Attraction between atoms or ions leads to a chemical bond. According to the types of bonds contained in a molecule, the physical properties including melting point, hardness, electrical and thermal conductivity and solubility are determined. Chemical bonds involve only the outermost or valence electrons of atoms. Using the example of the simplest element, hydrogen, its two atoms on approaching each other, cause electon-electron and proton-proton repulsions to attempt separation of the atoms. But counterbalance by proton-electron attraction fuses the two hydrogen atoms forming a bond.

This example demonstrates the gain, loss and sharing of electrons by atoms for possession of the same number of electrons as the noble gas in closest proximity on the periodic table. With eight valence electrons (s2p6), all noble gases are chemically stable in a phenomenon called the octet rule. But certain exceptions are possible. Among them, one group of atoms has less than eight electrons like hydrogen with only one. BeH2 has just four valence electrons around Be, with Beryllium contributing two electrons and each hydrogen with one. The second exception applies in elements in periods 4 and above. Their atoms can have over four surrounding valence pairs in certain compounds.

Types of Chemical Bonds
Familiarity with three types of chemical bonds is required for the SAT II Chemistry exam, ionic bonds, covalent bonds and metallic bonds.

Ionic Bonds
An electrostatic attraction between ions with opposite charges, cations and anions causes ionic bonds. They usually involve metals and nonmetals as elements active in ionic bonds are mostly from opposite ends of the periodic table with an electronegativity difference exceeding 1.67. Being very strong, ionic bonds in compounds increase melting points and take a solid form in normal conditions. Finally, an electron in an ionic bond is transferred from the less electronegative atom to the more electronegative element. A prime example of an ionic bond-content molecule is NaCl or table salt.

Covalent Bonds
Atoms may share electrons instead of transferring them from atom to atom, resulting in covalent bonds. But the sharing is rarely ever equal due to the difference in electronegativity value of each atom. The only exception is the bond between two atoms of the same element. Covalent bonds are said to be non-polar when the difference in electronegativity of two atoms ranges from 0 to 0.4. Polar refers to electronegativity difference between 0.4 and 1.67. For both non-polar and polar covalent bonds, higher electronegativity in an element results in stronger attraction of the electron pair. Carbon dioxide, CO2 molecules have two bonds which are covalent bonds.

It is possible for covalent bonds to be single, double or triple. A single bond occurs when only one pair of electrons is shared. This single bond is a sigma bond with the electron density being most pronounced along the line joining the two atoms.

Metallic Bonds
Metallic bonds are exclusive to metals alone, including aluminium, gold, copper and iron. Each atom in a metal shares a bond with a number of other metal atoms, thereby allowing their electrons unrestricted movement within the metal structure. It is this specific phenomenon that is behind the properties of metals being unique, an example being their high conductivity.

That basically sums up the essence of chemical bonds, the differences in various types and the reasons for molecular structures being formed.