learning outcomes essential chemistry - brian · pdf file1 of 55 © boardworks ltd 2007...

1

Boardworks GCSE Additional Science: Chemistry

Chemical Reactions

Spring 2007

1 of 55 © Boardworks Ltd 2007

Essential Chemistry

CEB Textbook Chapter 2, pages 22- 28

Mastering Biology, Chapter 2


Learning Outcomes

•After studying this topic you should be able to….

•Describe ionic and covalent bonds.

•Describe what a chemical reaction is, and distinguish between reactants and products.

NB: You are not required to study the sections on atomic number, mass number, or isotopes in the CEB textbook.


All substances are made of tiny particles called atoms.

Many substances are made up of different types of atoms.

iron, aluminium, silicon,

oxygen and boron atoms

carbon, nitrogen, hydrogen,

oxygen and sulfur atoms

hydrogen and oxygen atoms carbon and hydrogen atoms

All substances are made of atoms


Atomic zoom


Atoms – the building blocks

John Dalton had the first

ideas about the existence of

atoms over 200 years ago.

This image is highly magnified.

What could it be showing?

However, it is only relatively

recently that special microscopes

(called electron microscopes)

have been invented that can

actually ‘see’ atoms.

The yellow blobs are individual

gold atoms, as seen through

an electron microscope.


What are atoms made of?

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How big are atoms?

Atoms have a diameter of about 0.00000001 cm, which is far

too small to be seen with your eyes.

In one glass of water there are around:

However, microscopes

called Scanning Tunnelling

Microscopes allow scientists

to see the outlines of atoms.

12,000,000,000,000,000,000,000,000

oxygen atoms

24,000,000,000,000,000,000,000,000

hydrogen atoms.


An element is a substance made up of only one type of atom.

Atoms in elements

Copper is made

up of copper

atoms only.

Carbon is made

up of carbon

atoms only.

Helium is made

up of helium

atoms only.


What atoms are you made of?

Atoms are the smallest part of an element. Different elements

join together in different ways to make all the materials in the

Universe, from rocks to air.

Just like all materials

in the Universe,

humans are also

made of atoms.

Which four atoms

do you think are

most common in

the human body?

oxygen (65%)

other (%)

sulfur (0.25%)

potassium (0.35%)

phosphorus (1%)

calcium (2%)

nitrogen (3%)

carbon (18%)

hydrogen (10%)


What particles are atoms made of?

For some time, people thought that atoms were the

smallest particles and could not be broken into anything

smaller.

proton neutron electron

Scientists now know that atoms are actually made from

even smaller particles. There are three types:

How are these particles arranged inside the atom?


What is the structure of an atom?

Protons, neutrons and electrons are not evenly distributed in

an atom.

The electrons are

spread out around the

edge of the atom. They

orbit the nucleus in

layers called shells.

The protons and neutrons

exist in a dense core at the

centre of the atom. This is

called the nucleus.


Mass and electrical charge

There are two properties of protons, neutrons and electrons

that are especially important:

mass

electrical charge.

The atoms of an element contain equal numbers of protons

and electrons and so have no overall charge.

-1 almost 0 electron

0 1 neutron

+1 1 proton

Charge Mass Particle

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More about atomic number

Each element has a definite and fixed number of protons.

Changes in the number of particles

in the nucleus (protons or neutrons)

are very rare. They only take place

in nuclear processes such as:

radioactive decay

nuclear bombs

nuclear reactors.

If the number of protons changes, then the atom becomes

a different element.


How many electrons?

Atoms have no overall electrical charge and are

neutral.

The number

of electrons

is therefore

the same as

the atomic

number.

Atomic number is the number of protons rather than

the number of electrons, because atoms can lose or

gain electrons but do not normally lose or gain

protons.

This means atoms must have an equal number of

positive protons and negative electrons.

29

2

53

Electrons Protons Neutrons

helium

copper

iodine

Atoms

2 2

29 35

74 53


Atoms: true of false?


How are electrons arranged?

Electrons are not evenly spread but exist in layers called

shells. (The shells can also be called energy levels).

The arrangement of electrons in these shells is often

called the electron configuration.

