section 2 atomic structure_edited
TRANSCRIPT
-
7/29/2019 Section 2 Atomic Structure_edited
1/83
Atomic Structure
Cambridge A-level Chemistry
Centre of Pre-U Studies
Section
1
-
7/29/2019 Section 2 Atomic Structure_edited
2/83
Overview of Section 2
Content:
Describe the atom in terms of
protons and neutrons in the nucleusExplain and describe the electrons
in terms of energy levels, ionisation
energy, atomic orbitals andextranuclear structure
-
7/29/2019 Section 2 Atomic Structure_edited
3/83
Defining the Atom
The Greek philosopherDemocritus (460B.C. 370 B.C.) was among the first to
suggest the existence of atoms (from
the Greek word atomos) He believed that atoms were indivisible and
indestructible
His ideas did agree with later scientifictheory, but did not explain chemical
behavior, and was not based on the
scientific method but just philosophy
-
7/29/2019 Section 2 Atomic Structure_edited
4/83
Viewing Surface of metal using STM
Blue platinum
Blue Nickel
-
7/29/2019 Section 2 Atomic Structure_edited
5/83
Daltons Atomic Theory (experiment based)
3) Atoms of different elements combine in
simple whole-number ratios to formchemical compounds
4) In chemical reactions, atoms are combined,
separated, or rearranged but never
changed into atoms of another element.
1) All elements are composed of
tiny indivisible particles calledatoms
2) Atoms of the same element are
identical. Atoms of any one
element are different fromthose of any other element.
John Dalton
(1766 1844)
-
7/29/2019 Section 2 Atomic Structure_edited
6/83
Section 2
Structure of the Nuclear Atom
Review:
Discovery of subatomic
particles.
-
7/29/2019 Section 2 Atomic Structure_edited
7/83
Thomsons Atomic Model
Thomson believed that the electronswere like plums embedded in a
positively charged pudding, thus it
was called the plum pudding model.
J. J. Thomson
-
7/29/2019 Section 2 Atomic Structure_edited
8/83
Ernest Rutherfords
Gold Foil Experiment - 1911
Alpha particles are helium nuclei -
The alpha particles were fired at a thinsheet of gold foil
Particle that hit on the detecting
screen (film) are recorded
-
7/29/2019 Section 2 Atomic Structure_edited
9/83
Rutherfords Findings
a) The nucleus is smallb) The nucleus is dense
c) The nucleus is positively
charged
Most of the particles passed right through
A few particles were deflected VERY FEW were greatly deflected
Like howitzer shells bouncing
off of tissue paper!
Conclusions:
-
7/29/2019 Section 2 Atomic Structure_edited
10/83
The Rutherford Atomic Model Based on his experimental evidence:
The atom is mostly empty spaceAll the positive charge, and almost all
the mass is concentrated in a small area
in the center. He called this a nucleusThe nucleus is composed of protons
and neutrons
The electrons distributed around thenucleus, and occupy most of the volume
His model was called a nuclear model
-
7/29/2019 Section 2 Atomic Structure_edited
11/83
-
7/29/2019 Section 2 Atomic Structure_edited
12/83
Section 2
Structure of the Nuclear Atom
One change to Daltons atomic
theory is that atoms are divisible into
subatomic particles:Electrons, protons, and neutrons are
examples of these fundamental
particles
There are many other types of
particles, but we will study these three
-
7/29/2019 Section 2 Atomic Structure_edited
13/83
Discovery of the ElectronIn 1897, J.J. Thomson used a cathode ray
tube to deduce the presence of a negativelycharged particle: the electron
-
7/29/2019 Section 2 Atomic Structure_edited
14/83
-
7/29/2019 Section 2 Atomic Structure_edited
15/83
Question May 2002
Sir James Jean who was a great populariser ofscience, once described an atom of carbon asbeing like six bees buzzing around a space thesize of a football stadium.
(a)(i) suggest what were represented by the sixbees in this description.
