Topic 2Atoms, Elements and

Compounds

Learning Outcomes Identify how matter is classified based on its atoms. Differentiate between metals and non-metals. Write the chemical symbols for elements. Write the formula of molecules for elements and

compounds. Identify similarities and differences between

elements, compounds and mixtures. Describe alloys. Differentiate between solution, solute and solvent.

2.1 Nature of Atoms, Elements, Compounds and Mixtures Matter is anything that has mass and occupies

space.

Matter, whether it is living or non-living, is made up of atoms - the almost tiny and small building blocks of matter.

The properties of matter relate not only to the kinds of atoms it contains (composition) but also to the arrangement of these atoms (structure).

Nature of Atoms, Elements, Compounds and Mixtures Matter can be classified in two ways:

1. according to physical state - gas,liquid or solid

2. according to composition (either pure substances or mixtures)

Classification of Matter

A pure substance (usually referred to simply as a substance) is matter that has a fixed composition and distinct properties.

A substance cannot be further broken down or purified by physical means.

Examples: water, sodium chloride…

Pure substances can either be elements or compounds.

2.1.1 Pure Substances

Compound & Element A compound is a substance that can be broken

down into a simpler type of matter (elements) by chemical means (but not by physical means).

An element is a substance that cannot be broken down into simpler substances by chemical or physical methods.

Electrolysis of water

Element An element is the simplest substance with the

following three features:i. It consists of only one type of atom;ii. It cannot be broken down into simpler

substances either by physical or chemical means; and

iii. It can exist either as individual atoms or molecules.

Examples of elements that consist of individual atoms include Metals (Al, Zn, Fe, Ca, Au..) Noble gases (He, Ne, Ar..)

Elements

Diatomic molecules Elements that occur as diatomic molecules: hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, iodine

Note: A molecule consists of two or more atoms of the same element, or different elements, which are chemically bound together.

Polyatomic molecules(poly = more than one)

Some other elements exist as more complex molecules.

Molecules that contain two or more atoms are called polyatomic molecules.

Elements

Three group of elements - metals, non-metals and semi-metals

Elements

Metal elements

Examples: Potassium, mercury, lead, magnesium, silver and sodium.

Nonmetal elements

Examples: Hydrogen, chlorine, bromine, phosphorus, carbon and oxygen.

Semimetal elements (or metalloids)

Examples: Boron, silicon, germanium, arsenic, antimony, tellurium and astatine.

An element is the simplest substance with the following three features:1. It consists of only one type of atom.2. It cannot be broken down into simpler

substances either by physical or chemical means.

3. It can exist either as individual atoms or molecules.

Elements

Compounds

A compound is a substance which consists of two or more elements chemically combined together.

Compounds

We can determine a compound by inspecting these three features:

1. It can be broken down into a simpler type of matter (elements) by chemical means (but not by physical means);

2. It has properties that are different from its component elements;

3. It has a constant composition throughout and always contains the same ratio of its component atoms.

Molecules of Compounds

Molecules of compounds are composed of more than one kind of atom in a definite ratio.

Electrolysis of water

Electrolysis of water Water is converted into two simpler substances

– hydrogen & oxygen.

Hydrogen and oxygen are always present in the same ration by mass, 11.1% to 88.9%.

These observations allow us to identify water as a compound.

Mixtures are two or more substances that are mixed together but not chemically joined.

More detailed definition:Mixtures are combinations of two or more substances that are mixed physically in which each substance retains its own chemical identity and hence its own properties.

2.1.2 Mixtures

In any mixture,1. the composition can be varied2. each substance of the mixture retains its

own composition and properties3. may be separated into pure substances by

physical methods

The substances making up a mixture are called components of the mixture.

Mixtures

Mixture can be further classified as either homogenous or heterogeneous:

Heterogeneous - components are distinguishableExample: Vegetable soup, mixture of salt and charcoal

Homogeneous - components are indistinguishable; called a solutionExample: Salt water, alloys, air

Mixtures

2.1.3 Separating Components of Mixture

Filtration Evaporation Distillation Fractional distillation Crystallisation Chromatography

Filtration Suitable for an insoluble solid and liquid

mixture

Evaporation Evaporation is a method of separating a solid

that has been dissolved in a solvent.

