© boardworks ltd 2003 ks4: atoms. © boardworks ltd 2003 a slide contains teacher’s notes...
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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
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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
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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
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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
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“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.
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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
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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
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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
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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
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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.
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The Atom: Check It Out!
• Draw a labelled diagram of the atom showing the nucleus and labelling protons, neutrons and electrons
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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
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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
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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
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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.
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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
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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
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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
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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
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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.
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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
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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
ProtonsElectronsNeutrons
Hydrogen
ProtonsElectronsNeutrons
(Deuterium)
ProtonsElectronsNeutrons
(Tritium)
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Isotopes: Chlorine• About 75% of natural chlorine is 35Cl the rest is 37Cl.
Cl35
1775%
17ProtonsElectronsNeutrons
17
18ProtonsElectronsNeutrons
171720
Cl37
1725%
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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
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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
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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
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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
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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
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ActivityDrag the statements at the top onto the correct
side of the table
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ActivityDrag the statements at the top onto the correct
side of the table
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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.
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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
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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
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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
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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!
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The Electrons in Carbon
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The Electrons in Neon
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The Electrons in Silicon
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Electrons in Phosphorus
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The Electrons in Argon
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The Electrons in Sodium
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The Electrons in Fluorine
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The Electrons in Aluminium
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The Electrons in Nitrogen
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The Electrons in Sulfur
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The Electrons in Oxygen
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The Electrons in Chlorine
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The Electrons in Magnesium
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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
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• 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
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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
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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
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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
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The Boron Ion
B
5
2
3
1st Shell = 2: full
2nd Shell = 3: not full
2.3
3
•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? 2.(0)
Bye!
Bye!
Bye!
B3+
11
B5
Activity
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• 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
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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
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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
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The Oxide Ion16
O8
O
8
2
6
1st Shell = 2: full
2nd Shell = 6: not full
2.6
2
•How many electrons?
•How many electrons in the first shell?
•How many electrons in the second shell?
What electron arrangement?
How many electrons to gain?
New electron arrangement? 2.8
O2-
Activity
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Drag the words at the top to their correct places in the sentences.
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1. Which of the following is not a sub-atomic particle?
A. Proton.
B. Isotope.
C. Neutron.
D. Electron.
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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.
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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
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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
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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
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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
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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
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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
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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