Covalent Bonding is the tug-of-war between two atoms over electrons. Every atom has a positive nucleus and every atom has negative electrons. When two atoms approach each other, the nucleus of the first pulls on the electrons of the second atom. At the same time, the second nucleus pulls on the electrons of the first atom. If neither atom is strong enough to remove the electrons from the other, then they will be stuck together in an eternal tug-of-war that neither ever wins. The word eternal here is used loosely. It might literally mean eternal for a molecule of hydrogen gas that is floating out in space and may never bump into another molecule, or it might mean only a short while before some other atoms with its positive nucleus comes in and joins the fight for the electrons. What is true is that the bond will remain until some outside force disturbs it.
A note about sharing. Many books describe covalent bonding as the sharing of electrons between two atoms. This is NONSENSE. Sharing is a rational, mature act for mutual benefit. Atoms are not rational, nor mature and mutual benefit has no meaning to them. Not only does the idea of sharing imply that atoms act magnanimously, but it also makes the idea of polarity (coming later in this unit) more confusing. Covalent bonding is war (or at least tug-of-war).
Covalent bonds can be imagined as follows. Two atoms come together. For our purposes, let us imagine that they are both hydrogen atoms with a single electron in the 1s orbital. As they approach, the nucleus of the atom on the left begins to pull on the electron from the atom on the right and vice versa.
Eventually the electron cloud (or orbital) is pulled hard enough
that they begin to warp towards each other until
they wrap around both nuclei at the same time
This is a covalent bond. In simplest terms then, a covalent bond is a tug-o-war for electrons between two atoms that neither atom wins.
In some cases, more than two electrons are involved in the tug-o-war between two atoms. Almost all of the time (and certainly all of the time in this text) bonds involve electrons in pairs. In other words, bonds are made of two electrons or four or six, but not one, three or five. A bond made from two electrons is called a single bond. A bond made from four electrons (two pairs) is called a double bond and a bond made of six electrons (three pais) is called a triple bond.
Technical Term Alert:
The technical term for the first bond (first 2 electrons involved in the tug-o-war) is a sigma bond. Any additional bonds are called pi bonds. So, a double bond contains a sigma bond and a pi bond, while a triple bond contains one sigma bond and 2 pi bonds. The physical arrangements of the electron clouds in double and triple bonds, as well as the differences between sigma and pi bonds are beyond the scope of this text.