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How to Read the Periodic Table

The periodic table organizes all 118 known elements by atomic number. An element's position encodes its properties:

• Periods (rows) — indicate the number of electron shells. Period 1 (H, He) have 1 shell; Period 7 elements have 7 shells.
• Groups (columns) — indicate the number of valence (outermost) electrons, which determines chemical behavior. Elements in the same group react similarly.
• Atomic number — the top number; also equals the number of electrons in a neutral atom.
• Atomic mass — the weighted average mass of all natural isotopes in atomic mass units (u).

Element Categories

• Alkali metals (Group 1) — Li, Na, K, Rb, Cs, Fr. Soft, highly reactive; react violently with water.
• Noble gases (Group 18) — He, Ne, Ar, Kr, Xe, Rn. Fully stable; outer electron shells complete.
• Halogens (Group 17) — F, Cl, Br, I, At. One electron short of a full shell; highly reactive. Fluorine is the most electronegative element.
• Transition metals (Groups 3–12) — Fe, Cu, Ag, Au, Pt. Variable oxidation states and colorful compounds.
• Metalloids — B, Si, Ge, As, Sb, Te. Properties of both metals and nonmetals. Silicon is the basis of semiconductor technology.

The Most Abundant Elements

• Universe — Hydrogen (~75%), Helium (~24%); all others comprise ~1%
• Earth's crust — Oxygen (46%), Silicon (28%), Aluminum (8%), Iron (5%)
• Human body — Oxygen (65%), Carbon (18%), Hydrogen (10%), Nitrogen (3%). These four elements account for ~99% of body mass.
• Oceans — Hydrogen and Oxygen (water); dissolved Sodium and Chlorine (salt)

Element Names and Symbols — Origins

Symbols often look nothing like English names because many come from Latin:

• Fe (Iron) — from Latin "ferrum"
• Au (Gold) — from Latin "aurum"
• Ag (Silver) — from Latin "argentum"
• Pb (Lead) — from "plumbum" (source of the word "plumber" — Romans used lead pipes)
• Hg (Mercury) — from "hydrargyrum" meaning liquid silver

Named after places: Francium (France), Germanium (Germany), Americium, Berkelium (Berkeley, CA). Named after scientists: Curium (Marie and Pierre Curie), Einsteinium, Fermium.

Electron Configuration and Chemical Behavior

An atom's chemical behavior is almost entirely determined by its valence (outermost) electrons. The octet rule: atoms tend to gain, lose, or share electrons to achieve 8 valence electrons (the stable noble gas configuration). This drives all of chemistry. Sodium (1 valence electron) gives it up; Chlorine (7 valence electrons) desperately wants one more. Na donates to Cl; both reach stable configurations; ionic NaCl (table salt) forms.

Isotopes — Same Element, Different Mass

Isotopes are variants of the same element with the same protons but different neutrons. Carbon-12 (6p, 6n) and Carbon-14 (6p, 8n) are both carbon with identical chemistry — but very different nuclear stability. Carbon-14 decays at a known rate, enabling radiocarbon dating. Uranium-235 and U-238 are both uranium; U-235 undergoes fission more readily, making it the isotope used in nuclear reactors. The atomic mass on the periodic table is the weighted average of all natural isotopes.

The Newest Elements

The most recently confirmed elements are synthetic — created in particle accelerators and existing for only fractions of a second:

• Nihonium (Nh, 113) — confirmed 2015; named after Japan (Nihon)
• Moscovium (Mc, 115) — confirmed 2015; named after Moscow
• Tennessine (Ts, 117) — confirmed 2015; named after Tennessee
• Oganesson (Og, 118) — confirmed 2016; named after physicist Yuri Oganessian

Scientists are pursuing element 119, which would begin Period 8 of the periodic table.

Dmitri Mendeleev and the Discovery of the Periodic Table

In 1869, Russian chemist Dmitri Mendeleev published the first widely recognized periodic table, arranged by atomic weight. His crucial insight was leaving gaps for undiscovered elements and predicting their properties based on position. When gallium (1875), scandium (1879), and germanium (1886) were discovered with almost exactly Mendeleev's predicted properties, his table gained widespread acceptance.

The modern table was reorganized by atomic number after Henry Moseley's X-ray experiments in 1913, resolving inconsistencies in Mendeleev's ordering. New elements have been added to its outer edges as heavy synthetic elements are created.