Atom and Molecules – History, Differences, Quiz

Atoms and molecules are the known as the building blocks of matter. Different types of matter exist around us which are due to the different kinds of atoms and molecules present in them. Atoms are made up of 3 subatomic particles, electrons, protons and neutrons. At the centre of an atom which is the nucleus consists of proton and neutrons while outside the nucleus electrons are present. There are various models and theories proposed by scientists about the structure of an atom by different scientists during their times. This article talks about the history of these models and the characteristics of atom.

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Atoms and Molecules Definition

Atoms are extremely small in size to be seen; hence experiments to find their structure and behavior are conducted with large numbers of them. With the help of these experiments a model of an atom that behaves like the true atom can be found.

Molecules consist of a group of such atoms bound together by covalent (chemical) bonds. Atoms may be portrayed by circle shapes, each of which has a nucleus at the centre (consisting of protons and neutrons), surrounded by one or more concentric circles symbolizing the ‘shells’ or ‘levels’ in which the electrons surrounding the nucleus of the atom are placed. Markings are used to indicate the electron at each level. A molecule is the tiniest thing a substance can be divided into while continuing to be the same substance. It is made up of 2 or more atoms that are bound together by chemical bonding.

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Atom Definition Chemistry

The tiniest particle of an component, which may or may not have an independent existence but always takes place in a chemical reaction is known as an atom. An atom is defined as the tiniest unit that retains the properties of an element. An atom is composed of sub-atomic substances and these cannot be created or destroyed. All atoms of the same material are identical and different materials have different types of atoms. Chemical reactions take place when atoms are rearranged.

Atoms consist of 3 fundamental types of particles namely the protons, electrons and neutrons. Neutrons and protons have more or less the same mass and in opposition to this the mass of an electron is negligible. A proton has a positive charge, a neutron contains no charge and an electron is negatively charged. An atom has equal numbers of protons and electrons and therefore overall an atom has no charge. The nucleus of an atom has protons and neutrons only, and therefore is positively charged. The electrons reside the region of space around the nucleus. Therefore, most of the element’s mass is concentrated inside its nucleus.

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The nucleus is the centre of the atom. The nucleus contains neutrons and protons that give an atom its weight and positive charges. A neutron carries no charge but has a mass of 1 unit. A proton has a single positive charge and also has a mass of 1 unit, The atomic number of a material is equal to the number of protons or positive charges inside the nucleus. The atomic weight of an element is obtained by calculating the total number of protons and neutrons in the nucleus. An electron carries a single negative charge. If an atom of an element is to have no charge, it must have the same amount of electrons as protons. These electrons are placed in orbits around the nucleus of the atom like the layers in an anion.

What are Atoms made of?

An atom is composed of 3 particles, namely,  neutrons, protons and electrons with hydrogen as an exception without neutrons.

• Every atom has a nucleus that is surrounded by one or more electrons around it.
• The nucleus has usually a similar number of protons and neutrons which are together known as nucleons.
• The protons have positive charge, electrons have negative charges and neutrons are neutral.

What is a Molecule?

Molecule Definition

A molecule is defined as the smallest part of a compound that has the chemical properties of the compound.

Molecules are made up of a collection 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 present inside the nucleus of the atom and electrons revolve around this nucleus.

Protons are positively charged materials whereas electrons are negatively charged particles. Neutrons do not have any charge. So we say that the nucleus is positively charged as it contains protons. The nucleus is the major mass at the centre of an atom. Atoms are largely empty.

Each element has a certain atomic number. The atomic number of an element is characterized as the number of protons present in its nucleus. It is represented  by Z.

When we talk of the mass of atoms, the mass of their particles is taken into consideration. Electrons have minimal mass. Thus the mass of an atom can be obtained by adding the masses of protons and neutrons. The mass number is represented  as A.

A molecule is the smallest part (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 further parts, e.g. the atoms from which they are formed or the fragments of the molecule, each have several atoms or parts of atoms.

A molecule is defined as the tiniest unit of a compound that contains the chemical properties of the compound. Molecules are made up of collection of atoms. Employing the structure of an atom, an atom is also  further divided into smaller units. Proton, electrons, and neutrons are sub division of an atom. The protons and neutrons are present inside the nucleus of the atom and electrons revolve around the nucleus.