(ii) explain(in terms of an atom of carbon) whatstopped the bees from flying away from the
space of the football stadium.(iii) what is missing from Jeans description whenapplied to an atom of carbon?
-
7/29/2019 Section 2 Atomic Structure_edited
16/83
Atomic particles Section 2(a),(b),(c)
Identify and describe
protons,neutrons and electrons interms of their relative charges and
relative masses.Deduce the behavior of beams of
protons, neutrons and electrons in
electric fields.
Describe the distribution of mass
and charges within an atom
-
7/29/2019 Section 2 Atomic Structure_edited
17/83
Mass of the Electron
1916 Robert Millikan determines the mass
of the electron: 1/1840 the mass of a
hydrogen atom; has one unit of negative
charge
The oil drop apparatus
Mass of the
electron is
9.11 x 10-28 g
-
7/29/2019 Section 2 Atomic Structure_edited
18/83
Conclusions from the Study
of the Electron:
a) Cathode rays have identical properties
regardless of the element used to
produce them. All elements must contain
identically charged electrons.
b) Atoms are neutral, so there must be
positive particles in the atom to balance
the negative charge of the electronsc) Electrons have so little mass that atoms
must contain other particles that account
for most of the mass
-
7/29/2019 Section 2 Atomic Structure_edited
19/83
Conclusions from the Study
of the Electron:
Eugen Goldstein in 1886 observed
what is now called the proton -
particles with a positive charge, anda relative mass of 1 (or 1840 times
that of an electron)
1932James Chadwick confirmedthe existence of the neutron a
particle with no charge, but a mass
nearly equal to a proton
-
7/29/2019 Section 2 Atomic Structure_edited
20/83
Summary Table
Therefore
Modern picture of an atom, then, consist of three types of particles-electrons, protons
and neutron.
Electric Charge MassParticle SI (C ) Atomic SI (g) amu Located
Electron -1.602x10-19 -1 9.109x10-28 5.49x10-4 outside nucleus
Proton +1.602x10-19 +1 1.673x10-24 1.0073 in nucleus
Neutron 0 0 1.675x10-24 1.0087 in nucleus
-
7/29/2019 Section 2 Atomic Structure_edited
21/83
Subatomic Particles
Particle RelCharge
Rel Mass Location
Electron
(e-
) -1 1/1840 Electron
cloud
Proton
(p+)+1
1
NucleusNeutron
(no) 0 1 Nucleus
-
7/29/2019 Section 2 Atomic Structure_edited
22/83
Deflection of atomic particles
Interaction of charged particles in electricfield
Non-interaction of Non-charged particles
in electric field Direction of deflection depends on the
charge of the particle
Magnitude of deflection depends on themass of the atomic particles when themagnitude of charge is the same
-
7/29/2019 Section 2 Atomic Structure_edited
23/83
Question 1.Beams of particles traveling at the samespeed from different sources are subjected to an electricfield as shown in the diagram below. A beam of neutronshas already been drawn.
Sketch on the diagram above how beams of each of the following
particles are affected by the electric field: (i) protons; (ii) electrons; (iii)2
1H
Label each of the beams.
Explain briefly the position and shape of each beam: (i) protons;(ii)electrons and (iii) 21H
+-
n
-
7/29/2019 Section 2 Atomic Structure_edited
24/83
Section 2(d)
Learning Outcomes
Deduce the numbers of protons, neutrons and
electrons present in both atoms and ions
given proton and nucleon numbers( andcharge)
-
7/29/2019 Section 2 Atomic Structure_edited
25/83
Proton Number
Atoms are composed of protons,neutrons, and electrons
How then are atoms of one element
different from another element? Elements are different because they
contain different numbers ofPROTONS
The proton number of an element is
the number of protons(#) in the nucleus
# protons in an atom = # electrons
-
7/29/2019 Section 2 Atomic Structure_edited
26/83
Proton Number
Proton number (Z) of an element isthe number of protons in the nucleus
of each atom of that element.
