Atoms
Atoms are known as the fundamental building blocks and tiniest particle of an element. They are the basic units of matter and define the structure of any element. It is an indivisible, hard and dense sphere. However, as perceived in the past, that atom is the smallest particle is not a fact now. Scientists discovered that an atom itself is made up of three tiny particles called subatomic particles which are protons, neutrons, and electrons. Atoms may or may not exist freely. They have a spherical shape and are not visible to naked eyes and magnifying microscopes. Atoms are highly reactive and subject to certain exceptions. They form a nuclear bond.
Moreover, the nucleus of the atom is positively charged and is encircled by a negatively charged electron. Furthermore, the protons and neutrons have almost equal masses such as in N, H, He, O, K, elements etc.
Nucleus: The nucleus is a collection of particles called protons, which are positively charged, and neutrons, which are electrically neutral. Protons and neutrons are made up of particles called quarks. Atoms of the same element have the same number of protons, called the atomic number. Represented by Z. In a single element, the number of neutrons may vary, determining the isotope of that element. The total number of protons and neutrons determine the mass number. The number of neutrons relative to the protons determines the stability of the nucleus, with certain isotopes undergoing radioactive decay.
Electron Cloud: It is defined as the system of electrons surrounding the nucleus of an atom. They have a collective property of a particle and a wave. The electron cloud is a zone inside the potential well where each electron forms a type of three-dimensional standing wave that does not move near to the nucleus.
Molecules
A group of two or more than two atoms of the same or different elements that are chemically bonded together is called a molecule. For instance, two atoms of hydrogen and one atom of oxygen chemically combines to form one molecule of water. Molecules exist in a free state. It contains two or more than two different or identical atoms that are bonded together. They usually have a triangular, angular or linear shape. Same as atoms, molecules are not visible to naked eyes but are visible through a magnifying microscope. As compared to atoms, molecules are less reactive and make a covalent or ionic bond, for example, Co2, NaCl HCl NO2, O3, etc.
Some chemical reactions of molecules:
C + O2 → CO.
2Na + Cl2 → 2NaCl
2NO+O2 → 2NO2
Atoms and molecules both are different in many terms such as shape, nature, existence, visibility and reactivity.