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Examples of Molecules

A molecule is a collection of 2 or more atoms that make up the tiniest recognisable unit into which a pure material may be split while maintaining its property and chemical characteristics. Some e.g 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 that exist between atoms are those which originate as a result of electron transfer. A simple e.g is that of say sodium fluoride. The sodium atom has a nuclear charge of  plus eleven, with two electrons in the K shell, eight in the L shell and one in the M shell. The fluorine atom has a nuclear charge of nine with two electrons in the K shell and seven in the L shell.

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

Differences between Atoms and Molecules

Given below are the differences between atoms and molecules.

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History of Atomic Theory

The different atomic theories that were proposed from the end of 18^th century are discussed below:

Dalton’s Atomic Theory

• The matter is consists of minute particles called
• Atoms are indivisible particles that can neither be destroyed nor created through chemical reactions.
• All atoms of a single element will have identical chemical properties and mass whereas, atoms of distinct elements will have different chemical properties and masses.
• Atoms combine to form compounds in a ratio of small whole numbers.

Plum Pudding Model and Rutherford Model

• The model tried to explain 2 properties of atoms then known: that electrons are negatively-charged particles and that atoms have no net electric charge or is neutral.
• According to this Model, an element  consists of small spheres where the positive charge is spread in a regular manner throughout the volume of sphere called pudding and the negatively charged particles or Electrons form the Plums which are apportioned as point charges on the shells.

Bohr Model of the Atom

• According to this model electrons orbit the nucleus in orbits that have a predefined size and energy.
• The energy of the orbit depends on its size. The minimal energy is found in the smallest orbit.
• Radiation is absorbed or emitted when an electron jumps from one orbit to another.

Quantum Atomic Theory

• The energy of the negatively charged particle or an electron is quantized i.e. an electron can only have certain specific whole values of energy.
• The quantized energy of an electron is the result of wave like properties of electron.
• As per Heisenberg’s Uncertainty principle, the exact position and momentum of an electron cannot be found.

Atom and Molecules: Quiz

1. Who had proposed the model of an atom in 1903, due to which electrons and protons came to be known?
a) J.J. Thomson

b) Rutherford

c) Goldstein

d) Dalton
Ans. (a) J.J Thomson

2. ________ is the model of an atom which is similar to a Christmas pudding.
a) Dalton’s Model

b) Thomson’s model

c) Rutherford model

d) None of the above
Ans. (b) Thomson’s model

3. The existing concept of the structure of an atom is given by _____________ .
a) Goldstein

b) Niels Bohr

c) J.J. Thomson

d) Rutherford
Ans. (c) Niels Bohr

4. ________________ is the atom in which the nucleus of that atom does not contain any neutrons.
a) Hydrogen

b) Phosphorous

c) Sodium

d) Oxygen
Ans. (a) Hydrogen

5. Which model is not able to justify the stability of an atom?
a) Thomson’s Model

b) Bohr’s Model

c) Rutherford’s Model

d) None of the above
Ans. (c) Rutherford’s Model

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6. On passing high voltage electricity through gases at very low pressure resulted in the discovery of  _______  according to Goldstein’s experiment.
a)Neutron

b)Electron

c)Proton

d)Nucleus
Ans. (c) Proton

7. ______________ discovered electron.
a) Goldstein

b) J.J.Thomson

c) D. James Chadwick

d) Niels Bohr
Ans. (d) Niels Bohr

8. In 1932 ___________ had discovered neutron.

a) Rutherford

b) J.J Thomson

c) James Chadwick

d) Goldstein
Ans. (c) James Chadwick

9. _______________  describes that there is no change in the energy of electrons as long as they keep revolving in the same energy level and atoms remains stable.

a) J.J Thomson Model

b) Bohr’s Model

c) Rutherford Model

d) None of the above
Ans. (b) Bohr’s Model

10. _________ is the correct electronic configuration of sodium.
a) 2,8,1

b) 8,2,1

c) 2,1,8

d) 2,8,2
Ans. (a) 2,8,1

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