1.0 atomic structure

1.0 Atomic structureLister p 4 - 20

AQA AS SpecificationLessons Topics

1-2 Fundamental particles• be able to describe the properties of protons, neutrons and electrons in terms of relative charge and relative mass

3 Electron arrangement• know that early models of atomic structure predicted that atoms and ions with noble gas electron-arrangements should be stable

4-7 Mass number and isotopes• be able to recall the meaning of mass number (A) and atomic(proton) number (Z)• be able to explain the existence of isotopes• understand the principles of a simple mass spec,limited to ionisation, acceleration, deflection and detection, and its use for identifying elements and RMM

8-11 Electron arrangement• know the electron configurations of atoms and ions up to Z = 36 in terms of levels and sub-levels (orbitals) s, p and d• know the meaning of the term ionisation energy.• understand how ionisation energies in Period 3 (Na – Ar) and inGroup 2 (Be – Ba) give evidence for electron arrangement in sub-levels and in levels

The Atom

The atom consists of two parts:

1. The nucleus which contains:

2. Orbiting electrons.

protonsneutrons

Draw a model of an atom and label the main parts

Structure of an atom

• An atom consists of a central positively charged nucleus containing protons and neutrons (nucleons)

• Diameter approx. 10-15 m (1 femtometre)

• Electrons surround the nucleus

• Atomic diameter approx. 10-10 m roughly 100 000 x nucleus diameter nucleus diameter ~ 10 – 15 m

atomic diameter ~ 10 – 10 m

If a helium atom was the size of a full stop, then the average gerbil would be the size of the

Earth.

Atoms: How small?

Now let’s pretend that the helium atom on the right is the size of the Earth.

What’s wrong with this simple picture?

Atoms: very small

The helium atom is not in the right proportions. The three subatomic particles are wrongly enormous in comparison to

the atom’s radius.

How big is a nucleus?

Most of the atom is empty space!

If you imagine an atom being the size of Wembley stadium, the nucleus would be about the size of a

football on the centre spot.

The electrons would be two peas flying around the whole stadium. The rest of it: emptiness.

Properties of subatomic particles

Property Proton, p Neutron, n Electron, e-

Mass/ kg 1.673 x 10-27

1.675 x 10-27

0.911 x 10-31

Charge/C +1.602 x10-19 0 -1.602 x 10-

19

Position In the nucleus

In the nucleus

Around the nucleus

Subatomic

particleRelative charge

Relative mass

Common depictio

n

Proton +1 1

Neutron 0 1

Electron -1 1 10-5

+

-

Subatomic particles in more detail

1.2 Electron arrangement• How are electrons arranged in

atoms?They are arranged in shells

How do we know how many electrons are in each shell?

The shells are numbered outward from the nucleus.

The maximum number of electrons found in each shell can be calculated from 2n2 where n is the shell number.

The shorthand form for, eg, Nitrogen, is 2,5

Shell Number Maximum number of electrons

1 2 x 12 = 2 x 1 = 2

2 2 x 22 = 2 x 4 = 8

3 2 x 32 = 2 x 9 = 18

4 2 x 42 = 2 x 16 = 32

5 2 x 52 = 2 x 25 = 50

Task

Complete the following table:

Shell Number Maximum number of electrons

1

2

3

4

5

Now complete worksheet 1.1

1.3 Mass number, atomic number and isotopesHow can we describe an atom in terms of it’s subatomic structure?

What information do we need to know?

The number of protons is called the Atomic number .

What is significant about the number of protons in the nucleus?

The number of protonsThe number of neutronsThe number of electrons

It tells us what the element is and how many electrons are present in the neutral atom

The number of nucelons is called the Mass number .

What information can we get from this?

We can find out the number of neutrons

Li7

3

No. of protons + neutronsLithium

Number of protons

Number of electrons=

Atomic number does not always equal the number of neutrons.

