The two elements 11X and 19Y are in the same group because they have the same

Analyze the question: Two elements with atomic numbers 11 (X) and 19 (Y) are said to be in the same group. We need to find what property they share that puts them in that group.

Explain the concept: Elements in the same group of the periodic table have the same number of valence electrons (electrons in the outermost shell). This is why they have similar chemical properties. The period (row) tells you the number of shells, while the group (column) tells you the number of valence electrons.

Solution:

Write the electronic configurations:

• X (Z = 11): 2, 8, 1 → 3 shells, 1 valence electron (this is sodium, Na)
• Y (Z = 19): 2, 8, 8, 1 → 4 shells, 1 valence electron (this is potassium, K)

Now check each option:

• A. Valence electrons: Both have 1. ✔ Same.
• B. Ionization energy: It decreases down a group as atomic size increases, so Na and K differ. ✘
• C. Number of shells: Na has 3 and K has 4. ✘ Different.
• D. Atomic size: K is larger than Na. ✘ Different.

Answer: Both elements are in Group 1 (alkali metals) because they have the same number of valence electrons (1).

Tip: Remember the rule: Same group = same valence electrons. Same period = same number of shells. In WASSCE, if a question asks why elements are in the same group, think "valence electrons" first.