If a mixture is heated or left over a few days, the solvent or liquid evaporates, leaving the solid as residue.

Distillation Distillation is a process to separate a

substance (in the form of a solution) from its solvent.

This method is suitable for a homogenous

mixture or solution.

Distillation

Distillation The liquid to be separated is evaporated by

boiling, and its vapour is then collected through condensation.

The condensed vapour, which is in the form of purified liquid, is called the distillate.

Fractional Distillation Fractional distillation is a method to separate a

mixture of compounds by their boiling points. This is done by heating them to certain temperatures.

This method is suitable for liquid-liquid mixtures with different boiling points.

Fractional Distillation When heated, the component of the mixture

with the lower boiling point will evaporate first and be distilled, followed by the component with the next higher boiling point and so on.

Fractional Distillation

Process of fractional distillation of petroleum

Uses of the major fractions of petroleumFraction Boiling point range (°C ) Uses

Petroleum gas

-161 to 20 Fuel and cooking gas

Gasoline 30 to 180 Automobile fuels

Kerosene 170 to 290 Rocket and jet engine fuels, domestic heating

Diesel 260 to 350 Diesel fuel

Industrial fuel oil

300 to 370 Fuel for electricity production

Lubricating oil Non volatile fraction Lubricants

Paraffin wax Solids Candles, polishing

Residue Bitumen

Solids Asphalt and road surfaces

Crystallisation Crystallisation is a process of forming crystals

from a uniform solution.

Example: Copper(II) sulphate solution can be separated into its components (copper (II) sulphate and water): Heating the solution until it is concentrated. Cool down the concentrated solution to obtain

solid copper(II) sulphate in the form of crystals.

Chromatography The differing abilities of substances to adhere to

the surfaces of various solids such as paper and starch make it possible to separate mixtures. This is the basis of chromatography.

Chromatography refers to a set of methods used to separate different compounds which normally involve separating chemicals and identifying them by colour.

Chromatography Ink is a good example for this method. The components in ink, which is a dye mixture,

can be separated by paper chromatography

Activity 2.1

2.2 Differences between Metals and Non-Metals

Metals

Examples: Potassium, mercury, lead, magnesium, silver and sodium.

Metals Metals react with non-metals to form ionic

compounds.

Most metal oxides are basic oxides, which when dissolved in water react to form metal hydroxides.

Metals Metal oxides react with acid to form salt and

water.

Non-metals

Examples: Hydrogen, chlorine, bromine, phosphorus, carbon and oxygen.

Non-Metals

Non-Metals Non-metals react with metals to form ionic

compounds.

Compounds composed entirely of non-metals are molecular compounds.

Non-Metals Most non-metal oxides are acidic oxides,

which when dissolved in water react to form acids.Example: Carbon dioxide dissolves in water to form carbonic acid.

Non-metal oxides react with base to form salt and water.

2.3 Chemical Symbols of Elements

Chemical Symbols of Elements

Chemical Symbols of Elements

2.4 Formulae of Molecules for Elements and Compounds

Many elements found in nature are in molecular form.

Diatomic molecules Elements that occur as diatomic molecules:

hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, iodine

Polyatomic molecules(poly = more than one)

Some other elements exist as more complex molecules.

Molecules that contain two or more atoms are called polyatomic molecules.

Formulae of Molecules for Elements and Compounds

The chemical formula that indicates the actual number and types of atoms in the molecule is called the molecular formula.

Formulae of Molecules for Elements and Compounds

Formulae of Molecules for Elements and Compounds

Compounds that are composed of molecules are called molecular compounds and they contain more than one type of atom.These substances are composed only of non-metallic elements.

Formulae of Molecules for Elements and Compounds

2.5 Mixtures, Substances, Compounds, & Elements

2.6 ALLOYS

The structure of atoms in metal - packed together very closely.

As a result, most metals have a high density.

ALLOYS

The layers of atoms in a metal can slide over each other easily and cause the properties of metals such as being malleable and ductile.

ALLOYS

In an alloy, the atoms of different metals have different sizes.

