The periodic table arranges elements in order of increasing atomic number. Explain what is meant by the terms 'group' and 'period' in the periodic table, and describe how the number of electron shells and the number of electrons in the outer shell relate to an element's position in the periodic table.

A group is a vertical column of the periodic table; elements in the same group have the same number of electrons in their outermost shell, which gives them similar chemical properties.
A period is a horizontal row of the periodic table; elements in the same period have the same number of electron shells.
The number of electron shells increases by one as you move from left to right across a period.
The number of electrons in the outer shell is constant for a group, determining the element’s valence and reactivity.
Thus, an element’s position in the table reflects both its shell count (period) and its outer‑shell electron count (group).