Element # of protons Z
Carbon 6 6
Phosphorus 15 15
Gold 79 79
N l N b (A)
-
7/29/2019 Section 2 Atomic Structure_edited
27/83
Nucleon Number(A)
Nucleon number is the number of
protons and neutrons in the nucleus
of an isotope: A = p+ + n0
Nuclide p+ n0 e- A
Oxygen - 10
- 33 42
- 31 15
8 8 1818
Arsenic 75 33 75
Phosphorus 15 3116
-
7/29/2019 Section 2 Atomic Structure_edited
28/83
Complete Symbols
Contain the symbol of the element,the nucleon number and the proton
number.
XNucleonNumber(A)
Proton
Number(Z)Subscript
Superscript
-
7/29/2019 Section 2 Atomic Structure_edited
29/83
Symbols
Find each of these:a) number of protons
b) number ofneutrons
c) number of
electronsd) Proton number
e) Nucleon Number
Br8035
-
7/29/2019 Section 2 Atomic Structure_edited
30/83
Symbols
If an element has an protonnumber of 34 and a nucleon
number of 78, what is the:
a) number of protons
b) number of neutrons
c) number of electrons
d) complete symbol
-
7/29/2019 Section 2 Atomic Structure_edited
31/83
Symbols
If an element has 91protons and 140 neutrons
what is thea) Proton number
b) Nucleon numberc) number of electrons
d) complete symbol
-
7/29/2019 Section 2 Atomic Structure_edited
32/83
Symbols
If an element has 78electrons and 117 neutrons
what is thea) Proton number
b) Nucleon numberc) number of protons
d) complete symbol
-
7/29/2019 Section 2 Atomic Structure_edited
33/83
Note
In a neutral atom , the number of electronsis equal to the number of protons.
In a negative ion (anion), the number of
electrons is greater than the number ofprotons since a negative ion is obtained by
adding electron(s) to a neutral atom.
In a positive ion(cation), the number ofelectrons is less than the number of
protons since a positive ion is obtained by
removing electron(s) from a neutral atom.
-
7/29/2019 Section 2 Atomic Structure_edited
34/83
Ions
is an electrically charged particle obtained from an atom or
chemically bonded group of atoms lose or gain electrons.
The charge on an ion is equal to the # of protons minus the #of electrons. An atom that gains extra electrons becomes a
negatively charged ion, called an anion. An atom that loses
electrons becomes positively charged ion, called a cation.
EAZ
number p + number n
number p
+ ? number p - number
E.g. Determine numbers of electrons in Mg2+ cation and the S2-
anion?
Mg2+ 12-number e = +2 number e =10
S2- 16-number e = -2 number e =18
-
7/29/2019 Section 2 Atomic Structure_edited
35/83
May/June 2008
Skin cancer can be treated using a radioactive isotope ofphosphorus,3215P . A compound containing the phosphideion 3215P
3- , is wrapped in a plastic sheet, is strapped to theaffected area.
What is the composition of the phosphide ion, 3215P3 ?
P32
15
332
15P
332
15P
Protons Neutrons electrons
A 15 17 18
B 15 17 32C 17 15 17
D 32 17 15
-
7/29/2019 Section 2 Atomic Structure_edited
36/83
Section 2.3(e)(II)
Distinguishing Among Atoms
Learning Outcome:
Distinguish between isotopeson the basis of different
numbers of neutrons present
-
7/29/2019 Section 2 Atomic Structure_edited
37/83
Isotopes Dalton was wrong about all elements of
the same type being identicalAtoms of the same element can have
different numbers ofneutrons.( see
next slide) Thus, different nucleon numbers.
These are called isotopes.
Isotopes have the same chemicalproperties but different properties(e.gdenstiy, mass)
-
7/29/2019 Section 2 Atomic Structure_edited
38/83
IsotopesElements
occur innature as
mixtures of
isotopes.
