Question

When assigning electrons to orbitals, which would be the most likely 'address' for the next electron following the 5s2 electron? 5s3 5p1 4d1 5d1

Answer 1

the answer is number 3, 4d1.

Answer 2

Filling of electrons in an orbitals takes place by using following rules:

Aufbau rule: Filling of electrons takes place in an energy levels in the increasing order of energy that is the one having lowest energy will fill first.

Hunds rule: No second electron will be filled in an orbital until each orbital occupies a single electron.

Pauli exclusion principle: No two electrons can have same spin in an orbital.

So here also also the rules must be applied in filling electrons , by Aufbau rule, energy level that is having lower energy in this case after 5s it will be 4d¹, will fill first. So the answer is 4d¹.