L EARNING O UTCOMES Write formulae to represent ions and molecules Write balanced equations...

LEARNING OUTCOMES

Write formulae to represent ions and moleculesWrite balanced equations including state symbols to represent chemical reactions referred to in the syllabus

Chapter 4

Elements and Compounds

An element When mercuric oxide is heated, it

decomposes into oxygen and mercury.

However, both oxygen and mercury cannot be broken down into anything simpler.

Oxygen and mercury are examples of elements.

An element is a substance which cannot be broken down into any simpler substances by chemical means.

Elements and CompoundsChapter 4

Elements Elements are the fundamental building blocks of matter

in our universe. There are about 92 natural elements and more than 10

man-made elements. Each element has a name and a chemical symbol. A list of elements with their symbols is given in the

Periodic Table.

The Periodic Table of the Elements

http://www.chemicool.com/





Atoms of elements An element is made up of only

one kind of atom. For example, gold is made up of

only gold atoms. Oxygen is made up of molecules

each consisting of two oxygen atoms, while ozone is made up of molecules each containing three oxygen atoms.


Metals and non-metals Elements can be classified into metals and

non-metals. Metals are usually hard and shiny. They

are malleable and ductile and are good conductors of heat and electricity.

Non-metals are usually soft and brittle, and are poor conductors of heat and electricity.

There are more metals than non-metals.

Copper: a metal

Sulphur: a non- metal


Quick check 11. “Magnesium is an element”. Explain what this statement

means. 2. “A piece of copper can be broken down into very tiny

pieces, hence copper is not an element.” Explain what is wrong with this statement.

3. Give the symbol for each of the following elements. State whether it is a metal or non-metal.(a) Mercury, (b) Lead, (c) Silver, (d) Chlorine, (e) Strontium, (f) Tungsten.

SolutionSolution


Solution to Quick Check 1

1. Magnesium is an element because it cannot be broken down into simpler substances. Magnesium is made up of magnesium atoms and nothing else.

2. A piece of copper can be broken down into very tiny pieces but each tiny piece of copper is still made up of only copper atoms, hence copper is an element.

3. (a) Mercury: Hg (metal), (b) Lead: Pb (metal), (c) Silver: Ag (metal), (d) Chlorine: Cl (non-metal), (e) Strontium: Sr (metal), (f) tungsten: W (metal).

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A Compound

A compound is a substance made up of two or more elements chemically combined together.

Mercuric oxide is not an element because it is made up of mercury and oxygen.

It is called a compound.

Element + Element Compound


Atoms of compounds A compound is made up of molecules.

A molecule of a compound is made up of two or more different types of atoms chemically joined together.

water molecules carbon dioxide molecules

methane molecules


Formulae of compounds As a compound has a fixed composition,

it can be represented by a formula. The formula of a compound shows: the symbols of the elements present the ratio of the atoms present For example, water has the formula H2O. H2O

shows 2 hydrogen atoms and 1 oxygen atom


Formulae of some compoundsCompound Formula Ratio of atoms

Carbon dioxide CO2 1 carbon atom with 2 oxygen atoms

Methane CH4 1 carbon atom with 4 hydrogen atoms

Sulphuric acid H2SO4 2 hydrogen atoms with 1 sulphur atom and 4 oxygen atoms

Magnesium hydroxide Mg(OH)2 1 magnesium atom with 2 oxygen atoms and 2 hydrogen atoms


To help us write chemical formulae more easily, we can use the valency of an element.

The valency of an element can be treated as the “combining power” of an element.

The valency of an element is related to the electronic structure of the atom and the Group number of the element in the Periodic Table.

Valency of an element


Valencies of some common elements (metals)Metal Valency Symbol (cation)

(Hydrogen) 1 H+

Sodium 1 Na+

Potassium 1 K+

Copper(I) 1 Cu+

Calcium 2 Ca2+

Magnesium 2 Mg2+

Zinc 2 Zn2+

Copper(II) 2 Cu2+

Iron(II) 2 Fe2+

Aluminium 3 Al3+

Iron(III) 3 Fe3+


Valencies of some common elements (non-metals)

Non-metal Valency Symbol (anion)

Chlorine 1 Cl−

Bromine 1 Br−

Iodine 1 I−

Oxygen 2 O2−

Sulphur 2 S2−

Nitrogen 3 N3−


Valencies of some group atoms

Name Valency Symbol

Ammonium 1 NH4+

Hydroxide 1 OH−

Nitrate 1 NO3−

Hydrogen carbonate 1 HCO3−

Sulphate 2 SO42−

Carbonate 2 CO32−


Metals (or cations) can react with non-metals (or anions). When a formula is formed, the positive charges of the cation

have to be balanced by the negative charges of the anion. The total charge of a neutral compound must be zero. Examples:

1. Sodium chloride: Na+ + Cl− NaCl2. Copper(II) oxide: Cu2+ + O2− MgO

Writing formulae


Example 3: magnesium chlorideMg2+ + (Cl−)x 2 MgCl2

Example 4: aluminium oxide(Al3+)x2 + (O2−)x3 Al2O3

Example 5: ammonium sulphate(NH4

+)x2 + (SO42−) (NH4)2SO4

Writing formula


Quick check 3Write the formula for each of the following compounds:

