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Covalent bonds

Covalent bonding is the sharing of electrons between atoms.

Once two atoms bond, their behavior complete changes, for instance, water is made of hydrogen gas (which is very explosive) and oxygen gas (which makes almost everything else around it flammable) yet together they form water.


Consider mercury, sulfur, sodium, carbon, hydrogen, oxygen

By itself, mercury is poisonous, and sodium is explosive. Yet covalently bonded together in thiomersal, is a safe and very well-tested antiseptic and antifungal agent.

Some people believe that just because an element, by itself, is dangerous, that any molecular containing the element must also be dangerous, but that is not so.

The very process of combining atoms contently changes their properties completely.



Elements are not compounds thimerosal

Covalent bonding can occur :

Between two atoms of the same element

Between atoms of elements close to each other in the periodic table.

Between atoms that have similar electronegativities (affinities for electrons)

It mostly occurs in nonmetals.

Why does covalent bonding occur? When atoms meet, and their electron clouds overlap, they are unstable. So their electron clouds reconfigure into lower energy, more stable shapes. For common chemicals, most of the time, this produces an octet.

Remember that octets aren’t a law of nature; they’re “magic numberw. To learn more about octets, Lewis theory, and magic numbers, see: Lewis theory, Dot Diagrams and Magic Numbers

Example: Carbon does not form ionic bonds. Each has 4 valence electrons (half an octet.)
To form ionic bonds, Carbon molecules would have to either gain or lose 4 electrons. This is highly unfavorable (takes too much energy)  Takes less energy to share electrons.

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How does bonding occur?

A hydrogen atom has 1 p+ and 1 e-

If 2 H atoms get close enough, then the e- from each H feels the attraction from the proton of the other H atom
(shown by double headed arrow below)

The attractions are not strong enough to pull the e- completely away from its own proton. But they are strong enough to pull the 2 atoms close enough together – so that the e- feel the attraction from both protons.

When the e- are attracted to and shared by both atoms, the individual H atoms have bonded : they are now the H2 molecule.

This is a covalent bond.

 Two or more atoms covalently bonded are called a molecule.

Once bonded, the H2 molecule is more stable than the individual H atoms.

Atoms bond until their outer energy levels are full

Each H atom now feels 2 e- in its first energy level.

For any atom, the first energy level only accommodates 2e-

So for H2, each atom’s outer energy level is full.

Atoms will covalently bond until their outer energy level is full.

Once the outer levels are full, additional atoms will not covalently bond to it, for 2 reasons:

• An e- from a new atom would have to join an atom in the H2 molecule on the next energy level, further from the nucleus – where it would not feel a strong enough attraction.

• An e- from a H atom already in the H2 molecule – and close to the nucleus – would need to move further away to share with the new atom.

Both of these possibilities would make the molecule less stable, and so would not happen.

Covalent bonding also happens in a water molecule.

they share electrons = forms a covalent bond

Hydrogen and fluorine bonding

only showing valence electrons

from http://chemwiki.ucdavis.edu/Organic_Chemistry/Fundamentals/Introduction_to_Organic_Chemistry

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