© boardworks ltd 2003 ks4: atoms. © boardworks ltd 2003 a slide contains teacher’s notes...

© Boardworks Ltd 2003

KS4: ATOMS


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Atoms – the building blocks

• All substances are made from very tiny particles called atoms.

• John Dalton had ideas about the existence of atoms about 200 years ago but only recently have special microscopes been invented that can “see” them.

Water


Elements – different types of atom

• Elements are the simplest substances. There are about 100 different elements. Two of John Daltons ideas were:– Elements contain different

types of atom.

– Each element has only one type of atom.

NS

O

C

Fe

K


Atoms – How Small?

• Atoms are about 1/100,000,000 cm across. They are small.small.

• To make an atom the size of a football we would have to magnify it to around 3,000,000,000 times its normal size

• If we magnified the size of a football by the same amount it would stretch from the UK to the USA!

NX

3,000,000,000


Atoms – How Heavy?

• A single grain of sand contains billions of atoms of silicon and oxygen.

• It follows that each atom must have an extremely small mass.

• However, it is still possible to find out the relative masses of atoms.

SiO

Billions of these atoms join to form each tiny grain of sand

O


“Weighing” atoms: The mass spectrometer

+ +

+

+ +

+

+

+

Atoms are given a + charge

These charged atoms (ions) are accelerated past a magnet

The deflected ions are detected on a screen or film.

The magnet deflects this ion. The lighter the ion the more it is deflected

+ + + + + + ++

+

The more mass the atom has, the more nearly it travels in a straight line.


Relative Atomic Mass 1

• The deflection in the mass spectrometer varies with the mass of the atom.

• However, this does not tell us the mass in grams.

• What it tells us is the relative masses of atoms – or relative atomic mass (RAM)relative atomic mass (RAM)

• The element carbon is the atom against which the mass of all other atoms are compared. Carbon is given a RAM value of 12Carbon is given a RAM value of 12.

Relative atomic mass = 12C


Relative Atomic Mass 2• The lightest atom is hydrogen. It has one

twelfth the mass of carbon and so has a RAM of 1.

• Magnesium is twice as heavy as carbon. It has a RAM of 24.

Mg

C C

Mg

C C

Mg

C CMg C C

C

H HHH

HH

H HHH

HH

H HHH

HH

H HHH

HH

C

24 x 1 = 12 x 21 x 12 = 12 x 1


Relative Atomic Mass

• The Table shows the mass of various atoms relative to carbon.

• Calculate their relative atomic mass.

Element Symbol Times as heavy as carbon R.A.M

Helium He one third

Beryllium Be three quarters

Molybdenum Mo eight

Krypton Kr seven

Oxygen O one and one third

Silver Ag nine

Calcium Ca three and one third

4

9

96

84

16

108

40

Activity


Even Smaller Particles!

• For some time people thought atoms were the smallest particles and that they could not be broken into anything smaller.

• We now know that atoms are themselves made from even smaller and simpler particles.

• These particles are– Protons– Neutrons– Electrons


How Are the Particles Arranged?

• Protons, neutrons and electrons are NOT evenly distributed in atoms.

• The protons and neutrons exist in a dense core called the nucleus.

• Around the outside are very thinly spread electrons.

• These electrons exist in layers called shells.

The Nucleusa dense core

of protons and neutrons

containing nearly all the mass of the

atom

‘Shells’ of electronselectrons are really very very tiny

so the atom is mostly empty space.


The Atom: Check It Out!

• Draw a labelled diagram of the atom showing the nucleus and labelling protons, neutrons and electrons


Properties of Sub-atomic Particles

• There are two properties of sub-atomic particles that are especially important:– Mass– Electrical charge

ParticleParticle ChargeCharge Relative MassRelative Mass

Protons +1 1

Neutrons 0 1

Electrons -1 0.0005

Element atoms contain equal numbers of protons and electrons and so have no overall charge


Atomic Number

• The atom of any particular element alwaysalways contains the same number of protons. E.g.– Hydrogen atoms always contain 1 proton– Carbon atoms always contain 6 protons– Magnesium atoms always contain 12 protons

• The number of protons in an atom is known as its atomic number.

