Three types of bonding will be considered: ionic, covalent and dative covalent. In any atom it is the electrons in the highest energy orbitals that are involved in the process of bonding. These electrons are sometimes called valency electrons.
The most typical ionic compounds are formed when a metallic element from Group 1 or Group 2 reacts with a non-metal one from Group 6 or Group 7. When the reaction occurs, electrons are transferred from the metal to the non-metal until the outer electron shells of the resulting ions are identical to those of the nearest Noble Gas.
In sodium chloride, one electron in sodium is transferred to a chlorine atom leading to the formation of a positive sodium ion and a negative chloride ion. These ions are held together by strong electrostatic forces.
In magnesium oxide, each magnesium atom loses two electrons to form an ion with a charge of 2+. Each oxygen gains two electrons to form an ion with a charge of 2-.
In covalent bonds pairs of electrons are shared between the two atoms so that the atoms have full outer shells. Covalent bonds usually occur between non-metals. The simplest case of covalent bonding is in a hydrogen molecule. Each hydrogen atom has a single electron in a 1s orbital. Each atom donates its electron to form a shared pair of electrons.
In carbon dioxide, double bonds are formed by the sharing of four electrons, two contributed by each atom.
Triple bonds can be formed by the sharing of six electrons, three contributed by each atom.
In the above examples covalent bonding leads to the formation of small molecules. It is possible for covalent bonding to produce a giant structure, such as SiO2.
Dative covalent bonding is a type of covalent bonding in which a pair of electrons is shared between two atoms to form a bond but one atom supplies both electrons and the other supplies none.
In the ammonium ion, NH4+, a lone pair of electrons on the nitrogen atom is donated to the hydrogen and shared between the two.
In a molecule like fluorine, F2, where both atoms are the same, the electrons in the bond must be shared equally:
Both nuclei have a charge of 9+ but the inner electrons tend to shield or screen the nuclear charge so that the effective nuclear charge which is felt by the shared electrons is 9+ -2 = 7+.
So both "feel" an effective nuclear charge of 7+. However, in a diatomic molecule with different atoms, the sharing will not be equal. In hydrogen fluoride, HF, the shared electrons "feel" one positive charge from the hydrogen and seven from the fluorine.
The electrons will be attracted closer to the fluorine than to the hydrogen, or the electron charge cloud will be distorted towards the fluorine. This makes the fluorine end of the molecule relatively negative and the hydrogen end relatively positive. This can be represented as:
H - F
Molecules like this are said to be polar or to have a dipole. δ (the lowercase Greek letter Delta) means "a small amount of"; in this case it refers to charge.
Due to its greater electron attracting ability, fluorine is said to be more electronegative than hydrogen. It has a greater "pulling power". More electronegative elements attract electrons toward themselves. As we go across a period in the Periodic Table, the effective nuclear charge increases from group 1 to Group 7. On descending a group in the Periodic Table the effective nuclear charge remains the same but the outer electrons get further away from the nucleus, so the electronegativity decreases.
Thus the most electronegative elements are found at the top right-hand corner of the Periodic Table. The most electronegative elements are fluorine, oxygen, nitrogen followed by chlorine.
A scale of electronegativity values was proposed by Linus Pauling. The greater the number, the more electronegative the element. The greater the difference in electronegativity between two atoms shews how polar the bond between them is.
Polar bonds are covalent bonds with a bit of ionic character in them. The ionic and covalent models are extreme forms of bonding; polar bonds are somewhere between the two. The bigger the difference in electronegativity between the atoms, the more polar the bond and the greater the ionic character.
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