![]() The more strongly an atom attracts the electrons in its bonds, the larger its electronegativity. It determines how the shared electrons are distributed between the two atoms in a bond. Electronegativity is a measure of the tendency of an atom to attract electrons (or electron density) towards itself. Whether a bond is nonpolar or polar covalent is determined by a property of the bonding atoms called electronegativity. This symbolism is shown for the H–Cl molecule in Figure 2. We sometimes designate the positive and negative atoms in a polar covalent bond using a lowercase Greek letter “delta,” δ, with a plus sign or minus sign to indicate whether the atom has a partial positive charge (δ+) or a partial negative charge (δ–). Compare this to Figure 1, which shows the even distribution of electrons in the H 2 nonpolar bond. Note that the shaded area around Cl is much larger than it is around H. Thus, in an HCl molecule, the chlorine atom carries a partial negative charge and the hydrogen atom has a partial positive charge. Figure 2 shows the distribution of electrons in the H–Cl bond. For example, the electrons in the H–Cl bond of a hydrogen chloride molecule spend more time near the chlorine atom than near the hydrogen atom. The atom that attracts the electrons more strongly acquires the partial negative charge and vice versa. This unequal distribution of electrons is known as a polar covalent bond, characterized by a partial positive charge on one atom and a partial negative charge on the other. Instead, the bonding electrons are more attracted to one atom than the other, giving rise to a shift of electron density toward that atom. When the atoms linked by a covalent bond are different, the bonding electrons are shared, but no longer equally. ![]() (b) Symbols δ+ and δ– indicate the polarity of the H–Cl bond. The small, black dots indicate the location of the hydrogen and chlorine nuclei in the molecule. The electron density is greater around the chlorine nucleus. ![]() (a) The distribution of electron density in the HCl molecule is uneven. ![]()
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