Download - 03 chap 01 structure of matter
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Chapter 1
STRUCTURE OF MATTER
1.1 The Atom 2
Democritus (c460-371 BC, Abdera)
Leucippus(440 BC, Miletus)
‘Atom’ ← ‘Atomos’:‘a’ = not‘tomos’ = cut, slice
Ancient Greek Atomic Theory
1.1 The Atom 3
John Dalton (1766 – 1844) Modern Atomic Theory (1800’s) - John Dalton
In 1803, Dalton proposed the Atomic Theory which stated that :
(1) all matter was composed of small indivisible particles termed atoms,
(2) atoms of a given element possess unique characteristics and weight, and
(3) three types of atoms exist: simple (elements), compound (simple molecules), and complex (complex molecules).
All matter is composed of individual entities called elements, distinguishable from the others by the physical and chemical properties of its basic components-the atoms.
1.1 The Atom 4
In 1897, J.J.Thompson discovered electrons through cathode ray tube experiment at Cavendish Laboratory, Cambridge University.Electron is one of the basic constituents of the Atom.
Inner Structure of the Atom – Electrons
electrons
J.J.Thompson (1856-1940) and the cathode ray tube
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Cavendish Laboratory
Cambridge University
River Cam
1.1 The Atom 6
The ‘ plum pudding’ Model of the Atom (chocolate chip cookie model)
In this model, the atom is composed of electrons surrounded by a soup of positive charge, like plums surrounded by pudding. The electrons were thought to be positioned uniformly throughout the atom.
1.1 The Atom 7
In 1910, Rutherford’s investigations into the scattering of alpha rays and the nature of the inner structure of the atom which caused such scattering led to the postulation of his concept of the "nucleus",
Inner Structure of the Atom – The Nucleus Ernest Rutherford (1871-1937)
1.1 The Atom 8
Niels Bohr (1885-1962)
The Bohr Model of the Atom (1913)The Bohr model depicts the atom as a small, positively charged nucleus surrounded by electrons in orbit —similar in structure to the solar system.
~10-10m
~10-14m
1.2 The Nucleus 9
James Chadwick (1891-1974) The Discovery of Neutron
In 1932 Chadwick made a fundamental discovery in the domain of nuclear science: he discovered the particle in the nucleus of an atom that became known as the neutron because it has no electric charge.
The neutron has almost the same weight as the proton.
1.2 The Nucleus 10
The properties of an atom are determined by the constitution of its nucleus and the number of electrons orbiting around it.
An atom is specified by : (e.g. ) XAZ C12
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X = the chemical symbol for the element, A= the mass number, = # of nucleons (sum of neutrons and protons in the nucleus), Z = the atomic number (# of protons or electrons).
Particle Symbol Charge (unit) Weight (amu)
neutron N 0 1.00866
proton Z +1 1.00727
electron Z -1 0.000548
1 unit charge = 1.60 10-19 coulombs,
1 amu (atomic mass unit) = 1/12 of the mass of a nucleus = 1.66 10-27 kg C126
1.2 The Nucleus 11
On the basis of different proportions of neutrons and protons in the nuclei, atoms can be classified into different categories:
isotopes isotones isobars isomers
Same Z N A A, Z, N
Different A, N A, Z Z, Nenergy states
Example , , , ,Co5927 Co60
27 N147 O15
8 P3215 S32
16 Xe13154
Xem13154
1.2 The Nucleus 12
As the atomic mass increases (beyond Z=20), stable nuclei have more N than Z, for example, .
In light atoms, stable nuclei tend to have the same Z and N , for example, .
0
20
40
60
80
100
120
140
160
0 20 40 60 80 100
proton number (p)
neut
ron
num
ber
(n=
A-p
)
stable nuclei
n/p = 1
C126
Ca4020
Pb20682
U23892neutrons vs protons
in stale nucleiC126
Pb20682
More than half of the stable nuclei have even numbers of neutrons and protons (even-even nuclei).
About 20% of the stable nuclei have even Z and odd N and about the same proportion have odd Z and even N.
In contrast, only four stable nuclei have both odd Z and odd N.
1.3 Atomic Mass and Energy Units
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The mass of an atom expressed in terms of amu is known as atomic mass or atomic weight.
Gram atomic weight is the mass in grams numerically equal to the atomic weight.
Avogadro’s Law: every gram atomic weight of a substance contains the same number of atoms, 6.023 1023 atoms per gram atomic weight (known as the Avogadro’s number NA). For example, the atomic weight (AW) of helium is 4.0026. Therefore,
The number of atoms/g = NA / AW = 6.023 1023 / 4.0026 = 1.505 1023 /g.
Grams/atom = AW / NA = 6.646 10-24 g.
The number of electrons/g = NA Z / AW = 3.009 1023 /g.
