C 06J
General Chemistry
Course Outline (7 lectures)
The Structure of the Atom
Atomic particles: protons, neutron and electrons
Mass Relationships of Atoms
Atomic number, mass number, isotopes, mass and atomic spectroscopy
(line spectra- Rydberg equation and the Balmer series)
The Electronic Structure of the Atom
Bohr model
The wave nature of the electron
Quantum numbers, atomic orbitals, electronic configuration
The periodic table
Atomic Structure
Matter
Anything that has mass and occupies space
It exists in three states: solid, liquid and gas
Element
A substance that cannot be chemically broken down
further
For example, hydrogen, sodium, chlorine, silicon
Atomic Structure
The Greek philosopher proposed that matter
is made up tiny particles called atoms
Atom: derived from the Greek word Atomos
(indivisible)
Dalton’s Atomic Theory
John Dalton (1766-1844)
Elements are made up of tiny particles called atoms
Atoms of the same element have the same mass (definite
average mass), while atoms of different elements have
different masses
Atoms of the same element are chemically alike and atoms
different elements are chemically different. Elements
combine chemically in whole number ratios to form
different substances
Atoms are not changed during chemical reactions
This theory does not explain what the atom is made up of
Structure of the Atom
Evidence supporting the structure of the atom was
gathered using radioactive elements
Several scientists conducted experiments that led to
what is known today as the structure of the atom
The atom consists of a positively charged nucleus
The nucleus is very small and dense and has a
diameter of 10-12 cm
1 Angstrom (Å) = 10-8 cm
Therefore diameter of the nucleus is 10-4 Å
Diameter of the atom is 1-5 Å
Structure of the atom
Evidence supporting the
presence of a nucleus
Experiment conducted by English physicist Ernest Rutherford
In an evacuate tube, a beam of alpha particles were directed at a thin
gold sheet
Most of the particles passed through the sheet
A few were slightly deflected
Very few were greatly deflected (1 in 20,000)
It was assumed that the alpha particles bounced back if they approached
a positively charged nucleus head on
Those that were slighly deflected passed close to the nucleus
Most particles passed straight through the gold foil, which implied that
the atom consists of a lot of space
Rutherford’s Experiment
www.sol.sci.uop.edu/~jfalward/physics17/chapter13/chapter113.html
The Nucleus
Except for the hydrogen atom, the nucleus of every atom
consists of two types of particles: protons and neutrons
Protons
Positively charged
Mass: 1.673 x 10-24 g
The positive charge is equal in magnitude to the negative
charge on the electron
In each atom # of protons = # of electrons
As a result the atom is neutral
The Nucleus
Neutrons
Discovered by English scientist James C. Chadwick in 1932
Neutral particles (no electric charge)
Mass: 1.6375 x 10-24 g
Thomson’s Experiment
This experiment involved the use of a cathode ray tube
This is a glass tube from which the air has been removed and with two pieces of
metals called electrodes attached
An electric current flows through the tube when a sufficient voltage is applied
The current flow is from the cathode (negatively charged) to the anode
(positively charged)
If the tube is not fully evacuated and still contains small amounts of air or other
gases, the current flow is visible as a glow called a cathode ray
This beam is produced at the negative electrode and is deflected towards the
positive electrode
The beam can also be deflected by a magnet or an electrically charged plate
Thomson proposed that the cathode ray must consist of tiny negatively charged
particle called electrons
Being emitted from different kinds of metal, elements must contain electrons
Thomson’s Cathode Ray Tube Experiment
http://online.cctt.org.physicslab/content/phy2HON/lessonnotes/modern/electronbeams.asp
Electrons
Discovered by English scientist J. J.
