Transcript Views on Atomic Structure - North Toronto Collegiate Institute
Views on Atomic Structure
Classical View – electrons and properties of electrons Experiments with Light – Quantum Theory Quantum View – behavior of electrons in atoms 1
Cathode Rays
Cathode rays
are the carriers of electric current from cathode to anode inside a vacuumed tube Cathode rays travel in straight lines 2
Cathode Rays
Cause glass and other materials to fluoresce Deflect in a magnetic field similarly to negatively charged particles 3
J. J. Thomson’s Experiment
Devised an experiment to find the ratio of the cathode ray particle’s mass (
m e
) to the charge (
e
)
m e
/e = –5.686 × 10 –12 kg C –1
4
The Electron
coined the term “electron” Millikan measured the charge on an electron - the famous “oil drop” experiment 5
Determined Electron Values
Robert Millikan then determined a value for the charge
e
= –1.602 × 10 –19 C From
m/e
and the charge, the mass of an electron was determined to be
m = 9.109 × 10 –31 kg/electron
6
J. J. Thomson – Atomic Model
Thomson proposed an atom with a positively charged sphere containing equally spaced electrons inside
RAISIN BUN MODEL
7
Rutherford’s Model
Ernest Rutherford characterized
alpha
particles through an experiment and discovered the positive charge of an atom is concentrated in the center of an atom, the
nucleus
8
Rutherford’s Interpretation
9
Protons and Neutrons
From Rutherford’s experiments, he was able to determine the amount of positive nuclear charge The positive charge was carried by particles called
protons
Scientists introduced the
atomic number,
which represents the number of protons in the nucleus of an atom James Chadwick discovered
neutrons
in the nucleus, which have nearly the same mass as protons and no charge 10
Mass Spectrometer
If a stream of positive ions having equal velocities is brought into a magnetic field, the lightest ions are deflected the most, making a tighter circle 11
Wave Motion
Caused by a displacement in a medium Characterized by height of crest (or depth of trough) 12
The Wave Nature of Light
Electromagnetic waves
originate from the movement of electric charges The movement produces fluctuations in electric and magnetic fields 13
Characterizing Waves
Electromagnetic radiation is characterized by its wavelength, frequency, and amplitude
Wavelength
( l ) is the distance between any two identical points in consecutive cycles 14
Characterizing Waves
Frequency
of a wave is the number of cycles of the wave that pass through a point in a unit of time
Amplitude
of a wave is its height: the distance from a line of no disturbance through the center of the wave peak 15
The Electromagnetic Spectrum
The electromagnetic spectrum is largely
invisible
to the eye 16
The Electromagnetic Spectrum
• We can feel some radiation through other senses (infrared) • Sunburned skin is a sign of too much ultraviolet radiation • Materials vary in their ability to absorb or transmit different wavelengths – Our bodies absorb visible light, but transmit most X rays – Window glass transmits visible light, but absorbs ultraviolet radiation 17
Continuous Spectra
White light passed through a prism produces a
spectrum –
colors in
co nt in uo us
form. 18
The Continuous Spectrum
l l l l l ~ 650 nm ~ 575 nm ~ 500 nm ~ 480 nm ~ 450 nm The different colors of light correspond to different wavelengths and frequencies 19
Line Spectra
Light passed through a prism from an element produces a
discontinuous spectrum
specific
of colors 20
Line Spectra
The pattern of lines emitted by excited atoms of an element is unique =
atomic emission spectrum
21
Quantum Theory – Black Body Radiation
Planck proposed that the vibrating atoms in a heated solid could absorb or emit electromagnetic energy only in discrete amounts The smallest amount of energy, a
quantum,
is given by:
E
=
hv
where
h
is Planck’s constant: = 6.626 × 10 –34 J s Planck’s quantum hypothesis states that energy can be absorbed or emitted only as a quantum or as
whole multiples
of a quantum 22
Quantum Theory – Photoelectric Effect
Einstein considered electromagnetic energy to be bundled into little packets called
photons
Energy of photon is
E = hv
Photoelectric Effect Movie 23
Bohr’s Hydrogen Atom
Niels Bohr found that the electron energy (
E n
) was quantized, that is, that it can have only certain specified values Each specified energy value is called an
energy level
of the atom 24
The Bohr Model
E n
= and
n –B/n 2
where
B
is an integer is a constant = 2.179 × 10 –18 J The negative sign represents the forces of attraction The energy is zero when the electron is located infinitely far from the nucleus 25
