Chapter 4

Elements and Compounds
LEARNING OUTCOMES
Write formulae to represent ions and molecules
Write balanced equati...
Chapter 4

Elements and Compounds
An element
When mercuric oxide is heated, it
decomposes into oxygen and
mercury.
 Howev...
Chapter 4

Elements and Compounds
Elements
Elements are the fundamental building blocks of matter
in our universe.
 There...
Chapter 4

Elements and Compounds
Atoms of elements
An element is made up of only
one kind of atom.
 For example, gold is...
Chapter 4

Elements and Compounds
Metals and non-metals
Elements can be classified into metals and
non-metals.
 Metals ar...
Chapter 4

Elements and Compounds
Quick check 1
1.
2.

3.

“Magnesium is an element”. Explain what this statement
means.
“...
Chapter 4

Elements and Compounds
Solution to Quick Check 1
1.

2.

3.

Magnesium is an element because it cannot be broke...
Chapter 4

Elements and Compounds
A Compound
 Mercuric oxide is not an element because it is made

up of mercury and oxyg...
Chapter 4

Elements and Compounds
Atoms of compounds
 A compound is made up of molecules.
 A molecule of a compound is m...
Chapter 4

Elements and Compounds
Formulae of compounds
As a compound has a fixed composition,
it can be represented by a ...
Chapter 4

Elements and Compounds
Formulae of some compounds
Compound

Formula

Ratio of atoms

Carbon dioxide

CO2

1 car...
Chapter 4

Elements and Compounds
Valency of an element






To help us write chemical formulae more easily, we can
us...
Chapter 4

Elements and Compounds
Valencies of some common elements (metals)
Metal

Valency

Symbol (cation)

(Hydrogen)

...
Chapter 4

Elements and Compounds
Valencies of some common elements
(non-metals)
Non-metal

Valency

Symbol (anion)

Chlor...
Chapter 4

Elements and Compounds
Valencies of some group atoms
Name

Valency

Symbol

Ammonium

1

NH4+

Hydroxide

1

OH...
Chapter 4

Elements and Compounds
Writing formulae






Metals (or cations) can react with non-metals (or anions).
Wh...
Chapter 4

Elements and Compounds
Writing formula


Example 3: magnesium chloride
Mg2+ + (Cl−)x 2  MgCl2



Example 4: ...
Chapter 4

Elements and Compounds
Quick check 3
Write the formula for each of the following compounds:
Name

Formula

Sodi...
Chapter 4

Elements and Compounds
Solution to Quick check 3
Name

Formula

Sodium bromide

NaBr

Potassium sulphate

K2SO4...
Chapter 4

Elements and Compounds
Writing chemical equations
Word equation

Mercury + oxygen  mercuric oxide
[ Reactants ...
Chapter 4

Elements and Compounds
Writing chemical equations
Mercury + oxygen  mercuric oxide

Step 1: Write down the che...
Chapter 4

Elements and Compounds
Balancing chemical equations
The equation is not balanced because the right side has
1 l...
Chapter 4

Elements and Compounds
Writing chemical equations
Step 4: To balance the equation, add 2 in front of Hg,
and ag...
Chapter 4

Elements and Compounds
State symbols in chemical equations






The state symbols tell us the physical st...
Chapter 4

Elements and Compounds
Quick check 4
1.

Balance the following chemical equations by writing
numbers in the bla...
Chapter 4

Elements and Compounds
Quick check 4
2.

Write balanced chemical equations with state symbols
for the following...
Chapter 4

Elements and Compounds
Solution to Quick check 4
Q1.
a) 2_Ca + __ O2  2_ CaO
b) __ N2 + 3_H2  2_ NH3
c) __ C2...
Chapter 4

Elements and Compounds
Solution to Quick check 4
Q2.
a) 2Mg(s) + O2(g) 2MgO(s)
b) 2H2(g) + O2(g)  2H2O(l)
c) ...
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C04 elements and compounds