-
7/29/2019 Section 2 Atomic Structure_edited
39/83
Naming Isotopes
We can also put the nucleonnumberafterthe name of the
element:carbon-12
carbon-14uranium-235
-
7/29/2019 Section 2 Atomic Structure_edited
40/83
Isotopes are atoms of the same element having
different masses, due to varying numbers ofneutrons.
Isotope Protons Electrons Neutrons Nucleus
Hydrogen1
(protium) 1 1 0Hydrogen-2
(deuterium) 1 1 1
Hydrogen-3
(tritium)
1 1 2
-
7/29/2019 Section 2 Atomic Structure_edited
41/83
-
-
i) Define the term isotopes.
[2]
ii) Br has 2 main isotopes. Complete the table below show the atomic structure of these
isotopes. [2]
Isotope Protons Neutrons Electrons
79Br81Br
iii) The percentage composition by mass of the isotopes in part (b) is 50.5% 79Br and
49.5%81
Br. Use this data to calculate the relative atomic mass of bromine to threesignificant figures.
[2]
-
7/29/2019 Section 2 Atomic Structure_edited
42/83
Other types of particles
Our understanding of atom is still
in growing. The unknowns are
yet to be discovered.
-
7/29/2019 Section 2 Atomic Structure_edited
43/83
Question May/June 2007
John Daltons atomic theory, published in 1808,contained four predictions about atoms.
Which of his predictions is still considered to be
correct
A Atoms are very small in size
B No atom can be split into simpler parts
C All the atoms of a particular element have the
same massD All the atoms of one element are different in
mass from all the atoms of the other elements
-
7/29/2019 Section 2 Atomic Structure_edited
44/83
Question31 May/June 2007
Use of the Data Bookletis relevant to this
question.
The technetium-99 isotope( 99Tc) is radioactive
and has been found in lobsters and seaweedadjacent to nuclear fuel reprocessing plants
Which statements are correct about an atom of99Tc?
1. It has 13 more neutrons than protons.
2. It has 43 protons.
3. It has 99 nucleons.
-
7/29/2019 Section 2 Atomic Structure_edited
45/83
Orbitals Section 2(f)
Learning Outcomes
Describe the number and relative
energies of the s,p, and d orbitalsfor the principal quantum numbers
1, 2, and 3 and also the 4s and 4p
Describe the shapes of s and porbitals
-
7/29/2019 Section 2 Atomic Structure_edited
46/83
What is an orbital?
An atomic orbital is a region of space(3dimensional) round the nucleus in which theprobability of finding a particular electron( in afree atom) is the greatest.
Electrons can occupy four types of orbital, whichdiffer from each other in shape and in theirorientation in space. These are called s,p, d andf orbitals. s orbitals are spherical
p orbitals are dumb-bell-shaped and can be arrangedin different directions.
-
7/29/2019 Section 2 Atomic Structure_edited
47/83
Shapes of the orbitals
s,p and d- orbitals
-
7/29/2019 Section 2 Atomic Structure_edited
48/83
Question
Sketch the shapes of and spatial distributions of all
the occupied orbitals in nitrogen
-
7/29/2019 Section 2 Atomic Structure_edited
49/83
Answers
1s
2s
2pz 2px 2py
-
7/29/2019 Section 2 Atomic Structure_edited
50/83
Modern Atomic Theory
The Principal Quantum Number (n)
Represents the energy level of highest
probability.
n = 1
n = 2
n = 3
-
7/29/2019 Section 2 Atomic Structure_edited
51/83
Analogy
A ladder
A bookshelf
View video on atom
-
7/29/2019 Section 2 Atomic Structure_edited
52/83
What is a shell?
A shell is a group of orbitals that are about
the same distance out from the nucleus.
Shells are numbered starting with the shell
nearest to the nucleus and working outwards
Each successive shell has a different number
of orbitals in it. In the nth shell, there are n
sub-shells, n
2
orbitals and a maximum of 2n
2
electrons.