Thompson
These are negatively charged particles
They move about the nucleus with different
energies
Mass: 9.110 x 10-28 g
Radius: 2.818 x 10-13 cm
Structure of the Atom
Comparison of Subatomic
Particles
Particle
Mass/g
Mass/amu Charge/C Charge/e
Electron
9.109 x 10-28
5.486 x 10-4
-1.602 x 10-19
-1
Proton
1.673 x 10-24
1.007
+1.602 x 10-19
+1
Neutron
1.675 x 10-24
1.009
0
0
Atomic and Mass Number
Atomic Number (Z)
The number of protons present in the nucleus
The number of electrons around the nucleus
Mass number (A)
The sum of the number of protons (Z) and neutrons
(N) present in the nucleus
A=Z+N
Isotopes
The number of protons is not always equal to the number of
neutrons
Therefore atoms of the same element can have different
mass numbers
These are called isotopes
For example, Hydrogen has three isotopes
Isotopes behave almost identical in their chemical reactions,
the number of neutrons present has very little effect on the
atoms chemical property
The chemical property is determined by the number of
electrons present
Isotopes of Hydrogen
Protium
1 proton
0 neutron
1 electron
http:/encarta.msn.com/media_461531710_-1_1/Hydrogen_Isotopes.html
Exercise
How many protons, neutrons and electrons
are present in this isotope of uranium 23592U?
Answer
92 protons
92 electrons
143 neutrons
Atomic Weight
An element’s atomic weight is a weighted average of the
isotopic masses of the element’s naturally occurring
isotopes
The mass of an atom is extremely small in grams and so is
measured in atomic mass units (amu) instead
1 amu = 1/12 mass of carbon-12 = 1.66054 x 10-24 g
For example, carbon
Exercise
Copper metal has two naturally occurring
isotopes copper-63 (69.17%, isotopic mass
62.94 amu) and copper-65 (30.83%, isotopic
mass 64.93 amu). Calculate the atomic
weight of copper.
Electromagnetic Spectrum
Made up of a range of electromagnetic
radiation
Electromagnetic radiation is characterised by
frequency, wavelength and amplitude
www.cem.msu.edu/~reusch/virtualtext/spectrpy/UV-Vis/spectrum.htm
Visible Light/White Light
www.cem.msu.edu/~reusch/virtualtext/spectrpy/UV-Vis/spectrum.htm
www.cem.msu.edu/~reusch/virtualtext/spectrpy/UV-Vis/spectrum.htm
Wave Characteristics
Frequency (ν): number of wave peaks or
cycles per unit time. Units: s-1 or Hertz
Wavelength (λ): distance from one crest or
wave peak to another. Units: nm
Amplitude: the height of the wave from the
center line between peaks and troughs
Wave Characteristics
www.cem.msu.edu/~reusch/virtualtext/spectrpy/UV-Vis/spectrum.htm
Waves
The rate at which electromagnetic radiation travels
is referred to as the speed of light c
Speed of light = 2.997 x 108 m/s
Wavelength (m) x Frequency (s-1) = Speed (m/s)
c=λxν
λ and ν are inversely related
The light blue glow given off by mercury lamps has
a wavelength of 436 nm. What is its frequency?
Exercise
What is the wavelength (in meters) of an FM
radio wave with frequency ν = 102.5 MHz
and a medical X ray with ν = 9.55 x 1017 Hz?
Last Lecture
Structure of the atom
Subatomic particles
Rutherford’s experiment
Thomson’s experiment
Atomic number
Mass number, atomic mass unit
Isotopes
Electromagnetic spectrum
Waves characteristics (c = λ ν)……..λ and ν are inversely related
Atomic Spectra
Visible light consists of a continuous distribution of wavelengths, i.e.400
(violet)-700 (red) nm
These different wavelengths travel at different rates through a prism
resulting in the splitting of of white light into the various different
colours
When atoms are excited they give off visible light
This visible light given off by an excited atom is not in the form of a
continuous distribution as with light from the sun
Instead it consists of only a few wavelengths
This results in a series of lines and blank spaces called a line spectrum
Line Spectra
http://neon.chem.uidaho.edu/~honours/spectra
http://csep10.phys.utk.edu/astra162/lect/light/absorption.html
Atomic Spectra
Each element has its own line spectrum
Na: yellow light
K: purple light
For hydrogen, a spectrum of four lines is produced at 656.3
nm (red), 481.6 nm (blue-green), 434.0 nm (blue) and 410.1
nm (indigo).
