Topic 3

Periodic Trends: Atomic Size and Shielding Effect

Atomic Size

                Atomic size is a measure of the average distance between the nucleus and the outermost electrons of an atom. It's generally measured in picometers (pm).  

Trends in Atomic Size:

·         Across a Period:

o                        Decreases: As you move from left to right across a period, the number of protons in the nucleus increases, which increases the effective nuclear charge. This stronger attraction pulls the outer electrons closer to the nucleus, reducing atomic size.  

·         Down a Group:

o                        Increases: As you move down a group, a new energy level (shell) is added to the atom. This increased distance between the nucleus and the outermost electrons leads to a larger atomic size.

Shielding Effect

           The shielding effect is the reduction in the attractive force between the nucleus and the outermost electrons due to the presence of inner electrons.  

Factors Affecting Shielding Effect:


·         Number of Inner Electrons:

                  Atoms with more inner electrons have a greater shielding effect.  

·         Orbital Penetration: 

                Electrons in s orbitals penetrate closer to the nucleus than those in p, d, or f orbitals, leading to a stronger shielding effect.

Trends in Shielding Effect:


·         Across a Period:

o    Shielding effect remains relatively constant: 

               Within a period, the number of inner electrons remains the same, so the shielding effect doesn't change significantly.

·         Down a Group:

o    Increases: 

                As you move down a group, the number of inner electrons increases, leading to a stronger shielding effect.  

Relationship Between Atomic Size and Shielding Effect:

·                     A stronger shielding effect reduces the effective nuclear charge felt by the outermost electrons, making them less tightly bound to the nucleus. This results in a larger atomic size.  

Example:

·         Sodium (Na) vs. Potassium (K): Both sodium and potassium are in Group 1. Potassium has a larger atomic size than sodium because it has a greater number of inner electrons, which shield the outer electron more effectively, reducing the effective nuclear charge.  

In summary: 

        Atomic size decreases across a period due to increased effective nuclear charge, while it increases down a group due to the addition of new energy levels. The shielding effect, influenced by the number of inner electrons and orbital penetration, plays a significant role in determining atomic size.