Atoms and Molecules: Short Notes

In this post, we will discuss about atoms and molecules.

Law of chemical combination

There are, in all, 5 laws of chemical combination. Here, only two of them, which contributed significantly towards the development of chemistry are described.

What is the law of conservation of mass?

The law of conservation of mass states that, “during a chemical reaction, matter can neither be created nor be destroyed”.

In a chemical reaction, total mass of the products formed is equal to total mass of the reactants used.

Ex: 12 g carbon reacts with 32 g oxygen to produce carbon dioxide, then the mass of carbon dioxide produced must be (12 g + 32 g) = 44 g.

What is the law of constant proportions?

The law of constant proportions is also known as the law of definite proportion.

The law of constant proportion was postulated by Proust in 1797. 

According to this law, “a chemical compound always consists of the same elements combined together in the same ratio, independent of the method by which it is prepared, or the source from where it is obtained”.

For example, water (H₂O) is always found to contain only hydrogen and oxygen. The ratio in which hydrogen and oxygen are present in water is also found to be fixed at 1:8 by mass or 2:1 by volume, independent of the source of water.

Thus, if we decompose 9 gram of water, then 1 gram of hydrogen and 8 g of oxygen are obtained always, independent of the source of water.

Similarly, carbon dioxide (CO₂) is always found to contain carbon and oxygen combined in the ratio of 3:8 by mass. Thus, if 3 gram of carbon is burnt, it combines with 8 gram of oxygen to give 11 gram of carbon dioxide.

Atoms and Atomic Theory of Matter

The idea that matter can be divided and subdivided was considered long back in India and Greece. Maharishi Kannada was a great Indian sage in the vedic period. According to him, if a pure substance is broken down into smaller pieces,a stage is reached when no further subdivision is possible. He called the ultimate smallest particle of any pure substance anu. Each anu of a substance has the same properties.

He further believed that each anu may be made of two or more still smaller particles. He called these particles as parmanu. According to him, a parmanu is the ultimate smallest unit of matter.

Some Greek philosophers, around 400-500 BC, believed that matter is composed of tiny particles which cannot be destroyed. The Greek philosopher Democritus called such tiny particles atoms, from the Greek word atomos meaning uncut or indivisible. However, there was no experimental evidence to support this model.

In 1805, an English school teacher John Dalton postulated his famous atomic theory, known as Dalton’s atomic theory.

What are the postulates of Dalton’s atomic theory?

The major postulates of Dalton’s atomic theory are:

• All forms of matter, solid liquid or gas, and elements compounds or mixtures are made up of very small particles called atoms.
• Atoms cannot be created, divided or destroyed as a result of a chemical change.
• All atoms of an element consists of identical, and different from those of the other elements. Thus, hydrogen consists of only the atoms of hydrogen, while nitrogen consists of only the atoms of nitrogen. The atoms of hydrogen differ from atoms of nitrogen.
• Atoms of elements combine in the ratio of whole number to produce a large number of compound-atoms of a new substance. The compound-atoms of particular substance are identical in all properties, and differ from those of other substances.

According to Dalton’s atomic theory, an atom is the smallest, discrete and indivisible particle of matter. Modern researches have shown that the atom is not truly indivisible. The atom is made up of even small particles called electrons, protons and neutrons.

The discovery of isotope indicated that all atoms of the same element are not perfectly identical. At least, they differ in their masses.

Atoms of the same element having different masses are called isotopes.

What is an atom?

An atom is the smallest particle of an element that take part in chemical reactions and maintains its chemical identity throughout all chemical and physical changes.

Atoms of an element are different from those of any other element.

Free atoms, except those of noble gases, do not exist under normal conditions.

How big are atoms?

Atoms are building blocks of matter. Atoms are very small in size. Hydrogen atom is the smallest. The radius of hydrogen atom is nearly 0.1 nm or 10^-10 m.

The smallness of hydrogen atom (or atoms in general) can be seen when compared the size of some common objects.

