Final answer:
Shielding refers to the ability of electrons in an atom to be shielded or protected from the attraction of the positively charged nucleus by other electrons. The more shielding an electron experiences, the less strongly it is held by the nucleus.
Step-by-step explanation:
Shielding refers to the ability of electrons in an atom to be shielded or protected from the attraction of the positively charged nucleus by other electrons. The more shielding an electron experiences, the less strongly it is held by the nucleus. In general, shielding increases with the number of filled energy levels or shells that surround the nucleus.
Based on this information, the elements can be ranked from lowest to highest shielding as follows:
1) Ba (Barium): Barium has the highest shielding because it has a full shell of electrons in its inner energy level.
2) Br (Bromine): Bromine has the next highest shielding because it has a partially filled inner energy level.
3) Na (Sodium): Sodium has less shielding than Bromine because it has fewer filled energy levels.
4) O (Oxygen): Oxygen has the lowest shielding because it has the fewest filled energy levels.