Molecule:The smallest particle of a substance that retains the chemical and physical properties of the substance and is composed of two or more atoms; a group of like or different atoms held together by chemical forces.

Bonding: Compounds are formed when atoms of two, or more, elements are not just mixed together but chemically combined. This is known as Bonding.

Types of bonding•Ionic bonds•Covalent bonds•Van der Waals bonds•Hydrogen bonds

Ionic BondIonic bonding occurs between a pair of atoms when one of the atoms gives up its valence electrons to the other. The result is that both atoms have filled shells. Both atoms also end up with a charge, one negative, and the other positive. We call the positive charged atom a cation, and the negatively charged one, an anion

Covalent BondCovalent bonds often form between atoms with too many electrons in their valence shells to give away, but not enough to easily fill. Thus they share electrons with their neighbors, in such a way that including the shared electrons the shells are full.

Molecular Spectra• Molecules have energy levels that are associated with rotation

motion of a molecules.• As a whole and with vibrationtinal motion of the atoms relative

to each other. Just as transition Between different energy levels in atoms leads to atomic spectra, transitions between rotational and vibrational energy levels In molecules leads to molecular spectra.

The energy of a molecule can be divided into four categories.

1. Electronic energy

2. Translational energy

3. Rotational energy

4. Vibrational energy

E=Eel +Etrans +Erot +Evib

Infrared Region

repulsive between opposite electron cloudsrepulsive between opposite nuclei attractive between one electron cloud and the opposing nucleus

the repulsive force predominates at short distancesthe attractive force predominates at long distances From the graph one can see that when the molecules are close to each other the repulsive force predominates, while at greater distances the attractive force is larger.

Potential Energy

The system becomes more stable (the energy of the system decreases) as two hydrogen atoms move toward each other from r = ∞, until the energy reaches a minimum at r = r0 (the observed internuclear distance in H2 is 74 pm). Thus at intermediate distances, proton–electron attractive interactions dominate, but as the distance becomes very short, electron–electron and proton–proton repulsive interactions cause the energy of the system to increase rapidly.

• Molecular Rotation• Molecular Vibration