Bohr Explains Line Spectra
Bohr’s equation is most useful in determining the energy change (
E
level ) that accompanies the leap of an electron from one energy level to another For the final and initial levels:
E
f
n
f 2
B and E
i
B n
i 2 The energy difference between
n
f and
n
i
B
B
B
1
n
f 2
n
i 2
n
i 2 is: 1
n
f 2 26
Energy Levels and Spectral Lines for Hydrogen
27
Ground States and Excited States
Electrons in their lowest possible energy levels are in the
ground state
Electrons promoted to any level
n
> 1 are in an
excited state
Electrons are promoted by absorbing energy e.g., electric discharge, heat, lasers (photons) Electrons in an excited state eventually drop back down to the ground state “relaxation” 28
Electronic Transitions
Arrows represent transitions between energy levels
Upward
arrows (a) show energy
absorption
, electrons move to higher energy levels
Downward
arrows (b)–(d) represent energy release and
relaxation
The length of an arrow is inversely proportional to photon wavelength 29
Electronic Transitions
The length of an arrow is inversely proportional to photon wavelength
Shorter
wavelengths, higher energies
Longer
wavelengths, lower energies 30
De Broglie’s Equation
•Louis de Broglie speculated that matter can behave as both particles and waves, just like light •He proposed that a particle with a mass
m
moving at a speed
v
will have a wave nature consistent with a wavelength l
h mv
31
Wave Functions (
y
)
Quantum mechanics,
or wave mechanics, is the treatment of atomic structure through the wavelike properties of the electron Erwin Schrödinger developed an equation to describe the hydrogen atom A wave function is a solution to the Schrödinger equation and represents an energy state of the atom 32
Interpretation of a Wave Function
Wave mechanics provides a
probability
of where an electron will be in certain regions of an atom The Born interpretation: The square of a wave function ( y
2
) gives the
probability
of finding an electron in a small volume of space around the atom (
orbital
) The interpretation leads to the idea of a cloud of electron density rather than a discrete location 33
The Uncertainty Principle
Werner Heisenberg’s uncertainty principle states that we can’t
simultaneously
know exactly where a tiny particle like an electron is and exactly how it is moving 34
The Uncertainty Principle
In light of the uncertainty principle, Bohr’s model of the hydrogen atom fails, in part, because it tells more than we can know with certainty Electron is spread out like a wave; the wave which describes how the electron is distributed spacially is called a wave function) 35
Quantum Numbers and Atomic Orbitals
A wave function with a given set of these three quantum numbers is called an
atomic orbital
In quantum mechanics the atomic orbitals require
three integer quantum numbers
to completely describe the energy and the shape of the 3-D space occupied by the electron (
n, l
, and
m l
) 36
Principal Quantum Number (
n
)
• Is independent of the other two quantum numbers • Can only be a positive integer • indicates the size of an orbital (distance from the nucleus) and its electron energy •
n
can be 1, 2, 3, 4, … 37
Orbital Angular Momentum Quantum Number (
l
) (aka Azimuthal quantum number) • Determines the shape of the orbital:
s
,
p
,
d
,
f which corresponds to
values of
l
=
0
,
1
,
2
,
3
• Possible values: 0 to (
n
– 1); e.g., if
n
= 2,
l
can only be 0 or /1 • Each of these orbitals is a different region of space and a different shape •All the
‘l’
quantum values represent different
subshells
•When n = 1, there is only 1 “l” value meaning there is only one subshell in the first energy level; when n= 2; there are 2 values for ‘l’ indicating two subshells in the second energy level 38
Magnetic Quantum Number (
m l
)
Determines the orientation in space of the orbital; so named because in a magnetic field, these different orientations have different energies Possible values: –
l
to +
l
; e.g., if
l
= 2,
m l
can be –2, –1, 0, 1, 2 The magnetic quantum number defines the number of orbital in a shell. E.g. in the
l
= 0 subshell, there is only one
m l
value, therefore there is only orbital in this subshell; when
l
=1; there are 3 possible
m l
values (-1, 0, +1) 3 orbitals in this subshell 39
Quantum Numbers Summary
Taken together the three quantum numbers specific the orbital the electron occupies. Namely: the energy of the orbital, the shape of the orbital, and the orientation of the orbital 40 .