• It is the smaller of two numbers shown in most periodic tables

12

C6


How Many Protons?

• Here is some data about elements taken from the Periodic Table.

• What is the atomic number of the elements?

23

Na11

56

Fe26

119

Sn50

19

F9

27

Al13

11 26 50 9 13

Activity


How Many Protons?

• Note that any element has a definite and Note that any element has a definite and fixed number of protons.fixed number of protons.

• If we change the number of protons in an atom then this changes that atom into a different element.

• Changes in the number of particles in the nucleus (protons or neutrons) is very rareis very rare. It only takes place in nuclear processes such as radioactive decay, nuclear bombs or nuclear reactors.


Mass Number

• The mass of each atom results almost entirely from the number of protons and neutrons that are present. (Remember that electrons have a relatively tiny mass).

• The sum of the number of protons and The sum of the number of protons and neutrons in an atom is the mass numberneutrons in an atom is the mass number.

Atom Protons Neutrons Mass NumberHydrogen 1 0 1

Lithium 3 4 7

Aluminium 13 14 27


Mass Number

• Calculate the mass number of the atoms shown.

Atom Protons Neutrons Mass Number

Helium 2 2

Copper 29 35

Cobalt 27 32

Iodine 53 74

Germanium 32 41

4

64

59

127

73

MASS NUMBER = Number of protons + number of neutrons

Activity


How Many Neutrons

• Calculate the number of neutrons in these atoms.

Atom Mass Number

Atomic Number

Number of Neutrons

Helium 4 2

Fluorine 19 9

Strontium 88 38

Zirconium 91 40

Uranium 238 92

Number of Neutrons = mass number - atomic number

2

10

50

51

146

Activity


How Many Neutrons 2

• It is not strictly true to say that elements consist of one type of atom.

• Whilst atoms of a given element always have the same number of protons, they may have different numbers of neutrons.

• Atoms that differ in this way are called isotopes.isotopes.

Remember: The number of protons defines

the element


How Many Neutrons 3

• Isotopes are virtually identical in their chemical reactions. (There may be slight differences in speeds of reaction).

• This is because they have the same number of protons and the same number of electrons.

• The uncharged neutrons make no difference to chemical properties but do affect physical properties such as melting point and density.


Isotopes: Carbon• Natural samples of elements are often a mixture of

isotopes. About 1% of natural carbon is carbon-13.

ProtonsElectronsNeutrons

C12

699% C

13

61%

6

6

6

6

6

7


Isotopes: Hydrogen• Hydrogen exists as 3 isotopes although

Hydrogen-1 makes up the vast majority of the naturally occurring element.

H1

1 H2

1 H31


Hydrogen


(Deuterium)


(Tritium)


Isotopes: Chlorine• About 75% of natural chlorine is 35Cl the rest is 37Cl.

Cl35

1775%

17ProtonsElectronsNeutrons

17

18ProtonsElectronsNeutrons

171720

Cl37

1725%


Isotopes of Oxygen

• Almost all of natural oxygen is 16O but about 0.2% is 18O.

• Produce a Table showing the particles in each isotope.

Protons

Electrons

Neutrons

16

O8

Oxygen-18Oxygen-16

8

8

8

Protons

Electrons

Neutrons

8

8

10

Activity


Isotopes and Relative Atomic Mass

• Many natural elements are a mixture of isotopes.• This means that when we react atoms of an

element we are using a mixture of atoms with different mass numbers.

• The relative atomic mass given in the periodic table takes account of this.

E.g.. For 100 atoms of chlorine:Mass of 75 atoms of Chlorine 35: 75 x 35 =2625Mass of 25 atoms of Chlorine 37: 25 x 37 =925Total = 3550Average (divide by 100) = 35.535.5


Relative Atomic Mass

• Bromine contains 50.5% bromine-79 and 49.5% bromine-81.

• Calculate the relative atomic mass of natural bromine showing your working.

Egg. For 100 atoms of bromine:Mass of 50.5 atoms of bromine 79: 50.5 x 79 =3989.5Mass of 49.5 atoms of bromine 81: 49.5 x 81 =4009.5Total = 7999Average (divide by 100) = 80 80 (3 s.f.)

