A covalent bond is the type of attraction that holds two atoms together when they share electrons. Covalent bonds typically are only formed between pairs of nonmetal atoms, for example H with Br, C with H, Cl with H, C with Cl. The simplest covalent bond is the single bond. A single bond exists between two atoms that share one pair of electrons. There are also double and triple covalent bonds.

Graphing the energy between two H atoms.

Low energy favors stability. The more negative the energy the more stable the bond. When energies are negative betwen two atoms this favors bond formation. The graph below shows how the energy of two atoms changes when the atoms approach each other. At long distance the energy is negative and near zero. As the atoms get closer the energy decreases. The energy goes to a low point or minimum at a separation that matches the average bond length of 0.74 Angstroms. and a negative energy of -436 kilojoules/ mol. When the atoms get too close the energy increases and goes above zero. If the atoms get too close the energy of repulsion goes too big and crosses over to positive energy.

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Bringing atoms together: When a pair of atoms are brought close to each other electrical forces and energy changes result. Remember particles with the same sign electrical charge repel one another. This means the positively charged nuclei repel each other and the negatively charged electrons repel each other. The size of these energies and forces change as the atoms are brought closer and closer. The total energy reaches a minimum at a separation equal to the bond length distance or bond length IF A BOND FORMS. The separation or bond length depends on the size of the atoms and the number of shared electrons. The bond length is consistent for a specific pair of atoms. The C-H bond length is roughly the same 109 picometers in all organic molecules. This allows us to predict the size of molecules if we know their composition. When a pair of atoms don't form a bond, the energy simply gets bigger and bigger. The two atoms will not stay together, but will fly apart if left on their own.

The covalent bond exists because the two nonmetal atoms both get the shared electrons in their valence shell some of the time. That is what the sharing means. The formulas for compounds result from these limitations on bonding. Examples: hydrogen and oxygen only form H₂O and H₂O₂. There are no other combinations like H₂O₅, H₃O₄, etc.

Molecules are groups of atoms that are held together by covalent bonds. A molecule can have as few as two atoms like H₂ or as many as millions, like proteins and polymers in plastics