Which of the following properties generally decreases along a period?

The correct option is Metallic Character.

To understand why metallic character generally decreases along a period (from left to right) in the periodic table, we can analyze the periodic trends of the atomic properties step-by-step:

1. Understanding Metallic Character:
Metallic character refers to the ease with which an atom can lose electrons to form positive ions (cations). Elements that lose electrons easily are highly metallic (electropositive), whereas elements that tend to gain or share electrons are non-metallic (electronegative).

2. Trend in Atomic Size and Nuclear Charge:
As we move from left to right across a period:
• The number of protons in the nucleus increases, which increases the effective nuclear charge ($Z_{\text{eff}}$).
• Electrons are added to the same principal energy level, meaning the shielding effect remains relatively constant.
• Consequently, the stronger nuclear attraction pulls the outermost electrons closer to the nucleus, causing the atomic radius to decrease.

3. Trend in Ionization Energy:
Because the atomic size decreases and the effective nuclear charge increases along a period, the valence electrons are held more tightly by the nucleus. Therefore, the Ionization Energy (the energy required to remove an electron) generally increases across a period. This makes it harder for atoms to lose electrons, thereby decreasing their metallic character.

4. Trend in Electron Affinity:
As the nuclear charge increases and size decreases, atoms have a greater tendency to attract and accept incoming electrons. Thus, Electron Affinity generally becomes more negative (increases in magnitude) across a period.

5. Trend in Valency:
Valency (combining capacity) first increases and then decreases across a period. For example, with respect to hydrogen, it increases from 1 to 4 and then decreases to 0.

Conclusion:
Since ionization energy increases and atomic size decreases from left to right across a period, the tendency of elements to lose electrons decreases. As a result, the Metallic Character generally decreases along a period, transitioning from highly reactive metals on the far left to non-metals on the right.