# Number of elements

1. Feb 13, 2016

### erisedk

1. The problem statement, all variables and given/known data
If elements with principal quantum number n > 4 were not allowed in nature, the number of possible elements would be
(A) 60
(B) 32
(C) 4
(D) 64

2. Relevant equations

3. The attempt at a solution
I got 36 from the periodic table (2+8+8+18)

2. Feb 13, 2016

### Staff: Mentor

What are the rules that tell you how many electrons you can have for a given n?

3. Feb 13, 2016

### erisedk

2n^2

4. Feb 13, 2016

### erisedk

But looking from the periodic table, isn't my answer right?

5. Feb 13, 2016

### Staff: Mentor

If you take only elements which have at most $n=4$ electrons, you indeed get a different number, but this is not what the question is about. What you get from the periodic table depends on the fact that the energy ordering of orbitals doesn't depend only on n, which makes it such that 5s electrons are lower in energy than 4d electrons. But the question says that n is at most 4, so when the 4p orbitals are filled, you will fill the 4d orbitals (and not the 5s as is the case for real atoms). You end up with different electronic configurations.

6. Feb 14, 2016

### erisedk

Oh OK, so basically ignoring the energy order. Thank you!