In large species, there can be more than one central atom. Beyond that, the ca is the atom needing the most e- for octet, or the least electronegative atom. Once the ca is determined, you can draw a Lewis e- dot structure for it. When wanting to determine the stability of a dot structure drawn, one calculates the formal charge of all the atoms in the structure. The more atoms with zero formal charge, the more stable the structure drawn.
Bond length is the distance between the nuclei in a bond.
Bond energy or strength is the energy needed to break a covalent bond. Each molecule or ion has one structure, but we must draw several dot structures to represent the bonding. In such cases, the combination of all the possible formulas, or hybrid, gives us the true structure of that species. This is called resonance. The valence theories that we use to show bonding have limitations in that we must show e- localized on a particular atom.