Periodic Table Trends: Across a Period

The periodic table organizes elements based on their recurring chemical properties. Understanding how these properties change across a period (from left to right) is fundamental in chemistry.

Understanding Trends Across a Period

As we move from left to right across a period in the periodic table:

• The number of protons in the nucleus (atomic number) increases.
• Electrons are added to the same principal energy level.
• The nuclear charge experienced by the valence electrons increases.

These factors influence various atomic and chemical properties. Let's examine the options provided:

Analyzing the Options

1. Atomic Radius

Atomic radius generally decreases across a period. Although electrons are added, they go into the same outermost shell. However, the increasing number of protons in the nucleus exerts a stronger pull on these electrons, drawing the electron cloud closer to the nucleus and reducing the atomic size.

2. Ionisation Enthalpy

Ionisation enthalpy (also called ionization energy) is the energy required to remove the most loosely bound electron from a gaseous atom. This value generally increases across a period. As the nuclear charge increases, the attraction between the nucleus and the valence electrons becomes stronger. Consequently, more energy ($ \Delta H_{IE} $) is needed to overcome this attraction and remove an electron.

3. Metallic Character

Metallic character generally decreases across a period. Metals tend to lose electrons easily. As ionisation enthalpy increases across a period, it becomes harder for atoms to lose electrons. Elements on the left (metals) readily lose electrons, while elements on the right (non-metals) tend to gain electrons. Thus, metallic character diminishes from left to right.

4. Reduction

Reduction involves gaining electrons. As metallic character decreases and non-metallic character increases across a period, elements become more likely to gain electrons. While the tendency to be reduced might increase for some elements towards the right (e.g., non-metals readily accept electrons to form negative ions), ionisation enthalpy is a more direct and universally increasing trend related to electron removal energy across a period.

Conclusion on Periodic Trends

Based on the analysis, the property that generally increases as you move from left to right across a period in the periodic table is Ionisation enthalpy. This is due to the increasing effective nuclear charge experienced by the valence electrons, making them harder to remove.