ICSE Class 9 Chemistry Chapter 04 The Language of Chemistry

Chapter 4

The Language Of Chemistry

Latest Syllabus - Scope Of Syllabus - The Language Of Chemistry

The language of Chemistry

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 of radicals; formula and valencies of the radicals and formula of compounds.

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 partial equation method and hit and trial method].

A. Symbols

The Term

A symbol represents - A short form of an element.

The qualitative and quantitative meaning of the term - 'symbol'.

A symbol represents - a specific element or one atom of an element. e.g. 'S' represents one atom of the element sulphur.

A symbol also represents - the weight of the element equal to its atomic weight [ie. it represents how many times an atom is heavier than one atomic mass unit [a.m.u.] which is defined as \(\frac{1}{12}\)th the mass of a carbon atom C\(^{12}\)].

Introduction To Concept Of A Symbol - Symbols Of Elements Used Often

In 1807 - John Dalton suggested - figurative symbols for atoms of elements.

The above method was cumbersome and later abandoned.

In 1814 - Berzelius suggested - a simple system of representing elements with symbols. He put forward certain points for presentation.

In most cases the first letter of the name of an element - was taken as the symbol for that element and written in capitals.

C carbon

S ulphur

Ca lcium

Si licon

In cases where the first letter is already adopted - the initial letter in capital together with its small letter from its name were used.

The symbols for some metals were derived from their - Latin names.

Copper - Cuprum [Cu]

Iron - Ferrum [Fe]

Potassium - Kalium [K]

Gold - Aurum [Au]

Silver - Argentum [Ag]

Lead - Plumbum [Pb]

Sodium - Natrium [Na]

Mercury - Hydrargyrum [Hg]

Teacher's Note

Understanding chemical symbols is like learning a universal language - just as "Au" instantly tells chemists worldwide about gold, symbols help us communicate chemical information efficiently across borders.

B. Valency, Radicals And Chemical Formula

Valency

The number of hydrogen atoms which can combine with or displace - one atom of the element or radical so as to form a compound.

The valency of all metals and hydrogen - is considered positive.

The valency of all non-metals/groups of non-metals- is taken as negative.

Valency of a metal is the number of electrons - lost per atom of the metal.

K - 1e\(^-\) \(\to\) K\(^{1+}\) univalent; Mg - 2e\(^-\) \(\to\) Mg\(^{2+}\)divalent; Al - 3e\(^-\) \(\to\) Al\(^{3+}\)trivalent

Valency of a non-metal is the number of electrons - gained per atom of the non-metal.

Cl + 1e\(^-\) \(\to\) Cl\(^{1-}\) univalent; S + 2e\(^-\) \(\to\) S\(^{2-}\) divalent; N + 3e\(^-\) \(\to\) N\(^{3-}\)trivalent

Variable Valency

Certain metals exhibit more than one valency hence showing - variable valency.

Reasons for exhibiting variable valency - An atom of an element can sometimes lose more electrons than are present in its- valence shell i.e. loss from penultimate [i.e. last but one] shell and - hence exhibit - more than one or - variable valency.

e.g. Cuprous [Cu\(^{1+}\)] & Cupric [Cu\(^{2+}\)].

[Cu = 29 (2,8,18,1) The outermost shell has one electron & the penultimate shell contains 8 electrons. However the penultimate shell has not attained stability and one or more electrons sometimes jumps to the outermost shell thereby increasing the valency electron. Hence the atom has new configuration (2,8,17,2) ] Therefore copper exhibits Cu\(^{1+}\) & Cu\(^{2+}\) valency.

Radicals

A radical is a group of atoms of elements that behaves like a single unit & shows a valency. Positive radical - Ammonium [NH\(_4\)\(^{1+}\)]. Negative radical - Hydroxide [OH\(^{1-}\)]

Chemical Formula

Chemical formula - A molecule of a substance i.e. element or compound - could be represented by symbols. Representation known as 'chemical formula'.

An element is referred to as monatomic when it exists as a single atom & represented by a single symbol of that element.

Sodium [Na]

Potassium [K]

Hydrogen [H\(_2\)]

Oxygen [O\(_2\)]

When an element exists as a molecule a number representing its atomicity is written as a subscript after the symbol of the element, thereby representing its formula.

A compound contains whole number of atoms of the component elements, indicating that the elements are in chemical combination, thereby representing its molecular formula.

1 atom of calcium & carbon 3 atoms of oxygen Calcium Carbonate [CaCO\(_3\)]

A molecular formula also indicates the number of units of the radicals present in a compound with the proper subscript outside the unit of the radical, e.g. 2 outside Ca(OH)\(_2\)

1 atom of calcium 2 Hydroxide radicals Calcium Hydroxide [Ca(OH)\(_2\)]

Teacher's Note

Valency and radicals are the "grammar rules" of chemistry - just as grammar helps us construct meaningful sentences, understanding valency helps us write correct chemical formulas that represent real compounds.

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