Atoms are so small that these cannot be seen even by the most powerful microscope.

What is the definition of an atomic symbol?

All the elements can be described either by one or by two letters. Therefore, instead of writing the full name of an element, we can simply write a letter or two. Thus, the symbol of an element is an abbreviation for the full name of the element. It may also be defined as, the symbol of an element is a shorthand notation for its name.

Symbol of an element consists of one or two letter derived from the Common or Latin or Greek name of the element. For example, symbol of hydrogen is H that of helium is He.

What is the significance of a symbol?

The symbol of an element has both qualitative as well as quantitative significance. These are mentioned below:

• The symbol stands for the name of the element.
• The symbol stands for one atom of the element.
• The symbol represents quantity of the element equal in mass to atomic mass or gram atomic mass.
• The symbol also represents mass of the element which contains one Avogadro’s number of atoms of that element.

For example, the symbol O stands for:

• The element oxygen
• One atom of oxygen
• Mass of oxygen equal to its gram atomic mass, i.e., O represents 16 g of oxygen.
• The mass of oxygen which contain one Avogadro’s number of oxygen atoms.

What is atomic mass unit?

Atomic masses are very small. To express atomic masses, a unit called atomic mass unit (amu or u) is commonly used. The atomic mass unit is defined as follows:

The mass equal to 1/12th of the mass of a carbon (C-12) atom is called the atomic mass unit.

Is atomic mass unit a practical unit?

The atomic mass unit is not a practical unit, because it is equivalent to a very small mass. One atomic mass unit is equals to 1.6*10^-27 kg or 1.66*10^-24 g. Such small masses cannot be measured on any balance.

What is the atomic mass of an element?

The average mass of an atom of an element it called as its atomic mass. Atomic mass of an element is designated by A. Since atomic mass is actual mass, hence it has the unit of mass, i.e., gram, kilogram, atomic mass unit. The accepted unit for describing atomic mass is atomic mass unit. The average mass of an atom of any element in atomic mass unit is called its atomic mass. 

Atomic mass of an element, A = Average mass of an atom of that element in the atomic mass unit.

What is relative atomic mass of an element?

The relative atomic mass of an element is defined as the average relative mass of an atom of the element compared with an atom of carbon (C-12) taken as 12 u. Thus,

The relative atomic mass is denoted by A_r. The relative atomic mass is a pure number and hence it has no units. The relative atomic mass of an element indicates that number of time one atom of that element is heavier than 1/12th of carbon (C-12) atom. For example, the average relative atomic mass of chlorine is 35.5. This means that an atom of chlorine on average is 35.5/12 times heavier than one atom of C-12.

Molecules and their chemical formula

What is a molecule?

A molecule may be defined as, “the smallest particle of an element or of a compound which can exist freely under ordinary conditions, and shows all the properties of that substance (element or compound).

A molecule may contain atoms of the same or different elements. Each molecule is described by its chemical formula.

What are chemical formulae?

Each chemical compound is known by a specific name. Writing the full name of a compound repeatedly is time consuming and inconvenient. That’s why, in chemistry, each substance is denoted by its chemical formula.

There are two types of chemical formula:

1. Molecular formula
2. Empirical formula or stoichiometric formula

What is a molecular formula?

A molecular formula contains two or more chemically bonded atoms of the same or different type. Therefore, a molecule can be represented in terms of the symbol and number of atom of the elements present in it.

A shorthand notation of a molecule in terms of symbols and the number of atoms of each element present in it is called molecular formula.

The molecular formula of a substance may also be defined as follows:

The symbolic representation of a molecule of any substance representing the actual number of various atoms present in it is called its molecular formula. For example, the molecular formula of water is H₂O. Thus, one molecule of water (H₂O) contain two atom of hydrogen and one atom of oxygen.

Molecular formula may also be defined as, a shorthand notation for the molecule of a substance in terms of symbols and number of atoms of each element present in it is called its molecular formula.

What is an empirical formula?