Note that this diagram is not drawn to scale – the atom is

mostly empty space. If the electron shells were the size

shown, the nucleus would be too small to see.

1st shell

2nd shell

3rd shell


How many electrons per shell?

Each shell has a maximum number of electrons that it can

hold. Electrons will fill the shells nearest the nucleus first.

1st shell holds a maximum of 2 electrons

2nd shell holds

a maximum of

8 electrons

3rd shell holds

a maximum of

8 electrons

This electron arrangement is written as 2,8,8.


Summary: the atom so far

The nucleus is:

made up of protons and neutrons

positively charged because of the

protons

dense – it contains nearly all the

mass of the atom in a tiny space.

Electrons are:

very small and light, and negatively charged

able to be lost or gained in chemical reactions

found thinly spread around the outside of the nucleus,

orbiting in layers called shells.

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Summary: the atom so far


The atoms in some elements join together to make molecules.

Molecules of elements

One of the elements that exists like this is hydrogen.

Each hydrogen atom joins up with one other atom to make a

hydrogen molecule.

The formula of a hydrogen molecule is H2.

Other elements such as nitrogen, oxygen, and chlorine also

exist as molecules. What are the formulae of these elements?

= hydrogen atom hydrogen

molecule =


When two or more elements react in a chemical

reaction, the atoms do not just mix together, they

become joined to one another to make compounds.

Atoms joining to make compounds

In a chemical reaction, the atoms in the reactants

become joined in different ways and this is how

new compounds are formed.

hydrogen oxygen water +

+


Some compounds exist as molecules:

Molecules of compounds

Water is made of hydrogen

and oxygen atoms. What is

the formula of water?

Carbon dioxide is made of carbon

and oxygen atoms. What is the

formula of carbon dioxide?


Recognising elements and compounds


A compound is not a mixture

The molecules in a mixture

of hydrogen gas and

oxygen gas look like this:

But the molecules in water

look like this…

…because water is a compound, made when hydrogen and

oxygen react and their atoms become chemically joined to

each other.

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Making a compound – water

Water is produced from the reaction between hydrogen and

oxygen.

Two hydrogen atoms react with one oxygen atom to produce

two molecules of water.

Why are the properties of water different from the properties

of hydrogen and oxygen?

hydrogen oxygen water +

+


A compound always contains a particular amount of each

element. It has a fixed composition.

Composition of compounds

This is shown in its formula, H2O.

For example, water molecules always contain twice as much

hydrogen as oxygen.

What is the formula of carbon

dioxide?

How many of each atom

does each carbon dioxide

molecule contain?

two hydrogen atoms

one oxygen atom


What is a chemical reaction?

A chemical reaction is a change that takes place when

one or more substances (called reactants) form one or

more new substances (called products).

reactants products

For example:

carbon carbon dioxide oxygen

+

+


How can you spot a chemical reaction?

Chemical reactions can appear very different.

As you observe a chemical reaction, you may detect:

a colour change

precipitate (solid) forming

energy being produced (fizzing, burning)

an odour being produced.


Why doesn’t the mass change?

In a chemical reaction, no atoms are made or destroyed.

The reaction just changes how the atoms are bonded

together.


What is a word equation?

For example, when a piece of

sulfur is burned in oxygen gas

it produces a white solid called

sulfur dioxide.

A word equation uses the names of the reactants and

products to show what happens in a chemical reaction.

+ sulfur oxygen sulfur

dioxide

The word equation for this

reaction is:

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What is a symbol equation?

A symbol equation uses the formulae of the reactants and

products to show what happens in a chemical reaction.

This equation shows that one atom of sulfur (S) reacts

with one molecule of oxygen (O2) to make one molecule

of sulfur dioxide (SO2).

A symbol equation must be balanced to give the correct

ratio of reactants and products.

+ S O2 SO2

+


What are bonds?

Compounds are formed when different elements chemically

react and form bonds with each other.

Some compounds, like water (H20),

have small, simple structures with

just a few atoms bonded together.

Others compounds, like DNA, have large, complex structures

containing thousands or even millions of bonded atoms.

Elements are the simplest substances that exist on Earth.

Each element is made up of just one type of atom, usually

joined to other atoms of the same element by bonds.

