Phosphorus has five valence electrons. Its noble gas electron configuration is [Ne]3s23p3.
What is the ground-state electron configuration for silicon (Si)?
The ground-state electron configuration for silicon is 1s22s22p63s23p2.
In noble gas form, this would be [Ne] 3s23p2.
What is the actual ground-state electron configuration for copper (Cu)?
Copper is anomalous because it’s ground-state electron configuration isn’t what you’d expect it to be. The actual configuration is due to the fact that it’s more beneficial to fill the 3d orbital and leave a half-full 4s orbital than to have a full 4s orbital and a not-quite-full 3d orbital.
Copper’s actual ground-state electron configuration is 1s22s22p63s23p64s13d10. In noble gas form (which is considerably easier to read), it’s [Ar] 4s13d10.
How many 4d electrons are in Mo?
There are five 4d electrons in Mo. Molybdenum has an anomalous because configuration. It’s expected to be [Kr] 5s^2 4d^4, but experiment data show that it’s actually [Kr] 5s^1 4d^5.