LithiumElectrons

3

Protons 3Neutrons

4

Mass number

(No. of protons)

Atomic number or proton number

LithiumNumber of

protonsNumber of electrons=

LithiumElectrons

3

Protons 3Neutrons

4

This is because the atom is neutral. The charges balance out

-3 charge

+3 charge

But atoms can gain and lose electrons (they become ions). This changes the overall charge on the atom.

No charge

The number of protons “defines” an element – nothing else!

Atomic number does not always equal the number of neutrons. This can change, even in atoms of the same element. These are called isotopes.

7-Lithium (7Li)Electrons 3Protons 3Neutrons 4

Some isotopes of lithium:

4Li 4-Lithium 3 protons,1 neutron

6Li 6-Lithium 3 protons,3 neutrons



Lithium: always 3 protons!

Isotopes

Complete:Atom P n e-

NaRhphosphorusThe last of the halogensXeThe only liquid non-metalLi+F-

Carbon-14 (14C)

A helium atom

He4

2

2 protons2 electrons2 neutrons

Answers

Atom P N ENa 11 12 11Rh 45 58 45phosphorus 15 16 15The last of the halogens 85 125 85Xe 54 77 54The only liquid non-metal 35 45 35Li+ 3 4 2F- 9 10 10Carbon-14 (14C) 6 8 6

Chemical properties of isotopesWould you expect the isotopes of lithium to have the same chemical properties?

What is the Mass number, Z, of Chlorine?

Yes – chemistry is about the movement of electrons

How can you get a fraction of a nucleon?

35.5

The relative abundance of two chlorine isotopes is similar, hence the mass number on the PT is an average number determined by the abundances of the isotopes

1.4 Mass spectrometry

What does a mass spectrometer do?

It ionizes atoms and then sends them through an em field where they become deflected on the basis of their mass and charge

Why is it important that the instrument is under vacuum?

To prevent collisions of the ions with gas molecules

How are samples put into the machine?

Volatile liquids and gases can be injected directly, solids must be vapourised.

http://www.youtube.com/watch?v=J-wao0O0_qM&feature=related



Mass Spectrometer

ABCDEFG

Mass Spectrometry - summary

Sample vapourisedSample ionised positive ions+ve ions in beam accelerated by electric fieldVacuum pump to keep whole apparatus at v. low pressure+ve ions subjected to variable magnetic field+ve ions separated according to mass: charge ratio+ve ions detected and measured mass spectrum

LOWER m:z ratio

HIGHER m:z ratio

State what happens at each of the locations A-G

Calculating RAM of atoms

Calculate the relative atomic mass of boron.

The tallest “stick” is often (but not always) set at 100

boron-10 23 boron-11 100

(100 x 11) + (23 x 10)/123 = 10.8

http://www.chem.uoa.gr/applets/AppletMS/Appl_Ms2.html

QuestionHow many isotopes does this element have? What element is it?

Calculate the RAM

51.5

11.217.1 17.4

2.8

Answer

51.5

11.217.1 17.4

2.8

Step 1: Find the total mass of these 100 typical atoms:(51.5 x 90) + (11.2 x 91) + (17.1 x 92) + (17.4 x 94) + (2.8 x 96) = 9131.8

Step 2: find the average mass of these 100 atoms :9131.8 / 100 = 91.3 (to 3 sig fig).

91.3 is the relative atomic mass of zirconium.

The mass spectrum of uranium has 3 peaks: at 234 m/z, 235 m/z and 238 m/z. The abundance of the isotopes is 0.006%, 0.72% and 99.2% respectively. What is the average relative atomic mass of uranium?

240

237.0

237.8

238

Question

Question

Chlorine has two isotopes, 35Cl and 37Cl, in the approximate ratio of 3 atoms of 35Cl to 1 atom of 37Cl. Draw the stick diagram for Chlorine

Wrong!Why?

The problem is that chlorine consists of molecules, not individual atoms.

When chlorine is passed into the ionisation chamber, an electron is knocked off the molecule to give a molecular ion, Cl2+. Doubly charges ions can also form.

