The periodic table is organized to help us understand the properties of elements. Rows, called periods, show elements in order of increasing atomic number (the number of protons in the nucleus). Each new period represents a new electron shell around the nucleus. Moving from left to right across a period, each element has one more proton and electron than the last.
Columns, or groups, contain elements with similar properties because they have the same number of valence electrons (electrons in the outermost shell). For example, all elements in Group 1 have one valence electron, which gives them similar reactivity. Groups also help explain trends, like why elements at the top of a group are often smaller and more reactive than those at the bottom.
This organization makes it easier to understand periodic trends in atomic radius, ionization energy, and electronegativity-patterns that emerge across periods and down groups.
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