Transcript Topic 7: Atomic and nuclear physics 7.1 The atom

Topic 7: Atomic and nuclear physics
7.1 The atom
2 hours
Dalton
Thompson
Rutherford
Bohr
John Dalton’s Atom
~1808
– Each element is made up of tiny
particles called atoms.
– The atoms of a given element are
identical; the atoms of different
elements are different in some
fundamental way or ways.
– Chemical compounds are formed
when atoms combine with each
other. A given compound always
has the same relative numbers and
types of atoms.
– Chemical reactions involve
reorganization of the atoms-changes in the way they are bound
together. The atoms themselves are
not changed in a chemical reaction.
J.J. Thompson’s Atom
~1904
– The atom is composed of
electrons (which Thomson still
called "corpuscles", though G. J.
Stoney had proposed that atoms
of electricity be called electrons in
1894) surrounded by a soup of
positive charge to balance the
electron's negative charge, like
negatively-charged "plums"
surrounded by positively-charged
"pudding".
Geiger and Marsden (and Rutherford)
~1909
The majority of alpha (a)
particles are slightly
deflected, however, some
are scattered at very large
angles. Small deflections can
be explained by positively
charged a-particles passing
the nucleus at large
distances and being repelled.
The large deflections are
explained by the a-particles
passing the nucleus at small
distances. These large
scattering angles were
surprising to Geiger and
Marsden as they did not
know the nucleus existed.
Rutherford’s Interpretation
The large deflections indicated an enormous
force of repulsion between the positive aparticles and the positive charge component of
the atom. This means that the positive charge in
an atom must be concentrated in one area and
not spread out like a positive soup as Thompson
had suggested. The fact that gold atoms do not
recoil when the a-particles hit them means that
they body holding the positive charge is very
massive and yet small enough for the a-particle
to get very close to.
Rutherford’s Interpretation
The Rutherford Model of the Atom
At atom (according to Rutherford) consists of a
massive positively charged nucleus at its centre and
electrons orbiting this nucleus like planets orbit the
sun.
Problems with Rutherford’s Model:
1. Electrons that are accelerating (and here we have
centripetal acceleration) are known to radiate light energy.
The electrons in Rutherford’s model should, therefore lose energy and spiral into
the nucleus in a matter of nanoseconds. Rutherford’s model, then, cannot explain
why stable atoms exist.
2. Rutherford’s model did not explain the observed emission spectra
The Bohr Model of the Atom
Neils Bohr examined the simplest atom
(Hydrogen) and realized that its electron could
exist in certain specific states of definite energy
without radiating any energy away. The
electrons energy is discrete (or quantized) rather
than continuous and it can only lose energy by
making a transition from one allowed state to
another allowed state of lower energy. The
emitted energy is equal to the energy between
the initial and final states and is given off as
light.
Excited
States
Emission Spectra
Consider hydrogen (H). Under normal conditions the
electron in each hydrogen atom occupies the lowest
energy state (known as the ground state). If the atom
is somehow excited, then the electrons will leave the
ground state and move to a higher energy state. As
soon as this happens they transition back to lower
energy states radiating energy in the process.
The set of wavelengths of light emitted by the atoms of
the element is called its emission spectrum. The
absorbed wavelengths (which equal the emitted
wavelengths) make up the absorption spectrum.
The Balmer Series
• The Balmer series or Balmer lines in atomic physics, is
the designation of a set of the spectral line emissions
of the hydrogen atom. The Balmer series is calculated
using the Balmer formula, an empirical equation
discovered by Johann Balmer in 1885.
• The visible spectrum of light from hydrogen displays
four wavelengths, 410 nm, 434 nm, 486 nm, and 656
nm, that correspond to emissions of photons by
electrons in excited states transitioning to the quantum
level described by the principal quantum number n
equals 2. There are also a number of ultraviolet Balmer
lines with wavelengths shorter than 400 nm.
Nuclear Structure
• The word nucleon is used to describe a proton or
a neutron.
• The number of protons in the nucleus is denoted
by Z and is called the atomic number of the
element.
• The total number of nucleons (protons +
neutrons) is called the mass number, and is
denoted by A.
• A nucleus with a specific number of protons and
neutrons is known as a nuclide.
Special Particles
Particle
Alpha Particle (Helium-4 Nucleus)
Symbol
4
2
a
or
4
2
e
or
e
He

Electron
0
1
Proton
1
1
p
or
p
Neutron
1
0
n
or
n
Photon
0
0

or


or
1
1
H
The Forces Within The Nucleus
• Consider an a-particle (a Helium-4 Nucleus)
• From our studies of Coulomb’s Law we know that there
will be a strong repulsive force between the two
protons and neither an attractive or repulsive force
between the two neutrons or between the neutrons
and the protons. We also know that all of nucleons will
be very weakly attracted to each other by Newton’s
Law of Universal Gravitation.
So what holds it together?
• Shortly after the discovery of the neutron, Hideki
Yukawa, a Japanese physicist, postulated a strong force
of attraction between nucleons that overcomes the
Coulomb repulsion between protons. The existence of
the force postulated by Yukawa is now well established
and is known as the strong nuclear interaction.
• The force is independent of whether the particles
involved are protons or neutrons and at nucleon
separations of about 1.3 fm, the force is some 100
times stronger than the Coulomb force between
protons. At separation greater than 1.3 fm, the force
falls rapidly to zero. At smaller separations the force is
strongly repulsive thereby keeping the nucleons at an
average separation of about 1.3 fm. (1 femtometre =
10-15 m)
The Fundamental Forces
Interaction
Current Theory
Relative Strength
Range (m)
Strong
Quantum chromodynamics
(QCD)
1038
10−15
Electromagnetic
Quantum electrodynamics
(QED)
1036
(Infinite)
Weak
Electroweak Theory
1025
10−18
Gravitation
General Relativity
(GR)
1
(Infinite)
Read Tsokos, Page 367 – 372 and
Answer Questions 1 to 12 on Page 372