2. Screening As a free electron approaches a positive cation, it will be attracted to the nucleus. But, it is also repelled by any electrons orbiting the nucleus. The electron-electron repulsion is canceled by the attraction to the nucleus. This is referred to as screening. The higher the value of the l quantum number, the more that orbital is screened by electrons in smaller, more stable orbitals.

3. Electrons are only screened by inner electrons. What does this mean? An electron in the 2s orbital is screened only by the 1s. An electron in the 3s orbital is screened by the 1s and 2s.

4. Pauli Exclusion Principle In general, most systems in nature tend to form a stable state. So, we might expect all electrons to be in the most stable orbital. NOT the case! Why? Pauli Exclusion Principle: each electron in an atom has a unique set of four quantum numbers (n, l, m l, and m s ) Stated another way: no two electrons can have the same set of 4 quantum numbers (n, l, m l, and m s )

5. The Aufbau Principle But, we still want the electrons in the most stable state possible. Most stable --- electrons occupy lowest energy orbitals possible. The ground state of an atom is the most stable arrangement of its electrons. Aufbau principle: we place electrons in orbitals starting with the lowest energy orbital following Pauli Exclusion Principle.

6. n = 1, l = 0, m l = 0, m s = + ½1s 1 n = 1, l = 0, m l = 0, m s = + ½1s 2 n = 1, l = 0, m l = 0, m s = - ½ n = 1, l = 0, m l = 0, m s = + ½1s 2 2s 1 n = 1, l = 0, m l = 0, m s = - ½ n = 2, l = 0, m l = 0, m s = + ½