Periodic Table Group Trends: Understanding Constant Properties

When examining trends in the periodic table, moving down a vertical column (a group) reveals specific changes in atomic properties. Understanding these trends helps predict chemical behavior.

Changes Down a Group

As you move down a group in the periodic table:

• Atomic Size increases because each successive element adds a new electron shell (energy level).
• Metallic Character increases as atomic size increases and ionization energy decreases, making it easier to lose electrons.
• Number of Shells increases by one for each step down the group.

Constant Property Down a Group: Valency

The property that generally remains constant when moving down a group for the main group elements (Groups 1-2 and 13-18) is Valency.

Reasoning:

• Elements within the same group have the same number of valence electrons (electrons in the outermost shell).
• Valency is primarily determined by the number of valence electrons, representing the combining capacity of an atom.
• For example, all elements in Group 1 (like Li, Na, K) have one valence electron and typically exhibit a valency of 1. Similarly, Group 17 elements (like F, Cl, Br) have seven valence electrons and usually show a valency of 1 (in forming simple halides).

Therefore, valency is the property that does not change significantly as one moves down a group.