When elements combine to form compounds, chemical bonds form between the
atoms. bonds form when atoms share or exchange
in a way that gives
each atom a complete outer shell.
electrons
protons
neutrons
nucleus

electrons

Explanation:

When elements combine to form compounds, chemical bonds form between the  atoms. Bonds form when atoms share or exchange  electrons in a way that gives each atom a complete outer shell.

For example:

Ionic bond:

It is the bond which is formed by the transfer of electron from one atom to the atom of another element.  

For example:

Sodium chloride is ionic compound. The electronegativity of chlorine is 3.16 and for sodium is 0.93. There is large difference is present. That's why electron from sodium is transfer to the chlorine. Sodium becomes positive and chlorine becomes negative ion. Both atoms are bonded together electrostatic attraction occur between anion and cations and have complete octet.

Covalent bond:

It is formed by the sharing of electron pair between bonded atoms.  

For example:

In water the electronegativity of oxygen is 3.44 and hydrogen is 2.2. That's why electron pair attracted more towards oxygen, thus oxygen becomes partial negative and hydrogen becomes partial positive.  Both have complete octet.

water at room temperature is flowing through a pipe with pressure taps located 10 ft apart along one section of the pipe, which are connected to a mercury (s=13.6) manometer.

the enthalpy of formation of water is given as -285.8kj/mol, which states that the enthalpy of formation is negative.

negative enthalpy of formation indicates that the heat is released during the reaction process. the heat is released from reaction to the surroundings the enthalpy is expressed with a negative sign with the energy given out in the reaction process.

the formation of water takes place by giving out heat which is -285.8 kj/mol.