Atoms, Molecules and Mole Concept: Definition, History, Notes

Atoms, Molecules and Mole Concept: In Hindi, the matter is referred to as ‘padarth.’ Maharishi Kanad, an Indian philosopher, stated that if we continue to divide matter, we would end up with smaller and smaller particles of matter. Ultimately, we will arrive at the smallest particle of matter that can’t be divided any further. Based on this philosophy, Kanad was one of the first to claim that matter is made up of extremely microscopic particles known as “parmanu”.

Atoms were the name given to these particles by John Dalton. The word atom refers to something that is indivisible. Another Indian philosopher, Pakudha Katyayama, went one step farther and proposed that matter particles are normally combined and that different combinations of particles give us different sorts of matter. Molecules are the result of the combination of atoms.

What is Atom?

Atoms and molecules are tiny particles that make up all matter. This is because different types of atoms and molecules have different properties, different types of matter have different properties as well. As a result, the properties of matter are determined by the properties of the atoms and molecules that make it up.

The Idea of Divisibility by Greek Philosophers

Democritus and Leucippus proposed that if we continue to divide matter, there will come a point where no more particle division is possible. These particles are known as atoms, which means “invisible.”

However, until Antoine \({\rm{L}}{\rm{.}}\) Lavoisier published two principles of chemical combination, none of these concepts was supported by much experimental evidence.

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Law of Chemical Combination

There are three main important laws of chemical combination. These are:
1. Law of conservation of mass
2. Law of constant proportions
3. Laws of multiple proportions

The laws of chemical combination are experimental laws that scientists have created after conducting numerous experiments involving various types of chemical reactions.

Law of Conservation of Mass

This law was given by Antoine Lavoisier in \(1774.\) He is known as the father of chemistry. The law may be stated as:

The total mass of the products in a physical change or a chemical reaction is equal to the total mass of the reactants that have combined.
OR
The mass can neither be created nor destroyed in a physical change or a chemical reaction.

For example, it has been found by experiments that if \(100\,{\rm{grams}}\) of calcium carbonate are decomposed completely then \(56\,{\rm{grams}}\) of calcium oxide and \(44\,{\rm{grams}}\) of carbon dioxide are formed.

Law of Constant Proportion

This law was given by Proust in \(1779.\) The law may be stated as:

In a chemical reaction, compounds always contain the same elements present in definite proportions by mass irrespective of their source.

For example:
(i) \(18\,{\rm{grams}}\) of \({{\rm{H}}_2}{\rm{O}} = 2\,{\rm{grams}}\) of hydrogen \( + 16\,{\rm{grams}}\) of oxygen
\( \Rightarrow {\rm{mass}}\,{\rm{of}}\,{\rm{hydrogen}}/{\rm{mass}}\,{\rm{of}}\,{\rm{oxygen}} = \frac{2}{{16}} = \frac{1}{8}.\)
(ii) \(36\,{\rm{grams}}\) of \({{\rm{H}}_2}{\rm{O}} = 4\,{\rm{grams}}\) of hydrogen \( + 32\,{\rm{grams}}\) of oxygen
\( \Rightarrow {\rm{mass}}\,{\rm{of}}\,{\rm{hydrogen}}/{\rm{mass}}\,{\rm{of}}\,{\rm{oxygen}} = \frac{4}{{32}} = \frac{1}{8}.\)

In an attempt to explain the laws of chemical combination, Dalton put forward his atomic theory of matter. This is discussed below-

Dalton’s Atomic Theory

This theory was put forward in \(1808.\) The various postulates of Dalton’s atomic theory are as follows:

1. All matter is made of very tiny particles called atoms, which participate in chemical reactions.
2. Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
3. Atoms of a given element are identical in mass and chemical properties.
4. Atoms of different elements have different masses and chemical properties.
5. Atoms combine in the ratio of small whole numbers to form compounds.
6. The relative number and kinds of atoms are constant in a given compound.

