Introduction to Covalent Compounds

Covalent molecules are formed when atoms share electrons.

Molecular orbitals are formed when atomic orbitals overlap.

Covalent bonds occur at a distance at which the potential energy is at a minimum. Bonded atoms vibrate; the bond length is the average distance between the atoms. The energy required to break the bond and separate the atoms is known as the bond energy.

Electronegativity is the tendency of an atom to attract bonding atoms to itself when it bonds to another atom. Each atom is assigned an electronegativity value. The higher the value, the higher the tendency to attract an electron to itself.

Bond character can be predicted by subtracting the electronegativities of the two atoms involved in the bond. The greater the difference, the higher the degree of ionic character. At one extreme of the spectrum are ionic bonds, in which the electron has been transferred from one atom to the other. At the other extreme are nonpolar bonds, in which electrons are shared equally. The best example of this is a diatomic molecule such as H2. In between are polar covalent molecules, in which electrons are shared, but not equally.

Nonpolar Covalent Bond

Polar Covalent Bond

Ionic Bond

Bond character can be predicted by subtracting the electronegativities of the two atoms involved in the bond. Nonpolar covalent bond-difference of less than 0.5 Polar covalent bond-difference of 0.5-2.1 Ionic bond-difference of more than 2.1

Polar molecules have a positive end and a negative end. A molecule for which this is true is a dipole. The strength of this polarity is measured by the dipole moment. The higher the dipole moment, the higher the polarity. As the polarity of the molecule increases, so does its bond strength.

Naming binary covalent compounds Table 6-5 on p. 212 gives the prefixes used in naming covalent compounds. The first element named is usually the first one written in the formula-the least electronegative element The root word is the name of the element. The prefix is used to indicate how many atoms of that element are in the molecule. The prefix mono- is not used in the first word of the name.

Naming binary covalent compounds The last element in the compound is generally the element with the higher electronegativity. It is given the suffix –ide, just as in binary ionic compounds. The prefix is used to indicate how many atoms of that element are in the molecule. The prefix mono- is used for the second element when only one atom of that element is present.

Example: CO There is only one carbon. Since carbon is the first element, the prefix mono- is not used. There is only one oxygen, but since this is the second element, the prefix mono- is used. Because oxygen is the second element, we add the suffix -ide. The name of this compound is: Carbon monoxide

Example: P4O6 There is are 4 phosphorus atoms in this compound. The prefix for 4 is tetra-. There are 6 oxygen atoms in this compound. The prefix for 6 is hexa-. Because oxygen is the second element, we add the suffix -ide. The name of this compound is: Tetraphosphorus hexoxide