In case of ionic compounds (e.g., sodium chloride) and giant covalent compounds (e.g., diamond) having network structure, there is no discrete molecule. So, it is meaningless to talk of a molecule and molecular formula for such substances.

An ionic or a giant network covalent compound is described by its empirical formula.

Empirical formula of a substance is the simplest formula which gives the lowest whole number ratio between the number of atoms of different elements present in that substance.

For example,

• Crystalline or solid sodium chloride is a three dimensional structure containing sodium (Na⁺) and chloride (Cl^–) ions. So ,sodium chloride may be represented by the formula (Na⁺Cl^–)n, where n is a large number. The actual number of Na⁺ and Cl^– ion in sample of sodium chloride depends upon the size of the sample. But, in all samples, either small or big, the ratio between the number of Na⁺ ions and Cl^– ions is always 1:1. So, the simplest formula for sodium chloride is Na⁺Cl^– or only NaCl. Therefore, the empirical formula of sodium chloride is NaCl.
• A covalent compound, called benzene, has a molecular formula C₆H₆. There are 6 carbon atoms and 6 hydrogen atom in its molecule. The lowest whole number ratio between the number of carbon and hydrogen atom is 1:1. Therefore, the empirical formula of compound having molecular formula of C₆H₆ is CH.

What is meant by the atomicity of a molecule?

Molecules are generally classified in terms of number of atoms present in it. For example,

• The molecules of the gases like hydrogen, oxygen, nitrogen, etc., containing two atoms. Therefore, the molecule of hydrogen (H₂), oxygen (O₂) and nitrogen (N₂) are termed as diatomic molecule.
• The molecules of water (H₂O) and carbon dioxide (CO₂) contain three atoms each. Therefore, these are termed as triatomic molecules.
• The molecules of noble gases contain only one atom. So, noble gases are monoatomic, i.e., noble gas exist in the atomic form, e.g., He, Ne, Ar, Kr. 

The number of atoms of all the element in a molecule of any substance, element or compound, is called its atomicity.

Molecules of elements

Different elements exist in different states depending on the reactivity or electronic configuration. For this purpose the elements can be divided into following categories:

• Molecules of elements which do not combine: Many elements such as noble gas helium, neon, argon, krypton, etc., exists as atoms. Thus, the molecules of noble gases contain only one atom of the respective element, and hence, the atomicity of noble gases 1. So, we can say that the molecules of noble gases are monoatomic. Such molecules are presented by their atomic symbol, i.e., the molecular formula of argon is Ar.
• Molecules of element which combine to form discrete molecules: The atoms of most non-metallic elements combine to form discrete molecules. Such molecules contain two or more atoms of the element, and therefore, have the atomicity of two or more than two. For example,

1. Gases like hydrogen, nitrogen, oxygen, chlorine from diatomic molecules. Thus, the molecules of hydrogen, nitrogen, oxygen and chlorine are diatomic molecules, i.e., the atomicity of hydrogen (H₂), nitrogen (N₂), oxygen (O₂) and chlorine (Cl₂) is 2.
2. Yellow phosphorus forms a modular containing four Phosphorus atoms. Thus, the molecule of yellow phosphorus (P₄) is a tetraatomic molecule, i.e., the atomicity of yellow phosphorus is 4.
3. A molecule of sulphur contain 8 atom of sulphur forming a puckered ring structure. Thus, the molecules of sulphur (S₈) is an octaatomic molecule.

• Molecule of element which form network or giant molecules: The elements such as carbon form network structure. Such a structure consists of a very large number of atoms bonded together in a particular way. For example, in diamond and graphite, a very large number of carbon atom are bonded to each other. So, diamond and graphite may be represented by the formula Cn, and where n may be a number ranging from a few hundreds to millions, depending on the size of the sample. The molecule of diamond and graphite are described by the empirical formula or atomic symbol C.
• Molecules of metallic elements (or metals): In metals, atoms of element are packed together. Thus, atoms of metallic elements form large aggregates. There is no discrete molecule formed in such cases also. So, the metallic elements are also described by their atomic symbol or empirical formula. 