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C04 elements and compounds

  1. 1. Chapter 4 Elements and Compounds LEARNING OUTCOMES Write formulae to represent ions and molecules Write balanced equations including state symbols to represent chemical reactions referred to in the syllabus
  2. 2. 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.
  3. 3. Chapter 4 Elements and Compounds 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/
  4. 4. Chapter 4 Elements and Compounds 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. 
  5. 5. Chapter 4 Elements and Compounds 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
  6. 6. Chapter 4 Elements and Compounds Quick check 1 1. 2. 3. “Magnesium is an element”. Explain what this statement means. “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. 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. Solution
  7. 7. Chapter 4 Elements and Compounds Solution to Quick Check 1 1. 2. 3. Magnesium is an element because it cannot be broken down into simpler substances. Magnesium is made up of magnesium atoms and nothing else. 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. (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). Return
  8. 8. Chapter 4 Elements and Compounds A Compound  Mercuric oxide is not an element because it is made up of mercury and oxygen.  It is called a compound. A compound is a substance made up of two or more elements chemically combined together. Element + Element Compound
  9. 9. Chapter 4 Elements and Compounds 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
  10. 10. Chapter 4 Elements and Compounds 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
  11. 11. Chapter 4 Elements and Compounds Formulae of some compounds Compound 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 Magnesium hydroxide H2SO4 2 hydrogen atoms with 1 sulphur atom and 4 oxygen atoms Mg(OH)2 1 magnesium atom with 2 oxygen atoms and 2 hydrogen atoms
  12. 12. Chapter 4 Elements and Compounds Valency of an element    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.
  13. 13. Chapter 4 Elements and Compounds 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+
  14. 14. Chapter 4 Elements and Compounds 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−
  15. 15. Chapter 4 Elements and Compounds 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−
  16. 16. Chapter 4 Elements and Compounds Writing formulae     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−  NaCl 2. Copper(II) oxide: Cu2+ + O2−  CuO
  17. 17. Chapter 4 Elements and Compounds Writing formula  Example 3: magnesium chloride Mg2+ + (Cl−)x 2  MgCl2  Example 4: aluminium oxide (Al3+)x2 + (O2−)x3  Al2O3  Example 5: ammonium sulphate (NH4+)x2 + (SO42−)  (NH4)2SO4
  18. 18. Chapter 4 Elements and Compounds Quick check 3 Write the formula for each of the following compounds: Name Formula Sodium bromide Potassium sulphate Calcium hydrogen carbonate Magnesium nitrate Copper(I) oxide Copper(II) carbonate Iron(II) chloride Iron(III) chloride Aluminium sulphate Solution
  19. 19. Chapter 4 Elements and Compounds Solution to Quick check 3 Name Formula Sodium 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 FeCl2 Iron(III) chloride FeCl3 Aluminium sulphate Al2(SO4)3 Return
  20. 20. Chapter 4 Elements and Compounds Writing chemical equations Word equation Mercury + oxygen  mercuric oxide [ Reactants ]     [ product ] 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.
  21. 21. Chapter 4 Elements and Compounds Writing chemical equations Mercury + oxygen  mercuric oxide Step 1: Write down the chemical formula for each reactant and product: 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
  22. 22. Chapter 4 Elements and Compounds Balancing chemical equations The 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: Hg + O2  2 HgO Left side: 1 Hg atom + 2 O atoms Right side: 2 Hg atoms + 2 O atoms The equation is still not balanced because the left side has 1 less mercury atom.
  23. 23. Chapter 4 Elements and Compounds Writing chemical equations Step 4: To balance the equation, add 2 in front of Hg, and again count the number of atoms on both sides of the equation: 2 Hg + O2  2 HgO Left side: 2 Hg atom + 2 O atoms Right side: 2 Hg atoms + 2 O atoms The equation is now correctly balanced. 2 Hg + O2  2 HgO
  24. 24. Chapter 4 Elements and Compounds 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) E.g. Write a balanced chemical equation, with state symbols, for the reaction between dilute hydrochloric acid and limestone chips (calcium carbonate). 2 HCl(aq) + CaCO3(s)  CaCl2(aq) + H2O(l) + CO2(g)
  25. 25. Chapter 4 Elements and Compounds Quick check 4 1. 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
  26. 26. Chapter 4 Elements and Compounds Quick check 4 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
  27. 27. Chapter 4 Elements and Compounds 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
  28. 28. Chapter 4 Elements and Compounds Solution to Quick check 4 Q2. 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

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