Periodic Table (continued)
Groups
The modern periodic table is divided into 18 columns called
groups or families (see illustration). Elements in each family
tend to have similar properties. In column 1, each alkali metal is
soft, relatively low-melting, and highly reactive toward air and
water. Column 2 contains the alkaline earth metals, which have
higher melting points and are less reactive. Columns 3–12 are
filled by the transition metals, which are shiny and good con-
ductors of both heat and electricity. Columns 13–18 are often
discussed along with columns 1 and 2, and collectively they are
known as the main group or representative elements. Column
15, headed by nitrogen, is known as the pnictogens; column
16, beginning with oxygen, as the chalcogens; column 17,
starting with fluorine, as the halogens; and column 18, starting
with helium, as the noble gases.
Periods
The horizontal rows of the periodic table are called periods.
Atomic mass generally increases from left to right across a
period, while atomic size generally decreases. The decrease
in size is due to incomplete screening of the positive nuclear
charge by the valence electrons, which causes the outer
electron shells to contract. Other properties follow periodic
trends, including the ionization potential (the energy needed
to remove an electron), electron affinity (the energy released
on accepting an electron), and electronegativity (the ability of
an atom in a compound to attract electron density
After element 57 (lanthanum) comes a series of 14 metallic
elements numbered 58–71 with very closely related properties.
These originally were named the rare earths since they are all
nearly the same size, have similar chemical reactivity, and are
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Periodic table of the elements.
