The Language Of Chemistry

Syllabus

(i) Symbol of an element; valency; formulae of radicals and formulae of compounds. Balancing of simple chemical equations.

Symbol - definition; symbols of the elements used often.

Valency - definition; hydrogen combination and number of valence electrons of the metals and non-metals; mono, di, tri and tetra valent elements.

Radicals - definition; formulae and valencies

Compounds - name and formulae.

Chemical equation - definition and examples of chemical equations with one reactant and two or three products, two reactants and one product, two reactants and two products and two reactants and three or four products; balancing of equations. (by hit and trial method).

(ii) Relative Atomic Masses (atomic weights) and Relative Molecular Masses (molecular weights): either - standard H atom or 1/12th of carbon 12 atom.

Definitions

Calculation of Relative Molecular Mass and percentage composition of a compound.

Introduction

Chemistry is a branch of science in which we study about matter, i.e. what is matter made up of, what is its nature, structure and what changes are observed when it is subjected to different conditions.

Many theories were proposed regarding matter which helped us to discover the real essence of matter, i.e. matter is nothing but a complete relationship between elements. An element is a simple and pure form of matter which cannot be decomposed into simpler substances. All elements are made of atoms, the smallest particles of an element which represent all the properties of an element. A set of atoms of the same type together form the molecule of the element. Molecules can be monoatomic, diatomic and even polyatomic.

Monoatomic molecules - elements having only one atom in their molecules, e.g. helium, neon and other inert gas molecules.

Diatomic molecules - elements having two atoms in their molecules, e.g. hydrogen (H₂), Oxygen (O₂), Nitrogen (N₂).

Tetratomic molecules - elements having four atoms in their molecules, e.g. phosphorus (P₄).

Octatomic molecules - elements having eight atoms in their molecules, e.g. sulphur (S₈).

Thus, we can say that a molecule is the smallest particle that has capability to exist independently. The molecule of an element exhibits all the properties of that element.

When atoms or molecules of different elements combine, they form the molecule of a compound. To name a few, sodium chloride (NaCl), water (H₂O), ammonia (NH₃) are molecules of compounds.

The names of elements and compounds are abbreviated by using certain symbols and formulae.

Before 1600 A.D., alchemists tried to represent the substances that they used for their experiments by different kinds of pictographic symbols, such as a triangle for the earth, a crescent for silver, etc. Dalton used some other types of symbols to represent elements, such as a circle [O] for an oxygen atom, a circle with a dot in its centre [O] for hydrogen, etc.

Later, Johann Berzelius suggested that the initial letter of an element written in capitals should represent that particular element, such as O for oxygen, H for hydrogen, C for carbon, and so on. But in some cases, the suggested symbol did not agree with the English name of the element. This was because some of the symbols were based on the Latin names of the elements, as shown in Table 1.1.

Chemical Symbols

A symbol is the short form that stands for the atom of a specific element or the abbreviations used for the names of elements.

Each element is denoted by a symbol, which is usually the first letter of its name in English or Latin, written in capital.

Example: Sulphur, an element, is denoted by the symbol 'S'. Similarly, hydrogen is denoted by the symbol 'H'.

However, when the first letter of more than one element is the same, the elements are denoted by two letters. The first letter is written in capital, while the second one is written in small.

Example: Carbon, cobalt, copper, calcium, cadmium, chromium, and chlorine are elements whose first letter is C. Therefore, only carbon is denoted by the symbol 'C'. Cobalt is denoted by two letters 'Co'. Copper is denoted by the two letters 'Cu' [taken from its Latin name cuprum]. Calcium by Ca, Cadmium by Cd, Chromium by Cr and Chlorine by Cl.

A symbol is not merely an abbreviation for the name of an element but also has a significance.

Significance of a symbol:

It represents:

(i) Name of the element

(ii) One atom of the element

(iii) Quantity of the element equal in mass to its atomic mass or gram atomic mass.

For example, the symbol N stands for

(i) The element Nitrogen

(ii) One atom of Nitrogen

(iii) 14 parts by weight of Nitrogen. This weight being the atomic weight of the element.

Note: Be careful about capital and small alphabets. For example: Co means the element cobalt. CO means the compound carbon monoxide.

Formula

Atoms of elements combine to form molecules. So it is possible to represent the molecules in terms of symbols of the constituent atoms. The symbolic representation of a molecule is known as formula or molecular formula.

A molecular formula also known as chemical formula employs symbols to denote the molecule of an element or of a compound.

A molecule of an element may contain one or more atoms of it. For example a molecule of elements hydrogen, oxygen, nitrogen, chlorine, bromine, iodine, contains two atoms and are written as H₂, O₂, N₂, Cl₂, Br₂, and I₂ respectively.

