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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).