Drawbacks

Some of the drawbacks of Dalton’s atomic theory are as follows-

1. The indivisibility of an atom has been proven false: protons, neutrons, and electrons may all be subdivided further.
2. The masses and densities of various elements’ atoms differ. Isotopes are atoms that have varying masses. Chlorine, for example, has two isotopes with mass numbers of \(35\) and \(37.\)
3. The idea that atoms of various elements are identical in every way has been disproved in several cases: argon and calcium atoms both have an atomic mass of \(40\,{\rm{amu}}.\) Isobars are the atoms that make up this group.
4. The theory is unable to account for the occurrence of allotropes, as well as differences in the characteristics of charcoal, graphite, and diamond.

Atoms

Atoms are the building blocks of all matter, and according to modern atomic theory, an atom is the smallest particle of an element which takes part in a chemical reaction. They  are so small that they cannot be seen through a very powerful microscope. The atomic radius is measured in nanometres.
\(1\,{\rm{nm}} = \frac{1}{{{{10}^9}}}{\rm{m}}\)
\(1\,{\rm{m}} = {10^9}\,{\rm{nm}}.\)

Atomic Radii of Some Common Elements

Symbols of Elements

Dalton was the first scientist to use symbols to represent elements in a short way. Some of the symbols of elements given by Dalton are:

Swedish chemist Berzelius suggested the use of one or two letters of the name of the element to represent its atomic symbol. For example, symbol \({\rm{C}}\) for carbon, \({\rm{Al}}\) for aluminium, etc.

The atomic symbol is a shorthand notation of the chemical name of an element. The chemical name may be an English name of the element or its Latin name. The first letter of the symbol is always written as a capital letter and the second letter as a small letter. For example:

Atomic Mass

According to Dalton’s Atomic Theory, each element has a distinct atomic mass. The law of constant proportions could be simply explained using this theory.
However, because the size of an atom is so small, determining its mass is difficult.

As a result, scientists began comparing the mass of an atom to the mass of a reference atom in order to determine its mass.

Earlier, \(\frac{1}{{16}}\) of an oxygen atom’s mass was used as a reference for estimating the mass of other elements. Carbon – \(12\) is now regarded as a standard atom.

It has a mass of \(12\,{\rm{u}}\) (\(12\) atomic mass units). As a result, one atomic mass unit equals \(\frac{1}{{12}}\) of the mass of a carbon-\(12\) atom. The atomic masses of a few elements are listed below.

Existence of Atom

Only the atoms of noble gases such as \({\rm{He}},\,{\rm{Ne}},\,{\rm{Ar}},\,{\rm{Kr}},\,{\rm{Xe}}\) and \({\rm{Rn}}\) are chemically unreactive and can exist in the state as a single atom.
Atoms cannot survive on their own. As a result, atoms combine to form molecules or ions.

Molecules

A molecule is made up of a number of different or same types of atoms that combine chemically. Certain forces of attraction bind these atoms together. Molecules are formed when atoms of the same or different elements link together.

As a result, a molecule is the smallest particle of a substance that can exist independently and exhibits all of that substance’s properties.

1. Molecules of Elements

Combinations of similar types of atoms make up an element’s molecules. Helium \(\left( {{\rm{He}}} \right),\) for example, is made up of only one atom, whereas oxygen is made up of two atoms. Molecules may be monatomic, diatomic, triatomic or polyatomic.

Atomicity

The atomicity of an element refers to the number of atoms in its molecule.

2. Molecules of Compounds

Compound molecules are made up of atoms from various elements that combine in a specific proportion. Water, for example, is made up of two hydrogen atoms and one oxygen atom.

Molecules of some compounds:

Ions

Ions are charged particles (atoms) that develop when a positive or negative charge is acquired on them.
An ion that is negatively charged is known as an anion.
A positively charged ion is known as a cation.

An ion can be a single charged atom or a collection of charged atoms with a net charge on the molecule.
A polyatomic ion is formed when a collection of atoms in a chemical bears a charge.

