What is a group on the periodic table?

• A. A vertical column of elements.
• B. A horizontal row of elements.
• C. A set of elements with similar ionization energies.
• D. The number of protons in the nucleus.

Answer: (A)

A group on the periodic table is a vertical column of elements. Elements in the same group share the same number of valence electrons, so they tend to behave similarly in chemical reactions and form similar kinds of ions. This is why you see patterns like the highly reactive alkali metals in one group, or the inert noble gases in another. A horizontal row, by contrast, is a period, where properties change more gradually as you move across. Saying that elements in a group have similar ionization energies describes a trend across the table rather than what defines a group. The number of protons in the nucleus is the atomic number, which identifies the element itself, not its placement in a column.