According to Johann Balmer these wavelengths could be
expressed as:
1/λ = R {1/22 – 1/n2} (Balmer Equation)
R = Rydberg constant (1.097 x 10-2 nm-1)
n = integer greater then 2
Atomic Spectra
Line spectra are also present in the non-visible region of the
spectrum
These give rise to spectral line called Lyman series (far UV)
as well as Paschen, Brackett and Pfund series (IR region)
Those resulting from the visible region are referred to as the
Balmer series
Jahannes Rydberg used the Balmer equation to show that
every aspect of the spectrum can fit the equation
This resulted in the Balmer-Rydberg equation
Balmer-Rydberg Equation
1/λ = R {1/m2-1/n2}
Lyman series, m = 1
Balmer series, m = 2
Paschen series, m = 3
m and n are integers, where n is greater than
m
Points to note
Light can behave like waves as well as small
particles (so far we have looked at the wave characteristics)
Excited electrons do not give off/emit light of
continuous wavelength
As a result, this does not give a continuous
spectrum
Light of specific energy is emitted (quantum)
giving rise to a line spectrum
Questions
What are the two longest wavelengths lines
(in nm) in the Lyman series of the hydrogen
spectrum?
What is the shortest wavelength line (in nm)
in the Lyman series of the hydrogen
spectrum?
Exercise
The Balmer equation can be extended
beyond the visible portion of the
electromagnetic spectrum to include lines in
the ultraviolet. What is the wavelength (in
nanometers) of ultraviolet light in the Balmer
series corresponding to a value of n = 7?
Particle-like Properties of
Electromagnetic Radiation
Blackbody radiation: visible glow given off by
objects when heated
The intensity of black body radiation varies with
the wavelength of the light emitted
As an object is heated, the colour change is dull red
bright orange white
The intensity does not rise indefinitely
Instead, it reaches a maximum (λ = 500 nm) and
then falls rapidlyIntensity of Blackbody Radiation
vs. Wavelength
Explanation
The intensity of black body radiation does not rise
indefinitely
Therefore the energy given off cannot be
continuous
It is quantised, i.e. emitted in discrete units
called quanta
The energy of this radiation depends on the
frequency
Planck Equation
E = hν
h = Planck’s constant (6.626 x 10-34 Js)
We already know that ν = c/λ
Therefore E = hc/λ
Question
Calculate the energy of one quantum of red
light of frequency 4.62 x 1014 s-1 and
wavelength 649 nm.
Recall
Shorter wavelengths imply high frequency
Longer wavelengths imply low frequency
The energy of radiation/light depends on the
frequency NOT intensity.
Exercise
What is the energy of photons of FM
radiowaves with ν = 102.5 MHz
Particle-like Properties of
Electromagnetic Radiation
For an electron to be ejected from the surface of a
metal the light or energy supplied must be above a
certain threshold value
This threshold is different for every metal
Einstein assumed that a beam of light behaves like a
stream of particles with specific amounts of
energies/frequencies
These particles are called photons
If the frequency is not above the threshold value,
then no electron is ejected
Wave-like Properties of Matter
de Broglie suggested that if light can behave like
matter (having particle characteristics), then matter can also
exhibit some properties similar to those of light (i.e.
wave-like properties)
Both have wave and particle characteristics
Energy, E = mc2
(Einstein)
m = E/c2
m = mass of photon, c = speed of light
According to Planck, E = hc/λ
hc/ λ = mc2
de Broglie Equation
de Broglie suggested that this equation could be
applied to an electron by replacing the speed of
light (photon) c with that of the speed of the
electron, v
As a result, the wavelength of an electron can be
calculated
Hence:
de Broglie equation λ = h/mv
Question
What is the de Broglie wavelength of an
electron of mass 9.11 x 10-31 kg and velocity
2.2 x 106 ms-1?
Exercise
What is the de Broglie wavelength of a
pitched baseball with a mass of 120 g and a
speed of 100 mph (44.7 m/s)?
Exercise
What is the de Broglie wavelength (in
meters) of a small car with mass 1150 kg
travelling at a speed of 55.0 mph (24.6 m/s)?