In case of a compound, the molecule containing different atoms united in certain fixed ratio, is represented by placing symbols of the elements present in it side by side indicating their numbers written in subscript. Thus, NH₄Cl represents one molecule of ammonium chloride containing one atom of nitrogen four atoms of hydrogen and one atom of chlorine.

Na₂CO₃ denotes one molecule of sodium carbonate which contains two atoms of sodium, one atom of carbon and three atoms of oxygen.

2H₂O represents two molecules of water i.e. dihydrogen oxide, each molecule containing two atoms of hydrogen and one atom of oxygen.

Thus by looking at a formula, we understand the ratio in which the different atoms are united to form the molecule.

Significance of molecular formula:

The molecular formula of a compound has quantitative significance. It represents:

(i) both the molecule and the molecular mass of the compound.

(ii) the respective numbers of different atoms present in one molecule of the compound.

(iii) the ratios of the respective masses of the elements present in the compound.

For example, the formula CO₂ means that:

(i) the molecular formula of carbon dioxide is CO₂;

(ii) each molecule contains one carbon atom joined by chemical bonds with two oxygen atoms;

(iii) the molecular mass of carbon dioxide is 44, given that the atomic mass of carbon is 12 and that of oxygen is 16.

Valency

Valency is the combining capacity of an atom or of a radical.

Valency is measured in terms of hydrogen atoms or oxygen atoms. It is the number of hydrogen atoms or double the number of oxygen atoms that can combine with it.

Since no other element has combining capacity less than that of hydrogen, its valency is taken to be one (1) and is considered a standard.

Modern Definition Of Valency

The number of electrons, that atom can lose, gain or share during a chemical reaction is called its valency. (For details refer to chapter Atomic Structure).

Elements with one, two or three electrons in their outermost shell (valence shell) are usually metals. Electrons in the outermost shell are known as valence electrons. To attain stable electronic configuration, these atoms lose their valence electrons and form positive ions.

Na - e⁻ - Na⁺ (univalent)

2, 8, 1 - 2, 8

Mg - 2e⁻ - Mg²⁺ (divalent)

2, 8, 2 - 2, 8

Al - 3e⁻ - Al³⁺ (trivalent)

2, 8, 3 - 2, 8

Positive ions are known as cations.

Elements with five, six or seven electrons in their outermost shell are normally non-metals. To attain stability, these atoms have to have eight electrons in their outermost orbit; so they gain electrons.

Note: Out of the elements that have four electrons in the outermost orbit - Carbon is a non-metal, Silicon and Germanium are metalloid and rest are metals.

Cl + e⁻ - Cl⁻ (univalent)

(2, 8, 7) - 2, 8, 8

O + 2e⁻ - O²⁻ (divalent)

(2, 6) - 2, 8

N + 3e⁻ - N³⁻ (trivalent)

(2, 5) - 2, 8

Negative ions are known as anions.

The valency of an element or of a radical is the number of hydrogen atoms that will combine with or displace one atom of that element or radical.

Examples of sharing of atoms:

(i) One atom of chlorine combines with one hydrogen atom to form a molecule of hydrogen chloride. So, the valency of chlorine is one.

H - Cl

(ii) One atom of oxygen combines with 2 atoms of hydrogen to form a molecule of water. So, the valency of oxygen is two.

H - O - H

(iii) In an ammonia molecule, one atom of nitrogen combines with 3 atoms of hydrogen. So, the valency of nitrogen is three.

H - N - H - H

(iv) In a methane molecule, one carbon atom combines with 4 hydrogen atoms. So, the valency of carbon is four.

H - C - H - H - H

Variable Valency

Certain elements exhibit more than one valency, i.e. they show variable valency.

Reasons For Variable Valency

An atom of an element can sometimes lose more electrons than are present in its valence shell, i.e. there is a loss of electrons from the penultimate shell too. Therefore, such an element is said to exhibit variable valency.

If an element exhibits two different positive valencies, then we use the suffix "ous" for the lower valency and the suffix "ic" for the higher valency. Modern chemists use Roman numerals in place of these trivial names. For example, SnCl₂, i.e. stannous chloride is written as Tin (II) chloride, SnCl₄, i.e. stannic chloride is written as Tin (IV) chloride.

Non-metals like nitrogen, phosphorus and sulphur also show variable valency. Nitrogen and phosphorus exhibit valencies of 3 and 5 while sulphur exhibits valencies of 2, 4 and 6.

Teacher's Note

Understanding valency helps explain why salt crystals have specific structures and why elements bond in predictable patterns - similar to how puzzle pieces only fit together in certain ways.

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