• writing 3 quantum numbers to indicate every possible orbital an electron can occupy is cumbersome; instead do we do the following: • retain the numeric value of the principal quantum number and we use a letter to indicate the azimuthal quantum number: •
l
= 0 s;
l
= 1 p;
l
= 2 d;
l
= 3 d • When combined, they indicate an a specific orbital e.g. 1s orbital; 2s orbital; 2p orbital 41
Radial Distributions
Electrons are most likely to reside nearest the nucleus because of electrostatic attraction Probability of finding an electron decreases as distance (radius) from the nucleus increases 42
Electron Probabilities and the 1
s
Orbital
The 1
s
orbital looks very much like a fuzzy ball, that is, the orbital has
spherical
symmetry (the probability of finding an electron is the same in direction) The electrons are more concentrated near the center 43
Electron Probabilities and the 2
s
Orbital
The 2
s
orbital has two regions of high electron probability, both being spherical The region near the nucleus is separated from the outer region by a spherical
node
- a spherical shell in which the electron probability is zero EOS 44
The Three p Orbitals
There are 3 p orbital; each orbital is cylindrically symmetrical with respect to rotation around one of the 3 axes, x, y, or z Each ‘p’ orbital has two lobes of high probability density separated by a node (region of zero probability) 45
The Five
d
Orbitals
46
Electron Spin (
m
s
)
The
electron spin quantum number
explains some of the finer features of atomic emission spectra The spin refers to a magnetic field induced by the moving electric charge of the electron as it spins Only possible values = –1/2 to +1/2 EOS 47
The Stern-Gerlach Experiment
Interaction of the electron spin with the magnetic field caused a splitting of the observed signal EOS 48
Summary of Concepts
• Cathode rays are negatively charged fundamental particles of matter, now called electrons • An electron bears one fundamental unit of negative electric charge • A nucleus of an atom consists of protons and neutrons and contains practically all the mass of an atom • Mass spectrometry establishes atomic masses and relative abundances of the isotopes of an element 49
Summary of Concepts
• Electromagnetic radiation is an energy transmission in the form of oscillating electric and magnetic fields • The oscillations produce waves that are characterized by their frequencies (
v),
wavelengths ( l ), and velocity (
c
) • The complete span of possibilities for frequency and wavelength is described as the electromagnetic spectrum 50
Summary of Concepts
• Planck’s explanation of quantums gave us
E
=
hv
• The photoelectric effect is explained by thinking of quanta of energy as concentrated into particles of light called photons • Wave functions require the assignment of three quantum numbers: principal quantum number,
n,
orbital angular momentum quantum number,
l,
and magnetic quantum number, orbitals
m l.
• Wave functions with acceptable values of the three quantum numbers are called atomic 51
Summary of Concepts
• Orbitals describe regions in an atom that have a high probability of containing an electron or a high electronic charge density • Shapes associated with orbitals depend on the value of
l
. Thus, an
s
orbital (
l
= 0) is spherical and a
p
orbital (
l
= 1) is dumbbell-shaped • A fourth quantum number is also required to characterize an electron in an orbital - the spin quantum number,
m
s 52