The Electronic Structure of the
Atom
Bohr Model
Bohr described the atom as a nucleus with an electron
orbiting it, similar to the planets orbiting the sun
Certain orbits correspond to specific energy levels that are
available to the electron
Electrons have specific energy levels available to them
Model failed for atoms with more than one electron
The path is now considered to be less definite
Rather than a strict path, the electron seems to occupy
empty space around the nucleus (electron cloud)
Quantum Mechanical Model
This abandons the notion of an electron as a
small particle that moves in a specific path
around the nucleus
It focuses on the wavelike properties of the
electron
Werner Heisenburg showed that it is
impossible to know exactly where the
electron is and what path it follows
Wave Function/Orbital
Has a specific energy associated with it
It contains information about the electron’s position (3D)
For H atom the lowest energy level available to an electron
is associated with the wave function called a 1s orbital
An electron in a 1s orbital will occupy a spherical region of
space around the nucleus
The electron is most likely to be found closer to the nucleus,
and less likely to be found as you move further away from
the nucleus
The electron’s path or movement is uncertain
It assumes a wavelike description, rather than a particle like
description
Wave Function and Quantum
Numbers
A wave function or orbital describes the behaviour
of an electron (position & path)
Each wave function contains three variables called
quantum numbers
These are represented as n, l and ml
They describe the energy level of the orbital as well
as the shape and orientation of the region of space
in which an electron will be found
The Principal Quantum Number
(n)
This is a positive integer (n = 1,2,3…)
It determines the size and energy level of the orbital
As n increases the number of allowed orbitals
increase and the size of the orbitals become larger
This allows for electrons to be located far from the
nucleus
As n increases the energy of the electron in the
orbital also increases
According to n, orbitals are grouped into layers
called shells around the nucleus
The Principal Quantum Number
(n)
n=1
n=2
n=3
n=4
K shell
L shell
M shell
N shell
The Angular-momentum/Orbital
Quantum Number (l)
This defines the three dimensional shape of the
orbital in which the electron moves
The number of possible shapes is equal to the
principal quantum number
When the principal quantum number is n, the
angular-momentum quantum number has values
from 0 to n-1
According to l, orbitals are grouped into subshells
referred to as s, p, d, f
The Magnetic Quantum
Number (ml)
This defines the spatial orientation of the orbital
along a standard set of coordinate axes
When the angular-momentum quantum number is l,
the magnetic quantum number has values from –l to
+l
For a group of orbitals having the same principal
quantum number n, and the same shape l, the
different spatial orientations for the orbital is (2l+1)
The Spin Quantum Number ms
This indicates the property of the electron described
by two conditions
It can be thought of as being right-handed or lefthanded, clockwise or counter clockwise
Electron spin
It can have either of two values: +1/2 or –1/2
The spin quantum number is independent of the
first three quantum numbers
Quantum Numbers
The position of the orbital described by the
first three quantum numbers can be occupied
by two electrons, but these must have
opposite spins
No two electrons can have the same set of
four quantum numbers (Pauli Exclusion
Principle)
No two electrons can have the same energy
Question
Identify the shell and subshell of an electron
with the quantum numbers: n = 3, l = 1 and
ml = 1
Exercise
Give the possible combinations of quantum
numbers for a 4p orbital.
Exercise
Give the possible combinations of quantum
numbers for the following orbitals:
(a) 3s
(b) 2p
(c) 4d
Exercise
Give the orbital notations for electrons with
the following quantum numbers:
(a) n = 2, l = 1, ml = 1
(b) n = 4, l = 3, ml = -2
(c) n = 3, l = 2, ml = -1
Explanation of Line Spectrum using the
quantum mechanical model
Electrons occupy orbitals
Orbitals have specific energy levels
The energy available to electrons are quantized
When an atom is heated, the energy causes an electron to jump from one
energy level to another (i.e. from a lower energy orbital to a higher
energy orbital)
The excited atom is unstable and quickly returns to the lower energy
level
This is accompanied by emission of the amount of energy that was
added (i.e. the difference between the higher and lower energy orbitals)
The energy of the orbitals are quantized, and the energy emitted is also
quantized
As a result, we observe the emission of only specific frequenices of
radiation
The variables m and n in the Balmer –
Rydberg equation for hydrogen corresponds
to the principal quantum numbers of the two
orbitals involved in the electronic transition
Question
What is the energy difference (kJ/mol)
between the first and second shells of the
hydrogen atom if the first emission in the
Lyman series occurs at λ = 121.5 nm?
