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The periodic table is organized into horizontal rows called periods and vertical columns called groups (or families). Elements in the same period have the same number of electron energy levels (shells). Elements in the same group have similar chemical properties because they have the same number of outer (valence) electrons. There are 7 periods and 18 groups in the standard periodic table. Key groups include the alkali metals (Group 1), alkaline earth metals (Group 2), halogens (Group 17), and noble gases (Group 18).
Use a color-coded periodic table and trace across a period and down a group. Notice that moving across a period, elements change from metals to nonmetals. Moving down a group, elements get larger but behave similarly. Identify the named families (alkali metals, halogens, noble gases) and discuss what the elements in each family have in common.
You know the periodic table organizes elements by atomic number and reveals patterns in chemical behavior. Now it is time to understand the two structural features that create those patterns: periods (rows) and groups (columns).
Periods are the horizontal rows of the periodic table, numbered 1 through 7 from top to bottom. Each period represents a new electron energy level (also called a shell). Elements in Period 1 (hydrogen and helium) have electrons in only one energy level. Elements in Period 2 (lithium through neon) have electrons in two energy levels. Elements in Period 3 have three, and so on. As you move across a period from left to right, you are adding one proton and one electron at a time, filling up the current energy level. The properties of elements change dramatically across a period — from highly reactive metals on the left, through less reactive metals and nonmetals in the middle, to unreactive noble gases on the right.
Groups are the vertical columns, numbered 1 through 18 from left to right. Elements in the same group have the same number of valence electrons — the electrons in the outermost energy level. Since valence electrons are the ones involved in chemical bonding, elements with the same number of valence electrons tend to behave similarly in chemical reactions. This is why groups are also called families — the members share a family resemblance in their chemistry.
Several groups have special names that chemists use constantly. Group 1 elements (lithium, sodium, potassium, rubidium, cesium, francium) are the alkali metals. They are soft, silvery metals that react vigorously with water — each has just one valence electron that it loses easily. Group 2 elements (beryllium, magnesium, calcium, and others) are the alkaline earth metals. They have two valence electrons and are also reactive, though less dramatically than Group 1. Group 17 elements (fluorine, chlorine, bromine, iodine, astatine) are the halogens. They are reactive nonmetals that need one electron to complete their outer level, making them eager to bond. Group 18 elements (helium, neon, argon, krypton, xenon, radon) are the noble gases — they have completely filled outer levels and almost never react.
The combination of periods and groups creates a grid that lets you predict an element's behavior from its position. An element on the left side of the table is probably a reactive metal. An element on the right side (but not the last column) is probably a nonmetal. An element in the same group as one you already know will behave similarly. This predictive power is what makes the periodic table one of the most useful tools in science — it turns the overwhelming diversity of 118 elements into an organized, navigable map.