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Atoms chemically bond with other atoms in order to achieve a stable configuration of their electrons. The group of elements called the noble gases were once referred to as the inert gases because none of them are readily chemically reactive. The electronic structure of each member of the group is remarkably stable. During chemical bonding, atoms acquire the electronic structure of one of the noble gases. There are two main ways in which atoms bond with each other
Covalent bonding forms molecules. If two or more different atoms are bonded in the molecule then the molecule is a compound.
Covalent bonding involves the sharing of electrons. When atoms bond covanently the shared electrons count in the electronic structure of both of the atoms involved in the sharing. The number of electrons an atom shares is normally equal to the number of electrons needed to gain the stable electronic structure, like a noble gas. These shared electrons are often referred to as the valency electrons.
A molecule of hydrogen is formed when two hydrogen atoms are covalently bonded together. By sharing their electron, each atom now has the now gains the more stable electronic structure of the noble gas, helium.
Other covalently bonded molecules include oxygen gas, nitrogen gas, water, ammonia and carbon dioxide.
Some atoms covalently bond together to form a charged ion. Some examples are the nitrate ion, the carbonate ion, the hydrogen carbonate ion, the sulphate ion, the hydroxide ion.