A Level Chemistry OCR Practice Exam 2026 - Free Chemistry Practice Questions and Study Guide

The observed trend across a period in the Periodic Table is that electronegativity typically increases. As you move from left to right across a period, the atomic number increases, resulting in an increased positive charge in the nucleus. This stronger positive charge attracts the bonding electrons more effectively from the surrounding atoms, leading to a higher electronegativity value.

In contrast, other choices do not accurately describe trends across a period. The mass number does not consistently decrease; rather, it typically increases as you move from left to right due to the addition of protons and neutrons in the nucleus. Atomic size does not stay constant; it generally decreases because the increasing nuclear charge pulls the electrons closer to the nucleus. Lastly, the number of valence electrons increases across a period, starting from one valence electron in alkali metals to eight in the noble gases. This helps distinguish the correct trend of electronegativity increasing as elements become more efficient in attracting electrons.