The periodic table organizes over 100 known elements into 18 columns and seven rows. The columns are called groups (or families), and rows are called periods. The layout of the table—arranged by increasing atomic number (the number of protons in the nucleus)—shows trends and patterns within periods and groups, which can be used to predict the properties of an element given its place in the table.

Trends in properties of the elements can be explained by electron configurations. According to the atomic model, electrons orbit the nucleus at specific levels, or shells. As atomic number increases, so does the number of electrons around the nucleus; in the table, each element has one more electron than the element preceding it. The rows of the periodic table correspond to the number of shells needed to hold the electrons. As a result, within a group, all the elements have the same number of valence electrons—the electrons in the outermost shell. For example, the elements in group 18 (the far right column) have full shells, making them much less chemically reactive compared to other groups. They are known as the noble gases.

A number of other relationships can also be seen, including patterns in atomic radius, electronegativity, and ionization energy. As atomic number increases from left to right across a period, nuclear charge increases, which also increases the strength of attraction between a nucleus and its electrons. Thus, going from left to right across a period, as the nucleus holds onto its electrons more tightly, atomic radius—the measure of an atom's size—generally decreases. On the other hand, atomic radius tends to increase as you travel down a group because the number of electron shells increases down the group.

Electronegativity is the measure of an atom's ability to attract electrons in chemical bonds. In general, elements on the left side of the periodic table have low electronegativities whereas elements on the right side have high electronegativities. Elements with low electronegativity have outer shells that are almost empty; elements with high electronegativity have outer shells that are mostly full.

Ionization energy is the amount of energy needed to remove an outer electron from an atom. Within a period, ionization energy generally increases with atomic number: a higher nuclear charge results in a stronger attraction of electrons, thus making it more difficult for an electron to escape. However, as you travel down a group, ionization energy tends to decrease—as the distance between the nucleus and valence shell electrons increases, the outer electrons become easier to remove.

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