Name FormulaSodium bromide

Potassium sulphate

Calcium hydrogen carbonate

Magnesium nitrate

Copper(I) oxide

Copper(II) carbonate

Iron(II) chloride

Iron(III) chloride

Aluminium sulphate Solution


Solution to Quick check 3

Name FormulaSodium bromide NaBr

Potassium sulphate K2SO4

Calcium hydrogen carbonate Ca(HCO3)2

Magnesium nitrate Mg(NO3)2

Copper(I) oxide Cu2O

Copper(II) carbonate CuCO3

Iron(II) chloride FeCl2Iron(III) chloride FeCl3Aluminium sulphate Al2(SO4)3

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Writing chemical equations

A chemical equation tells us what chemical changes take place during a reaction.

It tells us what the reactants (things that react) and what the products (things that are formed) are.

A chemical equation must be balanced. This means that the total number and types of atoms on the

right side of the equation must be equal to those on the left side of the equation. This is because atoms cannot be created or destroyed.

Mercury + oxygen mercuric oxide[ Reactants ] [ product ]

Word equation


Writing chemical equations

Step 1: Write down the chemical formula for each reactant and product:

Mercury + oxygen mercuric oxide

Hg + O2 HgO

Step 2: Count the number of atoms on each side of the equation:

Left side: 1 Hg atom + 2 O atoms

Right side: 1 Hg atom + 1 O atom


Balancing chemical equationsThe equation is not balanced because the right side has 1 less oxygen atom.

Step 3: To balance the equation, add 2 in front of HgO, and again count the number of atoms on both sides of the equation:

Left side: 1 Hg atom + 2 O atomsRight side: 2 Hg atoms + 2 O atoms

Hg + O2 2 HgO

The equation is still not balanced because the left side has 1 less mercury atom.


Writing chemical equationsStep 4: To balance the equation, add 2 in front of Hg,

and again count the number of atoms on both sides of the equation:

Left side: 2 Hg atom + 2 O atomsRight side: 2 Hg atoms + 2 O atoms

2 Hg + O2 2 HgO

The equation is now correctly balanced.

2 Hg + O2 2 HgO


State symbols in chemical equations The state symbols tell us the physical states of the reactants

and products in a chemical reaction. (s) solid state (l) liquid state (g) gaseous state (aq) aqueous state (solution in water)

HCl(aq) + CaCO3(s) CaCl2(aq) + H2O(l) + CO2(g)

E.g. Write a balanced chemical equation, with state symbols, for the reaction between dilute hydrochloric acid and limestone chips (calcium carbonate).


2

Quick check 41. Balance the following chemical equations by writing numbers in the blank

spaces provided:

a) __ Ca + __ O2 __ CaO

b) __ N2 + __ H2 __ NH3

c) __ C2H4 + __ O2 __ CO2 + __ H2O

d) __ Mg + __ Fe2O3 __ MgO + __ Fe

e) __ NH3 + __ O2 __ N2 + __ H2O

f) __ Pb(NO3)2 __ PbO + __ NO2 + __ O2

g) __ KClO3 __ KCl + __ O2

h) __ CaCl2 + __ Na2CO3 __ CaCO3 + __ NaCl Solution


2. Write balanced chemical equations with state symbols for the following word equations:

a) Magnesium + Oxygen Magnesium oxide

b) Hydrogen + Oxygen Water

c) Mercuric(II) oxide Mercury + Oxygen

d) Sodium + Oxygen Sodium oxide

e) Ammonia + Sulphuric acid Ammonium sulphate

f) Ammonium chloride + Sodium hydroxide Sodium chloride + water + ammonia

g) Zinc + Hydrochloric acid Zinc chloride + hydrogen

Solution

Elements and Compounds

Quick check 4

Chapter 4

Solution to Quick check 4

Q1.

a) 2_Ca + __ O2 2_ CaO

b) __ N2 + 3_H2 2_ NH3

c) __ C2H4 + 3_O2 2_CO2 + 2_H2O

d) 3 Mg + __ Fe2O3 3_ MgO + 2_ Fe

e) 4_NH3 + 3_O2 2_N2 + 6 H2O

f) 2_ Pb(NO3)2 2_PbO + 4_ NO2 + __ O2

g) 2_ KClO3 2_KCl + 3_O2

h) __ CaCl2 + __ Na2CO3 __ CaCO3 + 2_NaCl Return


a) 2Mg(s) + O2(g) 2MgO(s)

b) 2H2(g) + O2(g) 2H2O(l)

c) 2HgO(s) 2Hg(l) + O2(g)

d) 4Na(s) + O2(g) 2Na2O(s)

e) 2NH3(g) + H2SO4(aq) (NH4)2SO4(aq)

f) NH4Cl(s) + NaOH(aq) NaCl(aq) + H2O(l) + NH3(g)

g) Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)Return

Elements and CompoundsSolution to Quick check 4

Chapter 4

Q2.

L EARNING O UTCOMES Write formulae to represent ions and molecules Write balanced equations...

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