-
7/29/2019 Section 2 Atomic Structure_edited
53/83
No. of shells and No. of electrons
shell No. of
orbitals(n2)
s p d f Max. no. of electrons
in shell( 2n2)
n=1 1 1 2
n=2 4 1 3 8
n=3 9 1 3 5 18
n-=4 16 1 3 5 7 32
-
7/29/2019 Section 2 Atomic Structure_edited
54/83
What is a sub-shell?
A sub-shell is a group of orbitals with the
same energy level, but differ in their
orientation in space, e.g. the second shell
(n=2) contains two sub-shells: a sub-shell containing one s orbital, and
a sub-shell containing three p orbitals:
2px,2py,2pz
-
7/29/2019 Section 2 Atomic Structure_edited
55/83
Section 2(h)
Learning Outcome
State the electronic configuration of atoms
and ions given the proton number( and
charge)
Wh t i t b l t i
-
7/29/2019 Section 2 Atomic Structure_edited
56/83
What is meant by electronic
configurations?
The electronic configuration of an element
describes how the element of its atoms
are arranged in their shells, sub-shells and
orbitals.
-
7/29/2019 Section 2 Atomic Structure_edited
57/83
Filling of orbitals
In an atom, the orbitals are filled in order
of increasing energy(starting from 1s)
according to the following rules.
-
7/29/2019 Section 2 Atomic Structure_edited
58/83
Electronic Energy Level
An Energy Diagram
n = 1 1s
n = 2
2s ___
2p ___ ___ ___
N = 3
3s ___
3p ___ ___ ___
3d ___ ___ ___ ___ ___
Rules for assigning electrons
1. Occupy lowest levels
first.(Aufbau Principle)
2. Allowed 2 electrons in
each orbital. If 2, must
have opposite spins. [Pauli
Exclusion Principle]
3. Fill sub-level before
moving up.(Hunds Rule of
Multiplicity)
4. 1 electron in each orbital of
the sameenergy(degenerate) before
doubling up. To keep
electron spins the same
before thay are occupied in
pair)
-
7/29/2019 Section 2 Atomic Structure_edited
59/83
Electronic Configurations
The Electron Configuration
A short hand method for notating the electronic
structure of atoms.
Write the electron configuration for sodium.
Sodium has 11 electrons. We know this
because it has an atomic number of 11 which
means it has 11 protons and thus must have 11
electrons.
1s22s2p63s1 or [Ne]3s1
-
7/29/2019 Section 2 Atomic Structure_edited
60/83
Modern Atomic Theory
An Energy Diagram
n = 1 1s
n = 2
2s ___
2p ___ ___ ___
n = 3
3s ___
3p ___ ___ ___
3d ___ ___ ___ ___ ___
1s22s22p63s1
-
7/29/2019 Section 2 Atomic Structure_edited
61/83
Question Oct 2001
Give the full electron configuration of the
following
(i) Mg
(ii) Mg 2+
(iii) O
(iv)
O
2-
-
7/29/2019 Section 2 Atomic Structure_edited
62/83
Question
Construct a schematic diagram representing the
number and relative energies of the orbitals of the
first three principal quantum numbers.
-
7/29/2019 Section 2 Atomic Structure_edited
63/83
Answers
Energy
3d
3p
3s
2p
2s
1s
-
7/29/2019 Section 2 Atomic Structure_edited
64/83
Question
Show the electronic configuration of
nitrogen using the diagram in the previous
answer
-
7/29/2019 Section 2 Atomic Structure_edited
65/83
Answer
Energy 2p
2s
1s
-
7/29/2019 Section 2 Atomic Structure_edited
66/83
Isoelectronic
Atoms that have the same number of
electrons are known as isoelectronic.
-
7/29/2019 Section 2 Atomic Structure_edited
67/83
Oct/Nov 2007
Use the Data Booklet is relevant to this question.In forming ionic compounds, elements
generally form an ion with the electronic
structure of a noble gas.
