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Chemical bonds form when the valence electrons of one atom interact with the valence electrons of another atom.
Since the valence electrons are the outermost electrons, they have the greatest opportunity to interact with the valence electrons of other atoms.
Therefore, the valence electrons have the most influence in forming bonds.
The number of electrons in an atom’s outermost valence shell governs its behaviour.
Therefore, we group whose atoms have the same number of valence electrons together in .
An atom with a noble gas configuration (corresponding to an ##”s”^2″p”^6##) tends to be chemically unreactive.
It does not tend to participate in bonding.
As a general rule, a Main Group element — an element in any of Groups 1, 2, and 13 to 17 — tends to react to get a noble gas electron configuration: ##”s”^2″p”^6##.
Hydrogen and helium are exceptions.
This tendency is called the , because the bonded atoms share eight valence electrons.
The most reactive kind of metallic element is an metal from Group 1 — an alkali metal (such as sodium or potassium).
Such an atom has only a single valence electron.
This one valence electron is easily lost to form a positive ion (cation) with a noble gas configuration (e.g., ##”Na”^+## or ##”K”^+##).
A metal from Group 2 (e.g., magnesium) is somewhat less reactive, because each atom must lose two valence electrons to form a positive ion with a noble gas configuration (e.g., ##”Mg”^(2+)##).
An atom of a nonmetal tends to attract additional valence electrons to attain a noble gas configuration.
One way to do this is to remove electrons from another atom.
The most reactive kind of nonmetal element is a halogen such as fluorine (##”F”##) or chlorine (##”Cl”##).
Such an atom has the electron configuration ##”s”^2″p”^5##.
It requires only one additional valence electron to achieve a noble gas configuration.
Thus, atoms in Groups 1 and 2 tend to react with atoms in Groups 16 and 17 to form .
The two ions are attracted to each other by electrostatic forces.
These attractions are called IONIC BONDS.
Atoms generally form ionic bonds when the difference between the two elements is large (1.7 or greater).
An atom of a noble gas configuration can also attain a noble gas configuration by sharing share electrons with a neighboring atom.
By sharing their outermost (valence) electrons, atoms can fill up their outer electron shells and gain stability by getting an octet of electrons.
Nonmetals readily form covalent bonds with other nonmetals.
If the two atoms are identical, as in ##”H—H”## or ##”F—F”##, the electrons are shared equally, and there is no separation of positive and negative charges.
If the electronegativity difference between the two elements is very small (0.4 or less), the electrons are shared almost equally. We say that such a bond is NONPOLAR.
It is simply a COVALENT BOND.
To form a covalent bond between, say, ##”H”## and ##”F”##, one electron from the ##”H”## and one electron from the ##”F”## form a shared pair.
For example, in the molecule ##”H—F”##, the dash represents a shared pair of valence electrons, one from ##”H”## and one from ##”F”##.
In this bond, the ##”F”## atom “wants” the electrons more than the ##”H”## does, but the ##”H”## won’t give up its electron completely.
It’s a case of unequal sharing.
The electrons spend more of their time near the ##”F”## atom.
This build-up of electron around the ##”F”## gives it a slight negative charge.
The loss of electron density around the ##”H”## gives the ##”H”## atom a slight positive charge.
The bond has a positive end and a negative end (or pole).
If the electronegativity difference is between 0.4 and 1.7, the bond is polar covalent.
We say that this is a POLAR COVALENT BOND.
When two metal atoms share electrons, we get a METALLIC BOND.
Unlike a covalent bond, in which valence electrons are shared between two atoms, the valence electrons in a metallic bond are shared among all of the metal atoms in the sample.
We visualize metals as an array of atomic cores (nuclei and inner electrons) or metal cations immersed in a “sea” of surrounding valence electrons.
Thus, the valence electrons are free to move around and are not associated with any particular metal atom.
Thus, the nature of the valence electrons determines whether we get, covalent, polar covalent, ionic, or .
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