Single covalent bonds occur when one pair of electrons is shared between atoms to create a compound. A single covalent bond can be represented by a single line between the two atoms.
An atomic orbital can be defined as the representation of the probability of finding an electron in an area of an atom's nucleus. Atomic orbitals can be categorized by their energy, angular momentum, and angular momentum components. However, for the sake of simplicity, the simple orbital names of s, p, d, and f refer to orbitals with angular momentum quantum numbers of 0, 1, 2, and 3, respectively.
Generally, orbital shapes are drawn to describe the region in space in which electrons are likely to be found. The principal quantum number (n) determines the size and energy of the orbital for a given nucleus, whereas the angular momentum (l) determines an orbital's shape. The single s orbitals (l = 0) are spherical in shape. These s orbitals house all of their electron density around the nucleus. In contrast, all of the other orbitals have angular momentum, and therefore their electron densities are not situated close to the nucleus. With a higher principal quantum number, the electron density probability becomes more complicated. For example, the 3 p orbitals (n = 2) are ellipsoid-shaped with a point of tangency at the nucleus.
Bonding occurs when two atomic orbitals come together in close proximity and their electron densities overlap, creating a molecular orbital. The strongest type of covalent bonds are sigma bonds, formed by the overlapping of atomic orbitals. Regardless of the atomic orbital type, sigma bonds can occur as long as the orbitals directly overlap between the nuclei of the atoms.