Atoms and Molecules

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.

Table of Content

• Atoms and Molecules Definition
• Recommended Videos
• Atom Definition Chemistry
• What is the Size of an Atom?
• Relative Sizes
• What are Atoms made of?
• What is Atomic Mass?
• Salient features of Dalton’s Atomic Theory
• What is a Molecule?
• Examples of Molecules
• Forces between Atoms and Molecules
• Frequently Asked Questions – FAQs

Atoms and Molecules Definition

Atoms are much too small to be seen; hence experiments to find out their structure and behavior have to be conducted with large numbers of them. From the results of these experiments we may attempt to construct a hypothetical model of an atom that behaves like the true atom.

Molecules consist of one or more atoms bound together by covalent (chemical) bonds. Atoms may be depicted by circle shapes, each of which has a nucleus at the center (containing protons and neutrons), surrounded by one or more concentric circles representing the ‘shells’ or ‘levels’ in which the electrons surrounding the nucleus of the atom are located and markings indicating the electron.at each level. A molecule is the smallest thing a substance can be divided into while remaining the same substance. It is made up of two or more atoms that are bound together by chemical bonding.

Atoms and Molecules

Recommended Videos

Matter Around Us

Atomic Structure

Atom Definition Chemistry

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 nano metres.

1 m =

Relative Sizes

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

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 is a Molecule?

Molecule Definition

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. Protons, 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.

A molecule is the smallest unit (particle) of a compound having the physical and chemical properties of that compound. This does not mean that molecules can not be broken down into smaller parts, e.g. the atoms from which they are formed or the fragments of the molecule, each consisting of several atoms or parts of atoms.

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.

Examples of Molecules

A molecule is a collection of two or more atoms that make up the smallest recognisable unit into which a pure material may be split while maintaining its makeup and chemical characteristics. Some examples of molecules are

• H2O (water)
• N2 (nitrogen)
• O3 (ozone)
• CaO (calcium oxide)
• C6H12O6 (glucose, a type of sugar)
• NaCl (table salt)

Forces between Atoms and Molecules

The simplest forces between atoms are those which arise as a result of electron transfer. A simple example is that of say sodium fluoride. The sodium atom has a nuclear charge of +11, with 2 electrons in the K shell, 8 in the L shell and 1 in the M shell. The fluorine atom has a nuclear charge of 9 with 2 electrons in the K shell and 7 in the L shell.

The outermost electron in the sodium atom may transfer readily to the fluorine atom; both atoms then have a complete shell but the sodium now has a net charge of +1 and the fluorine a net charge of -1. These ions, therefore, attract one another by direct coulombic interaction. The force between them is strong it varies as x-2, where x is the distance between the ions, and it acts in the direction of the line joining the ions.

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