Atoms and Molecules

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Atom Definition What is Atomic Mass? Daltons Atomic Theory Molecule Definition FAQs

What is an Atom?

Atoms are defined as “the basic building blocks of matter”.

It is the smallest constituent unit of matter that posses the properties of the chemical element. Atoms don’t exist independently, instead, they form ions and molecules which further combine in large numbers to form matter that we see, feel and touch.

Atoms and Molecules

Atoms and Molecules

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What is the size of an Atom?

The size of an atom is extremely small, much smaller than our imagination. A layer of an atom as thick as a thin sheet of paper is formed when more than millions of atoms are stacked together. It is impossible to measure the size of an isolated atom because it’s difficult to locate the positions of electrons surrounding the nucleus.

However, the size of an atom can be estimated by assuming that the distance between adjacent atoms is equal to half the radius of an atom. Atomic radius is generally measured in nanometres.

1 m = \(10^{9}nm\)

Relative Sizes

Examples Radii (m)
Atom of Hydrogen \(10^{-10}m\)
Molecule of water \(10^{-9}m\)
Molecule of hemoglobin \(10^{-8}m\)
Grain of Sand \(10^{-4}m\)

What are Atoms made of?

An atom is composed of three particles, namely,  neutrons, protons and electrons with hydrogen as an exception without neutrons.

  • Every atom has a nucleus that bounds one or more electrons around it.
  • The nucleus has typically a similar number of protons and neutrons which are together known as nucleons.
  • The protons are positively charged, electrons are negatively charged and neutrons are neutral.

What is Atomic Mass?

It is the mass of an atom in a chemical element. It is roughly equivalent to total neutrons and protons present in the atom. It is expressed in atomic mass units (denoted by u). 1amu is equal to the exactly one-twelfth the mass of 1 atom of C-12 and the relative atomic masses of elements is determined with respect to-12 atom.

Atomic masses of Some Elements

Elements Atomic Mass (u)
Hydrogen 1
Carbon 12
Nitrogen 14
Oxygen 16
Sodium 23
Magnesium 24
Sulfur 32
Chlorine 35.5
Calcium 40

Salient features of Dalton’s Atomic Theory

  • The matter is composed of minute particles known as atoms.
  • Atoms are indivisible particles that can’t be destroyed or created through chemical reactions.
  • All atoms of an element have identical chemical properties and mass whereas, atoms of different elements have different chemical properties and masses.
  • Atoms combine in a ratio of small whole numbers to form compounds.

The matter is anything and everything in our surrounding. It has basic structural and fundamental units. Let us examine the concept of matter with an example. Taking a storybook into consideration and dividing its structure. The book contains many pages, each page consists of paragraphs, and each paragraph has many sentences.

Each sentence will further have many words and each word will have characters. Therefore we have divided a storybook into characters. This is exactly the same case when we take the matter into account. The matter is made up of substances which contain molecules. The molecules, in turn, are made up of groups of atoms.

Atoms in simple terms are defined as the smallest unit of matter. In ancient times the scientists wondered whether the matter could be further divided or not. Around 500 BC the idea of divisibility of matter evolved in India. A scientist named Maharishi Kanad stated that matter can be divided into smaller and smaller units. The smallest unit of matter after which further division was not possible was known as parmanu.

What are Molecules?

A molecule is defined as the smallest unit of a compound that contains the chemical properties of the compound. Molecules are made up of groups of atoms. Describing the structure of an atom, an atom is also sub-divided into smaller units. Proton, electrons, and neutrons are sub-particles of an atom. The protons and neutrons are contained inside the nucleus of the atom and electrons revolve around the nucleus.

Protons are positively charged particles whereas electrons are negatively charged particles. Neutrons do not carry any charge. So we can say that the nucleus is positively charged due to the presence of protons. The nucleus is a bulk mass at the centre of an atom. Atoms are largely vacant.

Every element has a certain atomic number. The atomic number of an element is defined as the number of protons present in its nucleus. It is denoted by Z.

When we talk about the mass of atoms, the mass of its particles is taken into consideration. Electrons have negligible mass. Hence the mass of an atom is the sum of the mass of protons and neutrons. The mass number is denoted by A.


1. How do atoms become molecules?
Ans: As atoms come together to form molecules, chemical bonds bind them together. As a consequence of sharing or exchanging electrons between the atoms, these bonds form. It is only the electrons that are ever active in bonding in the outermost shell.

2. What are 5 types of atoms?
Ans: Atoms consist of three groups of smaller particles, called protons (which are charged positively), neutrons (which have no charge) and electrons (which are charged negatively). The protons and neutrons are heavier, so they remain in the core of the nucleus. The nucleus is called them.

3. What is a molecule example?
Ans: A molecule is the smallest particle that has the chemical properties of that element or compound in a chemical element or compound. Molecules consist of atoms that chemical bonds bind together.

4. What is a simple molecule?
Ans: Water is known to be a basic molecule consisting of a few atoms. Basic molecular substances are molecules in which strong covalent bonds bind the atoms. Nevertheless, weak forces bind the molecules together so that they have high melting and boiling points.

5. Is ozone a molecule?
Ans: Ozone is a molecule made up of three atoms of oxygen. The chemical ozone symbol is O3 as the oxygen atom symbol is O. Most of the ozone present in our atmosphere is produced by a sun-emitted association of oxygen molecules with ultraviolet radiation.

We have briefly seen the properties of atoms and molecules. Stay tuned with us for more information on Atoms and Molecules.

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