Which ion does not have a noble gas
electronic structure?
A. I- B Rb+ C Sn 2+ D Sr2+
Representing electronic
-
7/29/2019 Section 2 Atomic Structure_edited
68/83
Representing electronic
configuration
By using electrons-in-boxes
Using a noble gas core
Using the s, p, d and f notation
Using energy levels
-
7/29/2019 Section 2 Atomic Structure_edited
69/83
Ionisation Energy Sect.2(j)
Learning Outcomes
(i) explain and use of the term ionisation
energy
(ii) explain the factors influencing theionisation energies of elements
(iii) explain the trends in ionisation energies
across a period and down a group of the
Periodic Table.
-
7/29/2019 Section 2 Atomic Structure_edited
70/83
Definition of IE
The energy to remove an electron from
each atom/ion in a mole of atoms/ions of
an element in the gaseous state
Refer to data booklet for successive IEenergies
Consecutive IE of a single
-
7/29/2019 Section 2 Atomic Structure_edited
71/83
Consecutive IE of a singleelement or atom
Generally increase of IE is due to the
consecutive electron removed because it
is harder to remove electrons from the
more positive ion. Large rise of IE implies that electron are
removed from a different principal shell.
Small increase of IE represent change insubshell.
-
7/29/2019 Section 2 Atomic Structure_edited
72/83
Explanation of IE Explain large rise in IE. When all valence shell
electrons are removed, only the inner shellremains. The inner shell experience much lessshielding , resulting in a higher nuclear attractionand therefore lead to a much higher IE big
jump Explain small increase in IE. Electrons in higher
level subshell removed first; after whichelectrons from lower energy subshell removed.
For example, 3p electrons are removed before3s. Lower subshell electron experience slightlyhigher nuclear attraction and therefore slightlyhigher IE small jump
-
7/29/2019 Section 2 Atomic Structure_edited
73/83
Factors influencing IE
The size of the positive nuclear charge
The distance of the electron from the
nucleus
The shielding effect by electrons in filled
inner shells
-
7/29/2019 Section 2 Atomic Structure_edited
74/83
Explaining the IE trends
Increase across the period
Across the period, nuclear attraction on the
valence electron increases. This is due to the
increase in proton number,Z but a fairlyconstant shielding
Decrease down the group
Down the group, nuclear attraction on thevalence electron decreases. This is due to
great increase in Screening Effect,SE with
atom having more inner shells.
-
7/29/2019 Section 2 Atomic Structure_edited
75/83
IE trend ACROSS period
IE depends on the effect of the nuclear attraction on thevalence electron.
Across the period, IE increases since the proton numberincreases but the shielding effect remains fairly constant.
IE do not increase uniformly. Small kink may beproduced, where the IE is lower than normal. 2 possiblereasons(i) electron is removed from the higher energysubshell. Less energy is required to remove p-electron(ii) paired vs unpaired electron is removed. Paired
electron experiences repulsion makes a removal easier(use only in p orbital)
Always write out the electronic configuration to figure outthe reason for non-uniformity.
-
7/29/2019 Section 2 Atomic Structure_edited
76/83
IE down the group
IE down the group will decreasedramatically, since the effect of the nuclear
attraction on the valence electron
decreases significantly due to increase innumber of principal quantum electron
shells(inner shells) . Outer shell
experiences much higher shielding effectwhich outweighs(greater weight attached
to the effect) increase of nuclear charge.
SE > Z
-
7/29/2019 Section 2 Atomic Structure_edited
77/83
Importance of IE
To predict or confirm the simple electronicconfigurations of elements
An example. the first four IE of an element
are, in kJ per mol: 590,1150,4940,6480.Suggest the Group in the Periodic table to
which this element belongs
Oct/Nov 2007
-
7/29/2019 Section 2 Atomic Structure_edited
78/83
The graph show the first thirteen IE ionisation energies for
element X
What can be deduced about element X from the graph?
