Atoms and Molecules: Definition, Meaning, Properties, Difference, Structure, and Sample Questions

Atoms and Molecules

Atoms and molecules are considered as the building blocks of matter in the universe. Around 500 B.C Maharishi Kand gave a theory that if we continuously divide the matter, one stage will come when we obtain the smallest particle beyond which the further division will not be possible, known as "pramanu". Ancient Greek philosophers & Democritus and Leucippus called these particles atoms. So what are the atoms and molecules? This blog post comprises everything from scratch, from the definition to its properties. Keep reading to learn the concept in-depth.

Atoms and Molecules: Definition and Meaning

Everything around us is made up of matter, which comprises atoms and molecules. Matter is anything that occupies space and possesses mass. It comprises the unit of substances known as atoms, and more than one atom makes up the molecules.

Definition of Atoms

Atom is the smallest part of an element that makes up any compound. It is considered the fundamental, structural and functional unit of all compounds that may or may not have an independent existence. However, it takes part in the chemical reaction. In any chemical reaction, the atoms can only be transformed and cannot be broke down or synthesised. Their size is measured in the diameter, and their diameter is measured by the nanometer.

Atom is the smallest unit of the element, which is extremely small and cannot be predictable through the naked eyes. It consists of the three fundamental types of particles: protons, neutrons, and electrons. For example, N, H, He, O, K, etc.

Atom 1

Definition of Molecules

A group of atoms that are chemically bonded together are called molecules. It is defined as the smallest particle of the elements (e.g., O2) or compound (e.g., H2O) that can exist individually or independently. Molecules exist in a free state and are stable. Covalent chemical bonds bond the atoms in a molecule together.

atom 2

The molecules may not have the same chemical characteristics as the atoms that make them up. Sodium (Na) is a highly reactive metal, whereas chlorine (Cl) is a toxic gas. When these elements are united, Sodium Chloride (NaCl), often known as table salt, is formed, which is neither a metal nor a toxin.

Properties of Atoms and Molecules

Properties of Atoms

Nanometers (1m = 109 nm) measure the size of incredibly tiny atoms.

To put that in perspective, one teaspoon of sugar has the same number of atoms as all of the sand grains on all of the world's beaches combined. Example: A hydrogen atom has a radius of 10-10m.

The electrons orbit around the nucleus of atoms. Nuclear connections hold these subatomic particles together. Atoms are the smallest particles involved in chemical reactions. The number of electrons in an atom's outermost energy level determines whether it is stable or unstable.

These outermost electrons are either obtained or lost to link with other atoms. When an atom obtains or loses an electron, it becomes an ion with a charge. A cation is formed when an atom loses an electron and gains a positive charge. An atom obtains a negative charge and is termed an anion if it gains one electron to fill up the energy level.

Properties of Molecules

Properties of molecules mainly depend upon the intermolecular force of attraction.

Liquid and gases molecular compounds have low melting, and boiling points as the atoms are bonded by the covalent bond, and they can be separated with the energy when applied to them.

These are insoluble in water, but sometimes they can be sparingly soluble in water on the reaction it gives hydrogen bonds with them.

These are not dissociated into ions but dissolve into molecules, so they don't conduct electricity. These move freely in whole solid and liquid as there is not enough connection between molecules and electrons due to the delocalisation of electrons. 

Difference Between Atoms and Molecules

The following are the difference between atoms and molecules:

  • The smallest unit of an element that can or cannot exist freely, according to science, is the atom. On the other hand, a molecule is the smallest unit of a complex and is made up of a group of atoms bound together by a bond.
  • In addition, an atom can exist in a free state or not. Molecules, on the other hand, exist in a free state.
  • A nucleus (made up of neutrons and protons) and electrons are also present in an atom. On the other hand, a molecule is made up of two or more distinct or similar atoms that are chemically joined.
  • Furthermore, an atom has a spherical form. The form of a molecule, on the other hand, might be angular, rectangular, triangular, or linear.
  • Most importantly, atoms are very reactive and participate in chemical reactions without decomposing chemically. On the other hand, a molecule is less reactive and does not participate in chemical reactions.
  • Atoms also maintain their nuclear connection based on the electrical attraction between the electron and the nucleus. On the other hand, Molecules have covalent bonds, which means they share electrons to stay together.


The following chart shows the difference between the atom and molecules clearly:

Structure of Atoms

Three particles, such as neutrons, protons and electrons, make up the atoms, except hydrogen, which has no neutrons.

  • Electrons: these are negatively charged particles that revolve around the atoms. These electrons are called the energy of levels. They revolve around the nucleus in a series of levels.
  • Protons: these are positively charged particles. These protons, together with the nucleus, are called nucleons.
  • Neutrons: it is present inside the atom's nucleus and is neutral. The number of protons remains the same in the atoms, whereas the neutrons can change.

atom 3

Atoms and Molecules Sample Questions

The following are the sample questions for your practice once you are done with the entire chapter.

  • What is the atomicity of Sulphur, Ozone, Chlorine, Benzene, and Graphite?
  • What is the mass of 0.5 moles of water?
  • Is benzene the atom or molecule? Find out its mass.
  • What are polyatomic ions? Give examples.

Did you know about the following types of Radioactive Decay?

  • The process of alpha decay occurs when the nucleus emits an alpha particle, a helium nucleus with two protons and two neutrons. As a result of the emission, a new element with a lower atomic number develops.
  • The weak force is in charge of beta decay (and electron capture), which occurs when a neutron transforms into a proton, or a proton transforms into a neutron.
  • A shift in the nucleus's energy level to a lower state causes gamma decay, resulting in electromagnetic radiation emission.

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