Molecule

A molecule refers to two or more atoms of the same element or different elements bound together. In other words, when atoms combine to form covalent bonds, the resultant is a molecule. It is the simplest unit of a covalent compound, and there are various ways to represent and draw it. Atoms in some elements bond together, as in the case of chlorine and oxygen. However, atoms present in certain elements do not bond with other atoms, like argon and neon. A molecule can be homonuclear- consisting of atoms of one chemical element like oxygen (O₂). It can also be heteronuclear, i.e., composed of more than one element like water (H₂O).

Types of molecules

• Diatomic Molecules

A diatomic molecule comprises two atoms of similar or different chemical elements. Examples of diatomic molecules- O₂ and CO

• Polyatomic molecules

Molecules that comprise more than two atoms are called polyatomic molecules. Examples of polyatomic molecules- carbon dioxide (CO₂), water (H₂O)

• Heteronuclear Diatomic Molecules

A heteronuclear diatomic molecule has two atoms of the same elements chemically combined. The seven diatomic elements are – Hydrogen (H₂), Nitrogen (N₂), Oxygen (O₂), Fluorine (F₂), Chlorine (Cl₂), Iodine (l₂) and Bromine (Br₂). These elements are highly reactive and often form bonds with another atom of the same type.

• Homonuclear diatomic molecules

A homonuclear diatomic molecule has two atoms of different elements chemically combined. Examples of homonuclear diatomic molecules- carbon monoxide (CO), hydrochloric acid (HCl), and Hydrogen Fluoride (HF)

Electrostatic forces

Electrostatic forces are responsible for holding atoms in molecules. The electrostatic forces holding these atoms together in molecules are also responsible for causing static electricity. Examples of static electricity – when a person gets a shock from touching a doorknob or when someone’s hair raises on the end down a plastic slide.

Molecules and molecular compounds

The attractive interactions between atoms are known as chemical bonds. Atoms create chemical compounds when the attractive electrostatic interactions are stronger than the repulsive interactions between them. Chemical bonds are of two types- ionic and covalent. Ionic compounds have positively and negatively charged ions held together by strong electrostatic forces, while covalent compounds comprise molecules in which pairs of electrons are shared between bonded atoms. Here is a table listing the differences between ionic and covalent bonds.

Characteristic	Ionic Bonds	Covalent Bonds
Description	The bond between metal and nonmetal in which the nonmetal attracts the electron and metal donates its electron to it.	The bond between two nonmetals with similar electronegativities in which atoms share electrons in their outer orbitals.
Polarity	High	Low
Shape	No definite shape	Has a definite shape
Melting Point	High	Low
Boiling Point	High	Low
State at Room Temperature	Solid	Liquid or Gas
Examples	Sodium chloride (NaCl), Sulphuric Acid (H2SO4)	Methane (CH4), Hydrochloric acid (HCl)
Chemical Species	Metal and nonmetal	Two nonmetals

Polar and nonpolar molecules

If a molecule has no net electrical charge, the negative charge equals its positive charge. The forces experienced by such molecules rely on the arrangement of the positive and negative charges in space. If this arrangement is spherically symmetric, the molecule is called nonpolar. If there is a surplus of positive charge on one end of the molecule and a surplus of negative charge on the other, the molecule has a dipole moment. That is a measurable tendency to rotate in an electric or magnetic field called polar. When polar molecules rotate freely, they favour those orientations that lead to attractive forces. Nonpolar molecules are generally lipophilic (lipid-loving), while polar chemicals are hydrophilic (water-loving). Nonpolar molecules pass readily through a cell membrane as they dissolve in the hydrophobic, nonpolar portion of the lipid bilayer. Although permeable to water (a polar molecule), the nonpolar lipid bilayer of cell membranes is impermeable to several other polar molecules, like-charged ions or those with many polar side chains. Polar molecules pass via lipid membranes through specific transport systems.

Conclusion

Molecules are in motion all the time, irrespective of the state. In the case of solids and liquids, molecules are tightly packed. In the case of solids, the molecules move in rapid vibration. In the case of liquids, they move freely in a slithering fashion. For gases, since the density is less, the movement of molecules takes place freely. For a given compound in any state, be it solid, liquid, or gaseous, the speed of molecular motion increases as the absolute temperature increases.