1.3 Atomic Mass and Energy Units
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The mass of an electron = 0.000548 amu; proton = 1.00727 amu, neutron = 1.00866 amu. The mass of an electron is approximately 1/2000 of a proton or a neutron.
The mass of an atom is less than the sum of the masses of its constituents, because certain mass is converted into energy which ‘glues’ the nucleons together. The reduction in mass is called the mass defect or the binding energy.
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Energy unit in atomic and nuclear physics: 1 eV = the kinetic energy acquired by an electron in passing through a potential difference of 1 Volt.
eV = 1 V 1.60210-19 C = 1.60210-19 J.
Energy unit: 1 joule (J) is the work done when a force of 1 newton (kg-m/sec2) acts through a distance of 1 m.
The mass of an electron at rest (so-called rest mass) is 9.110-31 kg. Its energy equivalent, according to E = mc2 (c = 3108 m/sec), is 0.511 MeV. It can be shown that 1 amu = 931 MeV.
1.4 Distribution of Orbital Electrons
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KLMNO
The maximum number of electrons in an orbit is 2n2, where n is the orbit number. That is, the maximum number of electrons that can exist is 2, 8, and 18 in the orbit K,L,M respectively. For example, in an oxygen atom (Z=8), there are 2 electrons in the K-shell and 6 electrons in the L-shell.
1.5 Atomic Energy Levels 17
KL
MN
ground state (0)
-69,500 eV
-11,000 eV
-2,500 eVK series
L series
Tungsten atom
Electrons are bound to the nucleus by the coulomb force of the positive changes in the nucleus.
Electrons in inner orbits (shells) are more tightly bound than those in the outer orbits.
Each orbit has its own energy level (potential energy, or binding energy).
Moreover, higher Z atoms have greater binding energies.
When an electron falls from an outer shell (higher potential energy) into an inner shell (lower potential energy), the energy difference of the two levels is emitted as radiation (called characteristic X-rays).
1.6 Nuclear Forces 18
There are 4 different forces in nature, in the order of their strengths:(1) strong nuclear force, (2) electromagnetic force, (3) weak nuclear force, and (4) gravitational force.Strong nuclear force is a short range, attractive force that overcomes the
repulsive electrostatic force of the protons and keeps the nucleons together.Weak nuclear force is a force involved in nuclear –decay.
particle with no charge
charged particle
1.7 Nuclear Energy Levels 19
The Co-60 is produced in a nuclear reactor from Co-59 by 59Co(n,)60Co
Co6027 (5.26 y)
-(Emax=0.32 MeV), 99+%
-(Emax=1.48 MeV),
0.1%
Ni6028
2.50
1.33
(1.33 MeV)
(1.17 MeV)
The shell model of the nucleus is similar to that of the atomic model with each shell characterized by a distinct energy level. When energy is imparted to a stable nucleus, it may be raised to an excited state. When it returns to a lower energy state, it gives off energy equal to the energy difference of the two states. For example, the radioactive was created in a nuclear reactor by bombarding the stable with neutrons. The
is transformed to by a –decay and 2 successive release of –rays (1.17-MeV and 1.33-MeV).
Co6027
Co5927
Co6027 Ni60
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1.9 Particle Radiation 20
Energy propagated by traveling corpuscles that have a non-zero definitive rest mass. Examples are electrons, protons, neutrons. The distinction between particle radiation and electromagnetic wave became less sharp when de Broglie hypothesized the wave-particle duality nature of matter. Elementary particles have either zero or unit charge.
Particle Symbol Charge Mass
Electron e-, - -1 0.000548 amu
Positron e+, + +1 0.000548 amu
Proton p, +1 1.00727 amu
Neutron n, 0 1.00866 amu
neutrino 0 ~ zero
H11
n10
1.9 Electromagnetic Radiation 21
Electromagnetic wave has two components: magnetic and electric fields oscillating at right angle to each other.
Examples are light waves, heat waves, radio waves, microwaves, ultraviolet rays, x-rays, and –rays.
Energy is propagated with the speed of light (3108 m/sec).
The relationship between wavelength (), frequency (), and the speed of propagation (c) is given by: c = .
Electromagnetic Radiation - Wave Model
1.9 Electromagnetic Radiation 22
1.9 Electromagnetic Radiation 23
Electromagnetic Radiation - Quantum Model
In order to explain certain experimental results such as Compton scattering, electromagnetic radiation needs to be treated as particles (with zero mass).
The relation between the energy and its frequency is given by:
E = h, where h is the Planck’s constant (6.6210-34 J-sec),
E is the energy in joule (J).
By combining with the previous equation: E = hc/.
If E is expressed in eV and in meters (m), then E (eV) = 1.24 10-6/ (m).