This forms molecules such as chlorine (Cl2).


Why do atoms form bonds?

Bonds involve the electrons in the outer shells of atoms.

Filled electron shells are very stable.

1st shell holds a maximum of 2 electrons

2nd shell holds a maximum

of 8 electrons

3rd shell holds a

maximum of 8 electrons

Each shell has a maximum number of electrons that it can

hold. Electrons fill the shells nearest the nucleus first.


What are the types of bonding?

Different types of bonds are formed depending on the types

of atoms involved:

All bonds involve electrons and all bonding involve changes

to the number of electrons in the outer shells of atoms.

ionic bonding

– occurs between metal and non-metal atoms.

covalent bonding

– occurs between non-metals atoms only.

hydrogen bonding

-- occurs between polar molecules such as water (negative

and positive ends)

–

How do you think electrons are involved in ionic bonding?


How do atoms form ions?

An ion is an atom or group of atoms that has

an electrical charge, either positive or negative.

Atoms have an equal number of protons and electrons and

so do not have an overall charge.

Atoms with incomplete outer electron

shells are unstable. By either gaining or

losing electrons, atoms can obtain full

outer electron shells and become stable.

When this happens, atoms have an unequal number of

protons and electrons and so have an overall charge.

This is how atoms become ions.

How does an atom become a positive or negative ion?


Positive and negative ions?

The electron configuration of an atom shows how many

electrons it must lose or gain to have a filled outer shell.

An atom that loses electrons has more protons

than electrons and so has a positive overall charge.

This is called a positive ion.

An atom that gains electrons has more electrons

than protons and so has a negative overall charge.

This is called a negative ion.

Atoms with a nearly empty outer shell, will lose electrons

to obtain a full outer shell.

Atoms with a nearly full outer shell, will gain electrons

to obtain a full outer shell.

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How is a magnesium ion formed?

2.8.2

(partially full outer shell)

12 protons = +12

12 electrons = -12

Total charge = 0

Magnesium atom:

12 protons = +12

10 electrons = -10

Total charge = +2

Magnesium ion:

[2.8]2+

(full outer shell)

2+

Mg Mg

loses

2 electrons


How is a sodium ion formed?

2.8.1

(partially full outer shell)

11 protons = +11

11 electrons = -11

Total charge = 0

Sodium atom:

11 protons = +11

10 electrons = -10

Total charge = +1

Sodium ion:

loses

1 electron

+

[2.8]

(full outer shell)

Na Na


Ionic Bonds

• Example of an Ionic Bond is Na (sodium)

and Cl (chlorine)

• Na has lost an electron to gain a full outer

shell and Cl has gained an electron to gain

a full outer shell.

• The ionic bond is formed by the attraction

of the positive and negative charges

OPPOSITES ATTRACT!!


What is a covalent bond?

Non-metal elements usually just need one or two electrons

to fill their outer shells. So how do they form a bond?

The shared electrons join the atoms together. This is called

a covalent bond.

The two non-metal atoms cannot form a bond by transferring

electrons from one to another. Instead, they share electrons.

Each atom now

has a full, stable

outer shell.

incomplete outer shells

Cl

Cl

Cl

Cl


Two common ways to represent a covalent bond are:

Only outer shells of electrons are involved in bonding, so the

inner shells do not always have to be included in diagrams.

solid

line

Cl Cl –

– Cl Cl

A covalent bond consists of a shared pair of electrons.

covalent bond

Cl

simplified dot and cross

diagram

Cl

How is a covalent bond drawn?

Cl Cl


Carbon

Carbon is number 6 on the periodic table.

What does this mean in terms of the bonds it

can form?

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Carbon

It forms 4 Covalent bonds with Hydrogen!

Both carbon and Hydrogen now have stable outer shells!


Comparing covalent and ionic bonding


HOMEWORK

• Read ‘The Scientific Method’ booklet and

use the information to answer questions 1-

3 in your Unit Assessment 1.

• Complete Exercises in ‘The Scientific

Method’ booklet

• Revise your notes on Ions and Covalent

bonds and use it to answer questions 3

and 4 in your Unit assessment 1

learning outcomes essential chemistry - brian · pdf file1 of 55 © boardworks ltd 2007...

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