Which of the following ions has the smallest radii? Fe2+, Co2+, Ni2+, Cu2+
The Correct Answer and Explanation is:
To determine which of the ions has the smallest radius, we need to consider the following factors:
- Charge of the Ion: All the ions given have the same charge of +2 (Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺), meaning they each have lost two electrons. This creates a higher effective nuclear charge (the attractive force exerted by the nucleus on the remaining electrons) which tends to draw the electrons closer to the nucleus.
- Electron Configuration: The atomic radii decrease as you move across a period (from left to right) in the periodic table due to an increase in nuclear charge. The effective nuclear charge increases, pulling the electrons more tightly toward the nucleus, leading to a smaller atomic radius.
- Fe²⁺: The electron configuration of Fe (atomic number 26) is [Ar] 4s² 3d⁶. When it forms Fe²⁺, it loses two electrons from the 4s orbital, resulting in the configuration [Ar] 3d⁶.
- Co²⁺: The electron configuration of Co (atomic number 27) is [Ar] 4s² 3d⁷. When it forms Co²⁺, it loses two electrons from the 4s orbital, resulting in the configuration [Ar] 3d⁷.
- Ni²⁺: The electron configuration of Ni (atomic number 28) is [Ar] 4s² 3d⁸. When it forms Ni²⁺, it loses two electrons from the 4s orbital, resulting in the configuration [Ar] 3d⁸.
- Cu²⁺: The electron configuration of Cu (atomic number 29) is [Ar] 4s² 3d¹⁰. When it forms Cu²⁺, it loses two electrons from the 4s orbital, resulting in the configuration [Ar] 3d⁹.
- Trends in Ionic Radii: As we move across a period from Fe to Cu, the number of protons (and thus the nuclear charge) increases. This increased nuclear charge pulls the electrons more strongly, resulting in a smaller ionic radius. Therefore, Cu²⁺ has the smallest radius among these ions. The reason is that copper has the highest atomic number (29), and as a result, the increased nuclear charge in Cu²⁺ draws the electrons in more tightly than the ions of Fe²⁺, Co²⁺, or Ni²⁺, which have lower atomic numbers and less effective nuclear charge.
In conclusion, Cu²⁺ has the smallest ionic radius due to its higher nuclear charge, which results in a stronger attraction of electrons to the nucleus.