Activity


How Many Electrons

• The atoms of any element have an equal number of protons and electrons.

Atom Protons Neutrons Electrons

Helium 2 2 2

Copper 29 35 29

Cobalt 27 32 27

Iodine 53 74 53

Germanium 32 41 32

..and so overall charge equals zero


How Many Electrons

• Fill in the blank columns

Atom Protons Neutrons Electrons Atomic number

Mass Number

Boron 5 6

Potassium 19 20

Chromium 24 28

Mercury 80 121

Argon 18 22

5 5 11

19 19 39

24 24 52

80 80 201

18 18 40

Note – atomic number is defined as the number of protons rather than electrons because atoms can lose (or gain)

electrons but do not normally lose protons

Activity


ActivityDrag the statements at the top onto the correct

side of the table


Summary: The atom so far!The nucleus.• Dense: contains nearly all the mass of the

atom in a tiny space.• Made up of protons and neutrons.• Has a positive charge because of the protons.

The electrons.• Exist thinly spread around the outside of the

atom.• Very small and light.• Negatively charged.• Exist in layers called shells.• Can be lost or gained in chemical reactions.


How Are Electrons Arranged?• Electrons are not evenly spread.• The exist in layers known as shells.• The arrangement of electrons in these shells is

often called the electron configuration.electron configuration.

2nd Shell

1st Shell

3rd Shell

4th Shell


How Many Electrons per Shell?• Each shell has a maximum number of

electrons that it can hold.

1st Shell: 2 electrons

2nd Shell: 8 electrons

3rd Shell: 8 electrons

The maximum


Which Shells Do Electrons go into?

• Opposites attract.• Protons are + and electrons are – charged.• Electrons will occupy the shells nearest the

nucleus unless these shells are already full.

1st Shell: Fills this first

2nd Shell: Fill this next

3rd Shell: And so on


Working Out Electron Arrangements

1. How many electrons do the element atoms have? (This will equal the atomic number).

2. Keeping track of the total used, feed them into the shells working outwards until you have used them all up.

1st Shell: Fills this first

2nd Shell: Fill this next

Drawing neat diagrams helps you

keep track!


The Electrons in Carbon


The Electrons in Neon


The Electrons in Silicon


Electrons in Phosphorus


The Electrons in Argon


The Electrons in Sodium


The Electrons in Fluorine


The Electrons in Aluminium


The Electrons in Nitrogen


The Electrons in Sulfur


The Electrons in Oxygen


The Electrons in Chlorine


The Electrons in Magnesium


1+ ion ProtonsElectrons

1- ion ProtonsElectrons

Ions and Electron Structures

1. Ions are atoms that have either extra electrons added or electrons removed. e.g.

Atoms ProtonsElectrons

Lose 1 electron Gain 1 electron

So in ions the number of

electrons no longer equals the

number

of protons


• These are ions formed by the atom losinglosing one or more electrons.

• They are called cationscations. This is because during electrolysis they move towards the cathode. (Or because they are “pussytive”!).

• The charge on the ions is equal to the number of equal to the number of electrons that the atom has lostelectrons that the atom has lost.

• In equations the charge is usually shown above and to the right of the symbol. (E.g. Mg2+ ).

Positive Ions

Lost 1 e- +

Lost 2 e- 2+

Lost 3 e- 3+

It is nearly always metal

atoms that lose

electrons

Ions usually have

OUTER electron

shells that are either completely full or else

empty


Na Bye!

The Electrons in a Sodium IonIn the sodium atomatomAtomic number = number of protons = 11 Number of electrons = 11

Electron arrangement: 2.8.1 (Incomplete Shell)

Na+

Electron lost

Electron arrangement: 2.8 (Full Shells)

23

Na11


Mg

The Electrons in a Magnesium Ion

In the magnesium atomatomAtomic number = number of protons = 12 Number of electrons = 12

Electron arrangement = 2.8.2 (Incomplete shell)

Mg2+

Electron arrangement 2.8 (Full Shells)

Bye!