Chemical Formula

A chemical formula is a set of symbols that represents the composition of a compound. To write a chemical formula, one must know concepts:

1. Elements’ symbols
2. Valency

Valency

The combining capacity of an element is known as its valency. Valency is used to determine how an element’s atom will combine with another element’s atom to produce a chemical compound. Below given is a list of valencies of some elements

Rules of Writing Chemical Formula
(i) The valencies of the charge in the ion must be balanced.
(ii) When a combination contains both metal and non-metal constituents, the metal’s name is always written first in the chemical formula. Sodium chloride, for example, is written as \({\rm{NaCl}}.\)
(iii) When writing the number of ions associated with polyatomic ions, the ion is placed in brackets first.

How to Write Formula of a Compound?

(i) Write the symbols of the corresponding elements of the compound.
(ii) Write the valencies of the compound’s elements.
(iii) Crossover the valencies of the elements.

Examples of some compounds are as follows:

Molecular Mass and Mole Concept

Molecular Mass- It is equal to the sum of the atomic masses of all the atoms present in one molecule of that substance. It is expressed in atomic mass units \(\left( {{\rm{amu}}} \right).\)

For example- the molecular mass of \({\rm{HN}}{{\rm{O}}_3}\) can be calculated as:
Atomic mass of \({\rm{H}} = 1\,{\rm{u}}\)
Atomic mass of \({\rm{N}} = 14\,{\rm{u}}\)
Atomic mass of \({\rm{O}} = 16\,{\rm{u}}\)
Molecular mass of \({\rm{HN}}{{\rm{O}}_3} = 1 + 14 + \left( {16 \times 3} \right) = 63\,{\rm{u}}.\)

Formula unit mass- The sum of the atomic masses of all the atoms in a formula unit of a substance is called formula unit mass. It is calculated in the same manner as molecular mass.

The formula unit mass is used in case of substances that constitute ions. For example, formula unit mass of Sodium Chloride \(\left( {{\rm{NaCl}}} \right)\) can be calculated as: \(\left( {1 \times 23} \right) + \left( {1 \times 35.5} \right) = 58.5\,{\rm{u}}.\)

Mole Concept

Mole is a numerical quantity with a mass equal to that of a species’ atomic or molecular mass (atoms, molecules, ions or particles).
A mole of any substance equals \(6.022 \times {10^{23}}\) particles (atoms, ions or molecules)
The Avogadro number, often known as the Avogadro Constant, is written as \({{\rm{N}}_{\rm{O}}}.\)
It is named in honour of the Italian scientist, Amedeo Avogadro.

A substance’s atomic mass or molecular mass is given in grams is the same as its molecular mass.

The gram atomic mass of a substance is defined as the atomic mass of a substance when expressed in grams.

The gram molecular mass of a material is the molecular mass of a substance given in grams.

Sulphur, for example, has an atomic mass of \({\rm{32}}\,{\rm{u}}.\) Its gram atomic mass of \({\rm{32}}\,{\rm{g}}.\)
In addition, one atom of Sulphur is found in \({\rm{32}}\,{\rm{u}}\) of Sulphur. Sulphur has \(1\) mole atoms per \({\rm{32}}\,{\rm{g}},\) or \({\rm{6}}{\rm{.022}} \times {\rm{1}}{{\rm{0}}^{23}}\) atoms total.
Carbon Dioxide, on the other hand, has a gram molecular mass of \(44\,{\rm{g}}{\rm{.}}\)
However, we know that in a chemical equation, the measuring unit is the mole.
As a result, \(1\) mole equals \({\rm{6}}{\rm{.022}} \times {\rm{1}}{{\rm{0}}^{23}}\) number, which equals relative mass in grams.