Orbital Shapes
This corresponds to the angular-momentum
quantum number (ℓ)
The designations are s, p, d, f
s Orbital
These are spherical
The probability of finding an electron depends on
the distance from the nucleus, not on direction
There is only one possible orientation of a sphere:
mℓ = 0
There is one s orbital per shell
The size on the orbital increases as you go to a
higher shell
Beyond the first shell, there are several regions of
maximum probability
s orbital
http://www.chemsoc.org/ExemplarChem/entries/2004/dublin_fowler/sorbitals
p Orbital
These are dumbbell-shaped
The electrons are distributed in identical lobes on
either side of the nucleus
The lobes are separated by a nodal plane that cuts
through the nucleus
When ℓ = 1, ml has three possibilities, one along
each axis
As n increases, the size of the p orbital increases
p orbitals
px
py
pz
http://www.chemsoc.org/ExemplarChem/entries/2004/dublin_fowler/porbitals
d orbital
These are found in the third and higher shells
There are five d orbitals with two different shapes
Four of them are clover-leaf shaped, having four
lobes of maximum probability and two nodal planes
The fifth d orbital is dumbbell shaped along the z
axis with a donut region along the xy plane
All five have the same energy
d orbitals
dz2
dx2-y2
http://www.chemsoc.org/ExemplarChem/entries/2004/dublin_fowler/dorbitals
d orbital
dxy
dxz
dyz
http://www.chemsoc.org/ExemplarChem/entries/2004/dublin_fowler/dorbitals
f orbitals
There are seven f orbitals
Each having eight lobes of maximum
electron probability separated by three nodes
For hydrogen, the energy of an orbital is determined
by n
For multi-electron atom, the energy level of the
orbitals depend on the shell as well as the subshells
Within a given shell, the orbitals have slightly
different energies
This is due to electron-electron repulsion
As a result, outer shell electrons are shielded from
the effects of the nucleus by the inner shell
electrons
The overall/net nuclear charge felt by an
electron is called the effective nuclear charge
Zeff and is lower than the actual nuclear
charge Z
A lower angular momentum quantum
number corresponds to a higher Zeff and
therefore a lower energy level
Electronic Configuration
Guided by a set of rules called the aufbau principles
Rules of the aufbau principles
Lower energy level orbitals are filled before higher energy
level orbitals
An orbital can hold only two electrons, these two electrons
must have opposite spins (PAULI EXCLUSION
PRINCIPLE)
For degenerate orbitals, one electron is placed in each orbital
(with the same spin quantum number), making them half
full, then the second electron is added
Hund’s Rule
If two or more orbitals with the same energy
are available, one electron goes into each
until all are half-full. The electrons in the
half-filled orbitals all have the same value for
their spin quantum number.
Electron Configuration
For carbon and nitrogen, the electrons are in
different p orbitals i.e. px, py, pz
The singly occupied orbitals have the same value of
the spin quantum number +1/2 or –1/2
Orbital filling diagrams are used to specify electron
configuration
In these diagrams electron are represented as arrows
(pointing up or down)
A single arrow indicates a half filled orbital while
an up and down pair indicate a full orbital
The Periodic Table
The periodic table can be divided into four regions
or blocks of elements based on the orbitals being
filled
Elements to the left: s block
Elements in the middle: d block
Elements to the right: p block
The detached elements at the bottom are called
lanthanides and actinides, and these form the f
block elements
Periodic Table
Question
Give the ground state electron configuration
of arsenic (Z = 33). Draw orbital filling
diagram, indicating the electron as up or
down.
Exercise
Give the expected ground state electron
configuration for the following and draw
orbital filling diagrams for the first two
Ti (Z = 22)
Zn (Z = 30)
Sn (Z = 50)
Exercise
What is the likely ground state electron
configuration for the sodium ion Na+ formed
by loss of an electron from a neutral sodium
atom?
