin quantum number.

Lets explain possible value of n and l

When n = 1, 2, 3, 4 , 5 , …

Then l value

l = 0, 1, 2, ….(n-1)

from this we say that maximum value of l is one less than the value of n.

for example n = 1

the value of l is 0.

There are specific notation there for value of l. Lets show it.

l = 0 then the notation is S

l = 1 then notation is p

and so on…

Now we look at H.

Atomic number of 1

We know n starts from 1.

The formula to calculated number of maximum electrons in n shell is as follow.

Number of maximum electrons in n shell = 2n^2

Here is n is principal quantum number. so lest check for n = 1

Number of maximum electrons in n = 1

= 2 x 1 ^2 = 2

So there are maximum electrons H can take = 2

And He also have principal quantum number =1

So maximum capacity of electrons = 2

Same also can be explained by using l values

When n =1 then l = 0 and that is s orbital which can only accommodate 2 electrons.

Lets find look at Ne

Which has n = 2

Value of maximum electrons = 2 x 2^2 =2 x 4 = 8

So Ne can accommodate number of maximum electrons = 8