Important Formulae

\({\rm{Number}}\,{\rm{of}}\,{\rm{moles}}\,\left( {\rm{n}} \right) = \frac{{{\rm{Weight}}\,{\rm{of}}\,{\rm{the}}\,{\rm{substance}}}}{{{\rm{At}}{\rm{.}}\,{\rm{Wt}}\,{\rm{or}}\,{\rm{Mol}}{\rm{.}}\,{\rm{Wt}}}}\)
\({\rm{Number}}\,{\rm{of}}\,{\rm{moles}}\,\left( {\rm{n}} \right) = \frac{{{\rm{Volume}}\,{\rm{of}}\,{\rm{gas}}\,{\rm{in}}\,{\rm{d}}{{\rm{m}}^3}\,{\rm{at}}\,{\rm{NTP}}}}{{22.4\,{\rm{d}}{{\rm{m}}^3}}}\)
\({\rm{Number}}\,{\rm{of}}\,{\rm{moles}}\,\left( {\rm{n}} \right) = \frac{{{\rm{Number}}\,{\rm{of}}\,{\rm{particles}}}}{{6.023 \times {{10}^{23}}}}\)
\({\rm{Weight}}\,{\rm{of}}\,{\rm{one}}\,{\rm{atom}}\,{\rm{of}}\,{\rm{an}}\,{\rm{element}} = \frac{{{\rm{Atomic}}\,{\rm{weight}}}}{{6.023 \times {{10}^{23}}}}\)
\({\rm{Weight}}\,{\rm{of}}\,{\rm{one}}\,{\rm{molecule}}\,{\rm{of}}\,{\rm{a}}\,{\rm{compound}} = \frac{{{\rm{Molecular}}\,{\rm{weight}}}}{{6.023 \times {{10}^{23}}}}\)

Three Important Questions of Atoms and Molecules

Q.1. What are polyatomic ions? Give examples.
Ans: A polyatomic ion, often called a molecular ion, is a charged chemical species (ion) made up of two or more covalently linked atoms or a metal complex that acts as a single unit.
For example, \({\rm{SO}}_4^{2 – },\,{\rm{CO}}_3^{2 – }.\)

Q.2. Which element will be more reactive- the element with atomic number \(10\) or the one with atomic number \(11?\)
Ans: Element with atomic number \(11\) is more reactive because the electronic configuration of this element is \(2, 8, 1\) so, it has to lose one electron from its outermost shell, and the element with atomic number \(10\) has electronic configuration \(2, 8\) and it is an already stable element.

Q.3. What is the difference between cation and anion?
Ans: An anion is an ion that has acquired a negative charge on it due to the gaining of electrons, whereas a cation is an ion that has acquired a positive charge on it due to the loss of electrons.

Summary

In this article, we studied in detail the laws of chemical combination and how these laws played a major role in determining what atoms and molecules are. We studied the definition of atoms, various symbols to represent them as well as atomic mass. We also studied that molecules are formed when different atoms combine together. Now we know about the mole concept and that \(6.022 \times {10^{23}}\) is called Avogadro’s number.

FAQs

Q.1. What are atoms and molecules?
Ans: Atoms are the building blocks of all matter, and according to modern atomic theory, an atom is the smallest particle of an element that takes part in a chemical reaction. A molecule is made up of a number of different or familiar atoms that combine chemically. Certain forces of attraction bind these atoms together. Molecules are formed when atoms of the same or different elements link together.

Q.2. What is the difference between atom and molecule?
Ans: An atom is a tiny particle of a chemical element that may or may not exist independently. Molecules are the smallest unit in a complex, consisting of a group of atoms that the bond links together. Chemical bonds exist between two or more identical or different atoms.

Q.3. What are the examples of atoms and molecules?
Ans: Some of the examples of atoms are oxygen, hydrogen, carbon, argon and iron.
Molecules are formed when atoms of the same or different elements link together. Some of the examples of molecules are aerozone, water, carbon dioxide, hydroxide, etc.

Q.4. What is the size of an atom?
Ans: Atoms are so small that they cannot be seen through very powerful microscopes. The atomic radius is measured in nanometres.
\(1\,{\rm{nm}} = \frac{1}{{{{10}^9}}}{\rm{m}}.\)
\(1\,{\rm{m}} = {10^9}\,{\rm{nm}}.\)

Q.5. Can a molecule have one atom?
Ans: No, a molecule cannot have one atom as molecules are formed when atoms of different or similar elements link together.

Q.6. How is an atom made of?
Ans: The protons and neutrons are packed into the nucleus of the atom, while the much smaller electrons revolve around the exterior. When people draw images of atoms, they depict electrons whirling like satellites in orbits around the Earth.

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