Molecules of compounds

Elements combine with other elements in definite proportions to form compounds. These compounds may be of different types. Here, we consider the formation of simple covalent compounds from the constituent elements. During compound formation the constituent elements combine in a definite proportion.

Ions and Ionic Compounds 

What is an ion?

The charged species obtained when an atom loses or gain electrons is called an ion.

How is an ion formed?

When an atom gain or lose one or more electrons, an ion is formed. Positively charged ion is called cation and negatively charged ion is called anion.

• A cation, positively charged ion, is formed when an atom loses one or more electrons, e.g., when sodium atom loses an electron sodium ions formed. Sodium ion is positively charged, so it is a cation.
• An anion, negatively charged ion, is formed when an atom gains one or more electron, e.g., when chlorine atom gain one electron, a chloride ion is formed. Chloride ion is negatively charged, so it is called as anion.

Are the properties of an atom and an ion similar?

No, the properties of an atom of any element are quite different from those of its ion.

Ion is formed due to loss or gain of electrons from an atom. In doing so, each atom acquire an electronic configuration of the nearest noble gas. As a result, the properties of ions are different from those of parent element.

For example, chlorine gas is poisonous but chloride ion is not poisonous.

Ions differ from the corresponding atom as follows:

Property	Atom	Ion
Nature of charge	Atom is electrically neutral.	Ions are electrically charged. Ions given by metals carry positive, while those by non-metals carry negative charges.
Number of protons and electrons	An atom contains equal number of protons and electrons.	An ion contains unequal number of protons and electrons. The cations have lesser electrons than protons while the anions have more electrons than the electrons.
Size	Definite size.	For any element, the cation is smaller than the atoms, and the anion is bigger than the corresponding atom.

How are ionic compounds formed?

An ionic compound is formed when a metallic element reacts with a non metallic element.

The atom of any metallic element tend to lose electrons to form positively charged ions called cations.

The atom of any non metallic element tend to gain electrons to form a negatively charged ions called anions.

These positively charged and negatively charged ions combine to form an ionic compound.

Na → Na⁺ + e^–

Cl + e^– → Cl^–

Na⁺ + Cl^– → NaCl

Formation of sodium chloride from sodium and chlorine

When sodium metal reacts with chlorine gas, an ionic compound sodium chloride is formed.

During this reaction a sodium atom loses one electron from its outermost shell and the chlorine atom gains one electron. Actually, in this reaction, one electron is transferred from sodium atom to chlorine atom.

During the formation of an ionic compound, the two reacting elements combined in a definite proportion by mass and also by number of atoms. For example,

• In calcium oxide (CaO), calcium and oxygen are in the ratio 5:2 by mass and 1:1 by the number of atoms.
• In magnesium sulphide (MgS), magnesium and sulphur are in the ratio 3:4 by mass and 1:1 by number of atoms.
• In sodium chloride (NaCl), sodium and chlorine are in the ratio 23:35.5 by mass and 1:1 by number of atoms.

Valence electrons and Valency

In an atom, electrons revolve around the positive charged core, called the nucleus. These electrons are distributed in various shells around the nucleus.

What are valence electrons?

The electrons present in the outermost shell, generally termed as valence shell, are called valence electrons.

For example,

Valence electrons of H is 1, of O are 6, etc

What is valency?

The combining capacity of an element is described in terms of its valency.

The number of hydrogen or chlorine or double the number of oxygen atoms, which combine with one atom of the element is termed is its valency. For example, in NaCl the valency of Na is 1, because one sodium atom combine with one chlorine atom.

In terms of electronic structure of atom, valency may also be defined as the number of electrons which an atom loses, gains or shares with other atoms to attain the nearest noble gas configuration.

The elements which have valency of one are called monovalent. Those having a valency of two are called as divalent. Those having a valency of three are called as trivalent and those showing a valency of four are called as tetravalent.

Read More: Complete Beginner’s Guide to Mole Concept

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