1, the formation of covalent bondsA, B two atoms have a single electron, when A, B close to each other, the two electrons in the opposite way to form an electron pair, that is, two electrons where the atomic orbit can overlap each other, the system energy is reduced, The formation of chemical bonds, that is, a pair of electrons to form a covalent bond.The more the covalent bond is formed, the lower the energy of the system, the more stable the formation of the molecule, so that the unpaired electrons in the atoms form as much as possible the covalent bond.For example: H2, can form a covalent bond; HCl molecules, also formed a covalent bond .N2 molecules howIt is known that the electron structure of N atom is 2s22p3Each N atom has three single electrons, so the formation of N2 molecules, N and N atoms can form three covalent bonds.The formation of CO molecules, and N2 similar to the same three pairs of electrons, the formation of three covalent bonds. The difference is that one pair of electrons in the formation of covalent bonds with particularity, that is, C and O out of a 2p The tracks are overlapped, and the electrons are provided by O alone. Such covalent bonds are referred to as covalent coordination bonds. Thus, CO can be expressed as:Coordination bond formation conditions: there is a pair of atoms in the solitary electrons, while the other atoms can be orphaned electrons in the orbit overlap with each other in the empty orbit in the coordination compound, often see the coordination key.2, covalent bond characteristics - saturation, directionalityFor example: O has two single electrons, H has a single electron, so combined into water molecules, only a few pairs of electrons (including the original and excited to generate) Can form two covalent bonds; C can form up to 4 covalent bond with H.Direction: the atomic orbit in the spatial distribution is fixed, in order to meet the maximum overlap of the orbit, the atoms between the covalent bond, of course, to have a directional, such as: HClCl 3p and H of the 1s orbit overlap, to overlap along the z axis, so as to ensure maximum overlap, and does not change the original symmetry. Another example: Cl2 molecules, but also to maintain symmetry and maximum overlap