The ability of an atom in a moleculeA set of atoms joined by covalent bonds and having no net charge. to attract a shared electronA negatively charged, sub-atomic particle with charge of 1.602 x 10-19 coulombs and mass of9.109 x 1023 kilograms; electrons have both wave and particle properties; electrons occupy most of the volume of an atom but represent only a tiny fraction of an atom's mass. pair to itself, forming a polar covalent bond, is called its electronegativity. The negative side of a polar covalent bond corresponds to the more electronegative elementA substance containing only one kind of atom and that therefore cannot be broken down into component substances by chemical means.. Furthermore the more polar a bond, the larger the difference in electronegativity of the two atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. forming it.
Unfortunately there is no direct way of measuring electronegativity. DipoleIn an electrically neutral species, separated, equal positive and negative charges that consitute a positive and a negative pole; such a species tends to assume certain orientations more than others in an electric field.-moment measurements tell us about the electrical behavior of all electron pairs in the molecule, not just the bonding pairA pair of electrons between atoms joined by a covalent bond. in which we are interested. Also, the polarity of a bond depends on whether the bond is a single, double, or triple bondAttraction between two atoms (nuclei and core electrons) that results from sharing of three pairs of electrons between the atoms; a bond with bond order = 3. and on what the other atoms and electron pairs in a molecule are. Therefore the dipole momentThe magnitude of the separation of electrical charge in a molecule that makes the molecule polar; the partial positive charge times the partial negative charge divided by the distance by which the charges are separated. cannot tell us quantitatively the difference between the electronegativities of two bonded atoms. Various attempts have been made over the years to derive a scale of electronegativities for the elements, none of which is entirely satisfactory. Nevertheless most of these attempts agree in large measure in telling us which elements are more electronegative than others. The best-known of these scales was devised by the Nobel prize-winning California chemist Linus Pauling (1901 to 1994) and is shown in the periodic tableA chart showing the symbols of the elements arranged in order by atomic number and having chemically related elements appearing in columns. found below. In this scale a value of 4.0 is arbitrarily given to the most electronegative element, fluorine, and the other electronegativities are scaled relative to this value.
Electronegativities of the elements
|→ Atomic radius decreases → Ionization energy increases → Electronegativity increases →|
As can be seen from this table, elements with electronegativities of 2.5 or more are all nonmetals in the top right-hand comer of the periodic table. These have been color-coded dark red. By contrast, elements with negativities of 1.3 or less are all metals on the lower left of the table. These elements have been coded in dark gray. They are often referred to as the most electropositive elements, and they are the metals which invariably form binary ionic compounds. Between these two extremes we notice that most of the remaining metals (largely transition metals) have electronegativities between 1.4 and 1.9 (light gray), while most of the remaining nonmetals have electronegativities between 2.0 and 2.4 (light red). Another feature worth noting is the very large differences in electronegativities in the top right-hand comer of the table. Fluorine, with an electronegativity of 4, is by far the most electronegative element. At 3.5 oxygen is a distant second, while chlorine and nitrogen are tied for third place at 3.0.
If the electronegativity values of two atoms are very different, the bond between those atoms is largely ionic. In most of the typical ionic compounds discussed in the previous chapter, the difference is greater than 1.5, although it is dangerous to attach too much significance to this figure since electronegativity is only a semiquantitative concept. As the electronegativity difference becomes smaller, the bond becomes more covalent. An important example of an almost completely covalent bond between two different atoms is that between carbon (2.5) and hydrogen (2.1).
The properties of numerous compounds of hydrogen and carbon (hydrocarbons) are described in sections on organic chemistry. These properties indicate that the C―H bond has almost no polar character.
EXAMPLE 1 Without consulting the table of electronegativities (use the periodic table), arrange the following bonds in order of decreasing polarity: B—Cl, Ba—Cl, Be—Cl, Br—Cl, Cl—Cl.
SolutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture. We first need to arrange the elements in order of increasing electronegativity. Since the electronegativity increases in going up a column of the periodic table, we have the following relationships:
- Ba < Be and Br < Cl
Also since the electronegativity increases across the periodic table, we have
- Be < B
Since B is a group III element on the borderline between metals and non-metals, we easily guess that
- B < Br
which gives us the complete order
- Ba < Be < B < Br < Cl
Among the bonds listed, therefore, the Ba—Cl bond corresponds to the largest difference in electronegativity, i.e., to the most nearly ionic bondThe electrostatic attraction that holds together the positive and negative ions of an ionic compound.. The order of bond polarity is thus
- Ba—Cl > Br—Cl > B—Cl > Br—Cl > Cl—Cl
where the final bond, Cl—Cl,is, of course, purely covalent.