Molecule vs Atom
All matter in the world is made up of atoms and molecules hence they can be considered to be the building blocks of everything in this planet, including us. In layman’s terms, it can be said that the most basic and also the smallest unit of any element is atom. This atom has all the properties possessed by that element. For example, the smallest unit of oxygen gas is the atom of oxygen, also represented by letter O. However, this atom of oxygen does not exist independently and it is only when it combines chemically with another atom of oxygen that it becomes stable. It then becomes a molecule and is represented by the formula O2.
It is not necessary that molecules of a substance may be made up of same atoms. The most common example is of the molecule of water which is made up of 2 atoms of Hydrogen and a single atom of Oxygen. It is represented by the formula H2O.
Atom is the smallest particle of an element that cannot exist independently and take part in a chemical reaction. It cannot be seen with a naked eye. It can be seen only under the electron microscope that magnifies up to 200,000 times. Atoms are the basic units of any element and are made up of sub atomic particles called electrons, protons, and neutrons. Protons are positively charged, electrons are negatively charged, while neutrons have no charge. Protons and neutrons lie inside a nucleus while electrons revolve in orbits surrounding the nucleus. The imaginary path followed by the electrons is called orbits. Each orbit has certain energy level. Atoms are electrically neutral as in each and every atom the number of electrons is the same as the number of protons. This is called the atomic number of the element. Atoms of most elements leaving the inert gases are unstable thus they have less than 8 electrons in the outermost (valence) shell. Atoms of metallic elements have 1, 2, 3 electrons in the outermost shell while in the non-metallic elements 5, 6, 7 valence electrons are present.
Molecule is the smallest particle of an element that can exist independently. Atoms of the same or various elements combine together and form molecules. Molecules are bound together with an electrostatic force that holds the atoms together. The sum of the mass of all the atoms present in a molecule represents the molecular mass of the molecule. Molecules have a definite set of properties and are stable in nature and it is molecules that take part in a chemical reaction. When 2 atoms of oxygen combine to make a molecule of oxygen, it then becomes the gas that we breathe in. But interestingly, 3 atoms of oxygen can also combine together to make a molecule of ozone gas that has altogether different properties.
It is clear from the above analysis that atoms and molecules are present in all living and non living things. The physical and chemical properties of substances are dependent upon the arrangement of atoms and molecules. For example, in gases molecules are loosely packed and thus gas has no definite shape. In the case of liquids, molecules are a bit more compact and thus it can take the shape of the container it is kept. It is in solids that molecules are most compactly arranged which is why solids have a definite shape and volume.