Anomalies with Electronic
Configuration
There are a few anomalies concerning electronic
configuration
Most of these anomalies usually occur in the elements with
atomic number greater than 40
At this point the energy differences between the subshells
are small
For example Cr and Cu
An electron moves from a 4s to an energetically similar 3d
orbital
A transfer in this way allows for decreased electron-electron
repulsion and therefore lowers the overall energy of the
atom
Electron Configuration of Ions
Elements on the left side of the periodic table
(metals) tend to give up electrons to form cations
Halogens and some other non-metallic elements
accept electrons to form anions
Electrons given up by a metal come from the
highest energy occupied orbital
The electrons accepted by a non-metal go to the
lowest energy occupied orbital
Electron Configuration of Ions
Na: 1s2 2s2 2p6 3s1
Na+: 1s2 2s2 2p6 Ne configuration
Cl: 1s2 2s2 2p6 3s2 3p5
Cl–: 1s2 2s2 2p6 3s2 3p6 Ar configuration
Electron Configuration of Ions
For Main Group Elements
All the elements in group 1A of the periodic table form
singly charged positive ions (cations) by losing one electron
Likewise, the elements in group 2A form doubly positive
ions by losing two electrons
The halogens in group 7A gain one electron to form singly
charged negative ions
Group 6A non metals gain two electrons to form doubly
charged negative ions
Electron Configuration of Ions
For transition metals
These form cations by losing their valence s
electrons, then their d electrons
Mass Spectrometry
Used to determine a compound’s molecular
formula by determining its molecular weight
Mass spectrometer: an instrument used to
determine both atomic weight and molecular
weight
Mass Spectrometer
http://www.chemguide.co.uk/analysis/masspec/howitworks.html
Mass Spectrometry
The sample is vaporized
It is then injected (as a gas) into an evacuated chamber
In this chamber, it is bombarded with a beam of high energy
electrons
This beam of electrons is able to knock off other electrons
from the sample molecules making them positively charged
ions
Ions of different masses are produced since some ionized
molecules fragment into smaller ions
These different ions are accelerated by an electric field and
pass between the poles of a strong magnet
As a result they are deflected through a curved pipe
Mass Spectrometry
The radius of deflection depends on the mass of the ion
Lighter ions are deflected more strongly than the heavier
ones
Varying the strength of the magnetic field allows for
focusing of different ions through a slit and into a detector
This gives rise to a mass spectrum results, which is a plot of
ion mass versus intensity (molecular weight of various ions
versus the relative number of ions produced)
The heaviest ion is generally due to the ionized molecule
itself
By determining the mass of this ion the molecular weight of
the molecule/compound can be determined
Mass Spectrum
http://www.chemguide.co.uk/analysis/masspec/howitworks.html
Exercise
(a)Define the term atomic weight
A certain element has only two naturally occurring isotopes. One
isotope has an abundance of 57.3 % and isotoic mass of 120.904 amu.
Given that the atomic weight of the element is 121.757 amu,
determine the isotopic masss of the other isotope. What element is it
most likely to be?
(b) Briefly state the rules of the aufbau (building up) principle
(c) Draw the box and arrow diagram for the 3d, 4s and 4p orbitals of
the atom having the following electronic configuration:
[Ar] 3d10 4s2 4p2
Give the ground state electron configuration of the elements with the
following atomic numbers
(i) Z = 13
(ii) Z = 17
(iii) Z = 21
Exercise
(a) On the basis of modern atomic theory, explain
what is meant by an atomic orbital.
(b) What three quantum numbers are used to
distinguish one atomic orbital from another and
what range of values is allowed for each quantum
number?
(c) Write the set of four quantum numbers for
each of the electrons in a ground state neon atom.
Exercise
Write down the Balmer-Rydberg equation in
terms of whole numbers m and n.
Hence, given that the frequency of the emitted
light due to a transition from the n = 5 level to
amother level m is 2.969 x 1014 Hz, calculate the
value of m.
To which series of the atomic spectrum does the
transition belong?
Determine the energy of the radiation emitted.
Exercise
(a)The quantum mechanical wave function, ψ, for electrons in atoms
is a function of n, ℓ and mℓ
Write down the meaning of n, ℓ and mℓ.
List the possible values of n, ℓ and mℓ for all the electrons in Na+ and ClSketch the shape of the orbitals with l = 1 and l = 2.
(b) According to quantum mechanics the electron does not move in a
defined circular orbit around the nucleus. Briefly explain the quantum
mechanical view of the electron.
(c) Calculate the wavelength of the line with the longest wavelength
in the Lyman series of the hydrogen spectrum. (The Lyman series is
made up of electron transitions to the n = 1 level of the hydrogen
atom)
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