This makes the layers of atoms slide over each other even harder due to the disruption of the initial orderly layers of metal atoms.

Why make alloys?

1. To improve the appearance of the pure metal Metals have lustrous surface. However, the exposed metal surface quickly

loses its shine due to the formation of an oxide layer.

Alloying helps to prevent formation of the oxide layer and enables the metal to keep its surface shiny.

Example: Pewter

Why make alloys?2. To increase the strength and hardness of

the pure metal For example, carbon atoms which are smaller

than iron atoms are added into iron atoms during the making of steel.

As a result, the uniformity of the arrangement of iron atoms is disrupted and it is more difficult for the layers of the iron atoms to slide over one another. This makes steel harder and stronger than pure iron.

Why make alloys?

3. To increase the resistance to corrosion Most metals such as iron and copper corrode

readily in the air.

Alloying can prevent metals from corrosion.

Example: Carbon, chromium and nickel are added to iron to make stainless steel.

Why doesn't stainless steel rust?

Composition, properties and uses of alloysAlloy Composition Properties Uses

Steel Fe 99.5%, C 0.5%

High strength Car bodies, bridges, ships

Stainless steel

Fe 80.6%, C 0.4%, Cr 18%, Ni 1%

Great resistance to corrosion

Knives, sinks, cutlery

Pewter Sn 91%, antimony (Sb) 7%, Cu 2%

High strengthResistance to corrosionBright shiny surface

Ornaments, souvenirs

Bronze Cu 90%, Sn 10% High strengthResistance to corrosion

Medals, art objects

Duralumin Al 95%, Cu 4%, Mg 0.5%, Mn 0.5%

High strengthResistance to corrosionLow density

Aircraft, racing mountain bicycle

2.7 Solution A solution is a homogeneous mixture of two or

more substances.

A solution is formed when tiny individual particles (<1 mm in diameter) of one substance are uniformly dispersed among the individual particles of the other substance.

2.7.1 Solution, Solute and Solvent A solution is a mixture obtained by dissolving the

solute in solvent.

Types of Solutions

Solution alloys are homogeneous mixtures in which the components are dispersed uniformly.

There are two types of solution alloys: substitutional alloys interstitial alloys

Solution Alloys

Substitutional alloys Atoms must have similar

atomic radii.

Interstitial alloys Solute atoms smaller than

solvent atoms (in order to fit into the interstitial site), e.g. a nonmetal.

2.7.2 Saturated Solution

A solution can be categorised into three categories: dilute solution concentrated solution saturated solution

Saturated Solution A dilute solution has little solute particles dissolved in

the solvent. The solvent can dissolve the solute particles more easily.

When we add more solute into the solvent, it can still dissolve. At this level, we call this solution a concentrated solution.

If we keep on adding the solute into the solvent until it reaches a level where the solute cannot dissolve any more at that particular temperature, then we call this solution a saturated solution.

Saturated Solution

A saturated solution is obtained when the solution is in equilibrium with undissolved solid.

Additional solute will not dissolve if added to such a solution.

Saturated Solution

The amount of solute needed to form a saturated solution in a given quantity of solvent is known as the solubility of the solute.

This is usually expressed in grams of solute in 100 g of solvent.

Saturated Solution

For example, the solubility of NaCl in water is 36 g per 100 mL of water at 20°C.

This is the maximum amount of NaCl that can be dissolved in water to give a stable, equilibrium solution at that temperature.

If we dissolve 30.0 g of NaCl per 100 mL of water at 20°C, the solution is said to be unsaturated because it has the capacity to dissolve more solute.

2.7.3 Some Factors Affecting Solubility

When a solute dissolves in the solvent, a solution is formed.

When only a small amount (or none at all) of a solute can be dissolved in the water, the solute is insoluble.

When the solute is slightly dissolved in the water, we get a suspension.

Some Factors Affecting Solubility

How much solute can dissolve in a solvent? It all depends on the following three factors:

a) Size of the solute particlesb) Type of the solvent

For example, sugar will dissolve in water but may not dissolve in other types of solvent like paraffin.

Most solutes dissolve in water, hence water is a universal solvent.

c) Temperature of the solvent