These ions aren’t very stable, and some will fall apart to give a chlorine atom and a Cl+ ion. The term for this is fragmentation

Chlorine MS

Cl2+ Cl + Cl+

What can molecular chlorine ions (Cl2+ ) fragment into?

What happens to the Cl atom? If it doesn’t acquire a charge in the ionization chamber then it gets “lost” in the MS

What are the possible combinations of chlorine-35 and chlorine-37 atoms in a Cl2+ ion?

Both atoms could be 35Cl, both atoms could be 37Cl, or you could have one of each sort.

Masses of the Cl2+ ion: 35 + 35 = 7035 + 37 = 7237 + 37 = 74

What would the MS look like?

Chlorine MS …

Why is there no scale on the y-axis?

Because you cannot predict how the molecules will ionize and fragment

1.5 Electron configurations

Why is the periodic table broken up into sections? What links each of these sections?

The distribution of electrons within the shells is, in most cases, more complicated than simple spheres. The regions within the PT closely follow the patterns of these distributions – or probabilities of electron density

http://www.yellowtang.org/images/electrons_atoms_pos_c_la_784.jpg

The shells represent energy levels in atoms. Electrons can move between these levels, gaining or losing energy in the process.

SublevelsEach energy level is divided into one or more sublevels. These sublevels have energies that differ slightly from that of the shell energy.

How many types of sublevel are there?

(hint – think about he number of regions in the PT)

There are 4: s.p.d.fThe “s-block” comprises Groups 1 and 2The “p- block” comprises Groups 3 - 8

How many electrons can an s sublevel have in it?

How many electrons can a p sublevel have in it?

Edps

3

2

1

What is the significance of the order of the subshells?

Atomic orbitals

Sublevels aren’t all the same.

The s-sublevel has the lowest energy and so is filled first. It can hold a maximum of two electrons.

The s orbital is spherical and represents the probability of finding the electrons within its boundary

The p-, d- and f- sublevels are degenerate, ie further broken down into more sublevels of almost equivalent energy.

If a p-orbital can hold 6 electrons in total, how many degenerate orbitals are there?

Orbital shapes

Spin

Electron s are either spin up, or spin down – ie clockwise or anticlockwise. (corresponding to a spin quantum number of +1/2 and -1/2)

Degenerate orbitals of the same energy fill up first. Parallel spins go in first followed by antiparallel spin.

1s 2s 2px 2py 2pz

Nomenclature: the number of electrons in a particular orbital is denoted by superscript. e.g. 1s2 2s2 3p2

Electrons have a property called “spin”. This determines the way in which the degenerate levels are populated.

2

ELECTRONIC CONFIGURATIONthe arrangement of the electron in the atom.Electrons are arranged in Energy Levels or Shells around the nucleus of an atom.

nucleus

1 3 4

nlx

sd f

sp

sp d

ps

Atomic orbital

f = 7d = 5

p = 3s = 1

1 Atomic orbital = 2 e-

x 2 = 2x 2 = 6x 2 = 10x 2 = 14

2e- 8e- 32e-

Main energy level

Subenergy level

no.of electrons

18e-

Aufbau

Orbitals do not always fill up in the way expected

This is due to overlap in the energies of the sublevels

Distance from nucleus

Energy

1s

2s

2p

3s

3p

3d 4s

4p

4d

4f

Silicon (Si)

Look at the energy level diagram for Silicon. Which orbitals have energy levels which overlap?

The 4s orbital has an energy between that of the 3p and 3d orbitals. This means that the 4s orbital fills before the 3d orbital.

Filling orbitals

Electrons enter the lowest energy orbital available (Aufbau principle)

In the periodic table, the transition elements make up the “d-block”.

The first row in the d-block contains the 3d elements. These follow from the 4s elements, Potassium and Calcium.

Electrons prefer to occupy orbitals on their own, and only pair up when no empty orbitals of the same energy are available (Hund's Rule)

Complete worksheet 1.5

Questions1. Which orbital would the electrons fill first? The 2s or 2p orbital?