Bye!

2 electrons lost

24

Mg12


The Lithium Ion

Li

3

2

1

1st Shell = 2: full

2nd Shell = 1: not full

2.1

1

•How many electrons?

•How many electrons in the first shell?

•How many electrons in the second shell?

What electron arrangement?

How many electrons to lose?

New electron arrangement? Include a diagram

2.(0)

Bye!Li+

7

Li3

Activity


The Boron Ion

B

5

2

3

1st Shell = 2: full


2.3

3





How many electrons to lose?

New electron arrangement? 2.(0)

Bye!

Bye!

Bye!

B3+

11

B5

Activity


• These are ions formed by the atom gaininggaining one or more electrons.

• They are called anionsanions. This is because during electrolysis they move towards the anode.

• The charge on the ions is equal to the number of equal to the number of electrons that the atom has gainedelectrons that the atom has gained.

• In equations the charge is usually shown above and to the right of the symbol. (E.g.. O2- ).

Negative Ions.

Gain 1 e- -

Gain 2 e- 2-

Gain 3 e- 3-

It is nearly always

non-metal atoms that

gain electrons

Ions usually have

OUTER electron

shells that are either completely full or else

empty


The Electrons in a Sulphide Ion.In the sulphur atomatomAtomic number = number of protons = 16 Number of electrons = 16

Electron arrangement: 2.8.6 (incomplete shell)

Electron arrangement 2.8.8 (Full shells)

2 electrons gained

S S2-

32

S16


The Electrons in a Fluoride Ion.

In the fluorine atomatomAtomic number = number of protons = 9 Number of electrons = 9

Electron arrangement: 2.8.7 (incomplete shell)

Electron arrangement 2.8.8 (Full shells)

1 electron gained

F F2-

19

F9


The Oxide Ion16

O8

O

8

2

6

1st Shell = 2: full


2.6

2





How many electrons to gain?

New electron arrangement? 2.8

O2-

Activity


Drag the words at the top to their correct places in the sentences.


1. Which of the following is not a sub-atomic particle?

A. Proton.

B. Isotope.

C. Neutron.

D. Electron.


2. The element Cobalt has a relative atomic mass of 59 and an atomic number of 27. Which of these is a true statement about each neutral cobalt atom?

A. It contains 59 neutrons.

B. It contains 27 electrons.

C. It contains 32 protons.

D. It contains equal numbers of neutrons and electrons.


3. The Periodic Table displays iron as shown below. This indicates that Fe atoms:

A. contain 56 neutrons.

B. contain 30 electrons.

C. contain 26 protons.

D. contains more protons than neutrons.

56

Fe26


4. Bromine consists of a mixture of two isotopes: Bromine-79 and Bromine-81

• Which of the following is true:

A. Both isotopes contain 35 protons.

B. Bromine 79 contains 46 neutrons.

C. Bromine 81 contains 44 neutrons.

D. Bromine-81 is more reactive than bromine-79.

80Br35


5. Natural boron consists of approx. 20% boron-10 and 80% boron-11.

• What will the relative atomic mass of natural boron will be?

A. 10.0

B. 11.0

C. 10.5

D. 10.8

?B5


6. Which answer best describes the shell arrangement of the electrons in a sodium atom?

A. 2,8,1.

B. 2,2,7.

C. 2,8,8,3.

D. 2,8,8,1.

23Na11


7. Which answer best describes the shell arrangement of the electrons in an oxygen atom?

A. 2,8,6.

B. 2,8,8.

C. 2,8.

D. 2,6.

16O8


8. Which of these elements will have electron shells that are either full or empty (i.e. not partially full)?

14N7

A 40Ar18

B

40Ca20

C 27Al13

D


9. Which answer best describes the shell arrangement of the electrons in an oxide ion O2-?

A. 2,8,6.

B. 2,8,8.

C. 2,8.

D. 2,6.

16O8

© boardworks ltd 2003 ks4: atoms. © boardworks ltd 2003 a slide contains teacher’s notes...

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mass of various atoms

mass of carbon

small mass

relative masses of atoms

billions of atoms

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weighing atoms