Chemical bonds, also called intramolecular bonds, are attractions between atoms that allow for the creation of substances consisting of more than one atom. When all the chemically bonded atoms are the same element, the substance is known as a molecule. When two or more different elements bond together, the result is called a compound. (However, the word molecule is often used colloquially to refer to both types of substances.)
Common Molecules and Compounds
There are two major types of bonds, distinguished from one another based on whether electrons are shared or transferred between the atoms. A covalent bond involves a pair of atoms sharing electrons from their outer orbitals to fill their valence shells. These bonds form between non-metals with similar electronegativities.
In an ionic bond, one atom “gives” its electrons to the other, resulting in one positively and one negatively charged atom. The bond is a result of the attraction between ions. Ionic bonds form between atoms with very different electronegativities.
NEED ionic bonds in table salt figure in COLOR
Metals can form tightly packed arrays in which each atom is in close contact with many neighbors. So many atomic orbitals overlap with each atom that they form very large molecular orbitals that in turn overlap with each other creating a continuous band in which electrons can move. Any excitation, such as an electrical current, can cause the electrons to move throughout the array. The high electrical and thermal conductivity of metals is due to this ability of electrons to move throughout the lattice. This type of delocalized bonding is called metallic bonding. Metals are ductile or can be bent without breaking because the atoms can slide past each other without breaking the delocalized bonds.
Polarity is the difference in charge across a compound caused by the uneven partial charge distribution between the atoms. Ionic bonds have higher polarity than covalent bonds because they consist of ions of full opposite charges, meaning one side of the compound is very positive and one very negative. The charge distribution in covalent bonds is more variable, resulting in either polar covalent bonds or non-polar covalent bonds.
Non-polar covalent bonds have no uneven distribution of charge. This is because electrons are completely shared between the two atoms, meaning neither has a strong hold on the shared electrons. Non-polar covalent bonds generally arise between two non-metal atoms with equal electronegativity, for example, two hydrogen atoms.
Polar covalent bonds arise between two non-metal atoms with different electronegativities. In these bonds, electrons are shared unequally. Neither atom is a completely charged ion; instead, the more electronegative atom will hold onto the electron more often, creating a slightly negative charge. The other atom will thus have a slightly positive charge. These slight charges are called dipoles.
A dipole moment is a measure of the unequal charge distribution in a polar bond. It is possible for a polar molecule to have no net dipole moment if the dipole moments of each bond are equal in magnitude and opposing in direction. These covalent compounds have a symmetrical molecular geometry, meaning that the dipoles created by the polar bond cancel each other out.