2. Can you have an electron in between two orbitals?

3. How many d orbitals are there in the d subshell?

4. How many electrons can the p orbital hold?

5. Why can two electrons occupy the same orbital?

1. The 2s orbital would be filled before the 2p orbital because orbitals that are lower in energy are filled first and the 2s orbital is lower in energy than the 2p orbital.

2. You cannot have an electron in between two orbitals. The electron will either be in one orbital or the next.

3. There are 5 d orbitals in the d subshell.4. A p orbital can hold 6 electrons.5. Two electrons can occupy the same orbital because they each have a

different spin. There cannot be two electrons that have the same exact orbital configuration and spin.

1.6 Ionization energy

Draw the electron configuration of oxygen

1s2 2s2 2p4

If oxygen was ionized, which electron would be removed first?

The antiparallel spin electron has a slightly higher energy. Due to the repulsion from the other electron in the 2px orbital.

Why?

Will the next electron be easier to remove?

Which one will it be?

The energy needed to remove this electron is known as the First Ionisation Energy (IE)

Successive ionisations require more and more energy

A logarithmic plot is needed for successive ionisation energies due to the scale. log 1 = 10log 5 = 100,000

Successive Ionisations

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

0 2 4 6 8 10 12 14 16 18 20

electron removed

log10 of ionisation

energy

Notice the “jump” in energy needed to remove the 2nd electron

Successive ionisation of potassium

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

0 2 4 6 8 10 12 14 16 18 20

electron removed

log10 of ionisation

energy

Successive ionisation energies for potassium

The different “jumps” are evidence for the arrangement of electrons in energy levels and sub-levels

level 1

level 2

level 3

level 4

0200400600800

1000120014001600

Na Mg Al Si P S Cl Ar

1st i

onis

atio

n en

ergy

(k

J/m

ol)

Periodicity of ionisation energy

What trend would you expect ionisation energy to have as you move across a period? B

A

C

What does region “A” represent?

2 x s electrons

What does region “B” represent?

3 x p electrons

Which three p electrons are these?

px1 py

1 and pz1

What else do you notice about the

graph?

The slopes of A, B and C are almost the same

Across the periodic table

Describe the graph

What causes the change in the pattern at A = 21

Predict the shape of a graph showing the trend of first ionization energy down a group

Trends of first ionization energy in groups

0

200

400

600

800

1000

Be Mg Ca Sr Ba

1st i

onis

atio

n en

ergy

Group 2

Explain why the first ionisation energy decreases as you move down a group

Describe the graph

The initial decrease is steep, but then the graph flattens out

ShieldingAs you move down a group, the distance of the outer electrons from the nucleus increases

The inner electrons also shield the outer electrons from the full effect of the positive nuclear charge and repel each other.

They are less tightly bound to the nucleus and so are more easily removed

+

e_

2) nuclear charge

1) distance from nucleus

3) shielding (repulsion) by electrons in inner shells between nucleus and outer electron

Question

Identify the groups that these atoms belong to

Group 4 – the jump is to remove the 5th electron

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

0 1 2 3 4 5 6 7

electron removed

kJ/mol

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 1 2 3 4 5 6 7

electron removed

kJ/mol

Group 2 – the jump is to remove the 3rd electron

Question

Identify the groups that these atoms belong to



0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 1 2 3 4 5 6 7

electron removed

kJ/mol

0

2000

4000

6000

8000

10000

12000

14000

0 1 2 3 4 5 6 7

electron removed

kJ/mol

Question

Identify the group that this atom belongs to

Group 1 – the jump is to remove the 2nd electron

The number of the electron whose removal causes a jump is one more than the group number that the element belongs to.

0

2000

4000

6000

8000

10000

12000

0 1 2 3 4 5 6 7

electron removed

kJ/mol

Write a general rule for identifying groups from the pattern in ionisation energy

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1.0 atomic structure

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