As 4s orbitals have a lower energy than 3d orbitals, electrons are first filled in the 4s orbital and then in the 3d orbital. This is because the outer shell is maximum shielded from the influence of the nucleus, and hence lower energy is required to extract electrons from the outermost shell than from an inner shell.
Just so, why does 3d come before 4p?
We say that the 4s orbitals have a lower energy than the 3d, and so the 4s orbitals are filled first. We know that the 4s electrons are lost first during ionization. The electrons lost first will come from the highest energy level, furthest from the influence of the nucleus.
What is 1s 2s 2p 3s 3p?
The order of filling is 1s, 2s, 2p,3s, 3p, 4s, 3d, 4p,5s, 4d, 5p, 6s, 4f,5d, 6p, 7s, 5f, 6d,7p.
How do you determine the electron configuration of an element?
Method 1 Assigning Electrons Using a Periodic Table
- Find your atom's atomic number.
- Determine the charge of the atom.
- Memorize the basic list of orbitals.
- Understand electron configuration notation.
- Memorize the order of the orbitals.
- Fill in the orbitals according to the number of electrons in your atom.