Elements, Mixtures & Compounds

Syllabus objectives:

(a) Describe the differences between elements, compounds and mixtures

(b) State the symbols of the elements and formulae of the compounds mentioned in the syllabus.

(b) Deduce the formulae of simple compounds from the relative numbers of atoms present and vice versa.

Types of particles

ParticlesParticles

Molecules

IonsAtoms

Atoms and Molecules

• Matter is made up of tiny particles. These particles are called atoms.

• Atoms are the smallest particle of an element that can still have the chemical properties of that element.

• A molecule is made up of two or more atoms chemically joined together by covalent bond.(Can a molecule be joined together by ionic bonds?)

• Some substances exist as atoms while others exist as molecules.

Atoms and Molecules

• Examples of substances that exist as atoms• All metals e.g. Gold (Au), Copper (Cu)• Noble gases e.g. helium (He), neon (Ne)

E.g. helium

Atoms and Molecules

• Examples of substances that exist as molecules• Diatomic molecules eg.

• oxygen (O2)• hydrogen(H2)• hydrogen chloride (HCl)

• Tri-atomic molecules eg. • water (H2O),• Carbon dioxide (CO2)

Atoms and Molecules Atoms of the same element that are chemically

joined together form molecules of elements. Atoms of different elements that are chemically

joined together form molecules of compounds.

element compound elementcompound

Nitrogen gas (N2)Bromine liquid (Br2)

These areELEMENTS !

Examples of substances that exist as molecules :-

Hydrogen gas (H2) Oxygen gas (O2) Chlorine gas (Cl2)

Examples of substances that exist as molecules :-

Carbon dioxide gas(CO2) Water(H2O) Ammonia gas (NH3)

Methane gas(CH4)

These are COMPOUNDS !

Elements

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

• Elements contain only one type of atom.• Egs. Aluminium, carbon, sulfur

Elements• Elements are represented by symbolsExamples..

Al C Cu Hg S

Aluminium Carbon Copper Mercury Sulfur

Metals & Non-metals• Elements can be classified as metals or non-metals

Metals Non-metals

• Shiny• Solids at r.t.p. (except Hg)• Malleable and ductile• High melting pt. & boiling pt.• Good conductors of heat• Good conductors of

electricity

• dull• Either gases, liquids or

solids at r.t.p. • brittle• low melting pt. & boiling pt.

(except C)• Poor conductors of heat• Poor conductors of

electricity (except graphite)

Compounds

• A compound is a substance which is made up of 2 or more elements chemically combined together.

• Compounds contain only one type of molecule

• E.g.s. Water (H2O), common salt (NaCl), carbon dioxide (CO2)

Compounds

• A chemical reaction takes place when a compound is formed.

Compounds

oxygen (

(silvery solid)a bright white light;

ELEMENTS COMPOUND

magnesium oxide colourlessgas)

magnesium magnesium burns with

• A chemical reaction takes place when a compound is formed.

Compounds

• A chemical reaction takes place when a compound is formed.

ELEMENTS COMPOUND

(colourlessgas)oxygen

gas)

lighted splint

heat, light water

hydrogencolourless

Mixtures

• A mixture contains 2 or more substances which can be easily separated by physical means.

• Mixtures contain two or more types of atoms or molecules (physical means

include : filtration, evaporation, distillation, chromatography etc.)

Representing atoms, molecules, elements, mixtures and compounds:

Representing atoms, molecules, elements, mixtures and compounds:

Representing atoms, molecules, elements, mixtures and compounds:

same properties as its components

unique physical and chemical propertiesproperties

can be easily separated by physical means

cannot be easily separated by physical meansseparation

a mixture of two elementsa compound of two elements

arrangement of atoms

variable melting and boiling points

fixed melting and boiling pointsmelting and

boiling points

variable composition by massfixed composition by masscomposition

MixtureMixtureSingle CompoundSingle Compound

mixtures vs compoundsmixtures vs compounds

Comparison between a single compound and a mixture:Comparison between a single compound and a mixture:

A good way to distinguish between a mixture and a compound is through using their melting and boiling points

A good way to distinguish between a mixture and a compound is through using their melting and boiling points

Molecular FormulaeMolecular Formulae

• With the presence of both metallic and non-metallic elements, write the metallic element first.

• E.g. CaO, NaCl, FeS.• Number of atoms is written as a subscript

except for “1”.• E.g. FeCl2 (not Fe1Cl2); K2Cr2O7 (not K2Cr2O7)• Write oxygen atom at the end of the

formula.E.g. H2O, CuO, MgCO3

How do we deduce the formulae?

• -ide: The compound has only 2 elements.

Sodium chloride (NaCl)

Iron (II) Sulfide (FeS)• -Hydroxide: The compound has OH- ion.

Sodium hydroxide (NaOH)

Ammonium hydroxide (NH4OH)

• -ate or -ite: The compound has element oxygen. The former one has more oxygen atoms than the latter one.

Sodium nitrate (NaNO3) & sodium nitrite (NaNO2)

Naming of compounds

Chemical Formula

provides information about• the type of element present in the

compound• the relative number of atoms

(ratio) present

• Eg. (NH4)2SO4 contains..

CuSO4.5H2O contains…