Metal atoms lose electrons to nonmetal atoms because metals typically have relatively low ionization energies. Metals at the bottom of a group lose electrons more easily than those at the top. That is, ionization energies tend to decrease in going from the top to the bottom of a group. Nonmetals, which are found in the right-hand region of the periodic table, have relatively large ionization energies and therefore tend to gain electrons. Ionization energies generally increase in going from left to right across a given period. Thus, the elements that appear in the lower left-hand region of the periodic table have the lowest ionization energies (and are therefore the most chemically active metals), while the elements that occur in the upper right-hand region of the periodic table have the highest ionization energies (and are thus the most chemically active nonmetals).
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As mentioned above, when a nonmetallic element reacts with a metallic element, electrons are transferred from the atoms of the metal to the atoms of the nonmetal, forming positive ions (cations) and negative ions (anions), respectively. This produces an ionic compound. For example, lithium and fluorine (F) react to form lithium fluoride (LiF), which contains Li+ and F− ions.
In contrast, when two nonmetallic elements react, the atoms combine to form molecules by sharing electrons. Bonds formed by electron sharing between atoms are called covalent bonds. The electrons are shared rather than transferred, because the two nonmetal atoms have comparable attractive powers for the electrons in the bond. For example, fluorine gas consists of F2 molecules in which the fluorine atoms are bound together by sharing a pair of electrons, one contributed by each atom. In addition, hydrogen and fluorine react to form hydrogen fluoride, which contains HF molecules. The hydrogen and fluorine atoms are bound together by a pair of electrons, one electron contributed by the hydrogen atom and one by the fluorine atom. Although the electrons are shared between the hydrogen and the fluorine atoms, in this case they are not shared equally. This is clear from the fact that the HF molecule is polar; the hydrogen atom has a partial positive charge (δ+), while the fluorine atom has a partial negative charge (δ−): H―F δ+ δ− (In this example the symbol δ stands for a number less than one.) This electrical polarity occurs because the shared electrons spend more time close to the fluorine atom than to the hydrogen atom. That is, fluorine has greater affinity for the shared electrons than does hydrogen. This leads to a polar covalent bond.
The ability of an atom to attract the electrons shared with another atom is termed its electronegativity. The relative electronegativities of the various atoms can be determined by measuring the polarities of the bonds involving the atoms in question. Fluorine has the greatest electronegativity value (4.0, according to the Pauling scale), and cesium and francium have the smallest values (0.79 and 0.7, respectively). In general, nonmetal atoms have higher electronegativities than metal atoms. In the periodic table, electronegativity typically increases in moving across a period and decreases in going down a group. When elements with very different electronegativities (such as fluorine and cesium) react, one or more electrons are transferred to form an ionic compound. For example, cesium and fluorine react to form CsF, which contains Cs+ and F− ions. When nonmetal atoms with differing electronegativities react, they form molecules with polar covalent bonds.
Each element has an electronegativity value, which is a measure of the ability of an atom to attract and share electron pairs of another atom.
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Another important atomic property is atomic size. The sizes of atoms vary; atoms generally tend to become larger in going down a group on the periodic table and smaller in going from left to right across a period.