Periodic Table Explorer

Every cell in the periodic table contains four key pieces of information about an element.

• Atomic number (Z) at the top tells you the number of protons in the nucleus. Hydrogen has Z = 1, Helium has Z = 2, and so on.
• Symbol is the one- or two-letter abbreviation (H for Hydrogen, Fe for Iron).
• Name identifies the element in full.
• Atomic mass at the bottom is the weighted average of all stable isotopes, measured in atomic mass units (u or amu).

Elements are arranged left-to-right by increasing atomic number. Each row is called a period and each column is a group. Elements in the same group share similar chemical behavior because they have the same number of valence electrons.

The 118 elements fall into several broad categories based on their physical and chemical properties.

• Metals include alkali metals (Li, Na, K), alkaline earth metals (Be, Mg, Ca), transition metals (Fe, Cu, Au), and post-transition metals (Al, Sn, Pb). They are generally shiny, conductive, and form positive ions.
• Nonmetals (C, N, O, S) are poor conductors and tend to form negative ions or share electrons.
• Metalloids (B, Si, Ge, As) sit on the boundary and have intermediate properties, making them useful as semiconductors.
• Noble gases (He, Ne, Ar, Kr, Xe) have full valence shells and are extremely unreactive.
• Halogens (F, Cl, Br, I) are highly reactive nonmetals that readily form salts with metals.

Several properties change in predictable patterns across the periodic table.

Electronegativity measures how strongly an atom attracts shared electrons. It increases left to right across a period and decreases top to bottom down a group. Fluorine (3.98) is the most electronegative element.

Atomic radius decreases across a period because additional protons pull the electron cloud inward. It increases down a group as new electron shells are added.

Ionization energy is the energy required to remove the outermost electron. It follows the same trend as electronegativity, increasing across periods and decreasing down groups.

Electron configuration describes how electrons fill orbitals around the nucleus. The Aufbau principle says electrons fill from lowest energy to highest.

For example, Sodium (Z = 11) has the configuration 1s² 2s² 2p⁶ 3s¹, or in shorthand [Ne] 3s¹. The shorthand replaces the inner-shell electrons with the symbol of the previous noble gas.

There are a few well-known exceptions to the filling order. Chromium (Z = 24) is [Ar] 3d⁵ 4s¹ instead of [Ar] 3d⁴ 4s² because a half-filled d subshell provides extra stability. Copper (Z = 29) is [Ar] 3d¹⁰ 4s¹ for the same reason, preferring a fully filled d subshell.