A It is in the second period (Li-Ne) of the Periodic Table
B It is a d-block element
C It is in the Group II of the Periodic Table
D It is in Group III of the Periodic Table
i) Define the term first ionisation energyand write an equation to represent the first
-
7/29/2019 Section 2 Atomic Structure_edited
79/83
) gy q p
ionisation energy of chlorine.
[3]
Definition
ii) Write an equation to show the fourthionisation energy of nitrogen.
iii) State and explain the trend in first ionisation energies down group 2 in the Periodic
Table.
Equation [2]
Equation [2]
[4]
The successive ionisation energies for nitrogen are show below.
-
7/29/2019 Section 2 Atomic Structure_edited
80/83
g g
Ionisation number 1st 2nd 3rd 4th 5th 6th 7th
Ionisation energy / kJmol1 1402 2856 4578 7475 9445 53268 64362
i) Explain why the successive ionisation energies show a general increase.
ii) What can be deduced about the electronic structure of the nitrogen atom? Explain
your deductions carefully.
[2]
[4]
i) Define the term isotope.
ii) B h 2 i i C l h bl b l h h i f h
-
7/29/2019 Section 2 Atomic Structure_edited
81/83
Mark schemei) Substances consisting of atoms with same atomic number / same number of protons / of the
same element 1but with different mass number / different numbers of nucleons / different neutron numbers
1
ii)
2 marks all correct 1 mark1 row or 2 columns correct
iii) Ar= (79 x 0.505) + (81 x 0.495) 1
= 79.99% 80.0% to 3 sig. figs. 1
Isotope Protons Neutrons Electrons
79Br 35 7935 = 44 35
81Br 35 8135 = 46 35
ii) Br has 2 main isotopes. Complete the table below show the atomic structure of these
isotopes.
iii) The percentage composition by mass of the isotopes in part (b) is 50.5% 79Br and
49.5% 81Br. Use this data to calculate the relative atomic mass of bromine to three
significant figures.
i) Define the term first ionisation energyand write an equation to represent the first
-
7/29/2019 Section 2 Atomic Structure_edited
82/83
ionisation energy of chlorine. [5]
ii) Write an equation to show the fourthionisation energy of nitrogen. [2]
iii) State and explain the trend in first ionisation energies down group 2 in the Periodic
Table. [3]
Mark schemei) Definition: Energy required to remove first electron from each atom 1
in one mole of 1
gaseous atoms (to form one mole of gaseous+1 ions) 1Equation: Cl (g)Cl + (g) + e 1 for species, 1 for state symbols
ii) N +3 (g)+4 (g) + e 1 for species, 1 for state symbolsiii) First ionisation energies decrease as group 2 is descended 1
Number of shells and hence atomic radii increase down the group 1
Greater number of shells means shielding increases 1
Overall, the outer shell electrons are further from the nucleus and less stronglyattracted to it 1
These factors are more important than the increase in nuclear charge 1
Maximum 4 marks
The successive ionisation energies for nitrogen are show below.
-
7/29/2019 Section 2 Atomic Structure_edited
83/83
Mark scheme
i) Negative electrons are being removed from an increasingly positive ion / there is agreater proton to electron ratio. 1
The attraction between the nucleus and the remaining electrons increases. 1
Also, the repulsion amongst the remaining electrons decreased 1
Maximum 2 marks
ii) There is a large increase in ionisation energy when the 6th electron is removed. 1 The 6th
electron is removed from a new shell 1 that is closerto the nucleus with less shielding. 1 Within a
shell, the increase in ionisation energy is much smaller. 1 So nitrogen
has 5 electrons in outer shell and 2 in shell closer to the nucleus. 1
Maximum 4 marks
i) Explain why the successive ionisation energies show a general increase. [2]
ii) What can be deduced about the electronic structure of the nitrogen atom? Explain
your deductions carefully. [4]
643625326894457475457828561402Ionisation energy / kJmol1
7th6th5th4th3rd2nd1stIonisation number