C H A P T E R O N E

A T O M I C A N D N U C L E A R S T R U C T U R E

To begin with, the term "nuclear chemistry" is a paradox. Chemical

changes involve the transfer or sharing of valence electrons. However, the

usual properties of nuclei—mass, atomic number, and nuclear structure—

bear no direct relation to chemical reactions. Consequently, there can be

no direct relationship between the chemical and nuclear properties of

atoms.

On the other hand, a wide variety of phenomena are of interest to

chemists either because certain nuclear properties provide new chemical

tools or because differences in nuclei actually produce subtleties of

chemical behavior. Thus, nuclear chemistry includes any of the

relationships between atomic and nuclear structure of importance to a

chemist.

In general, the nuclear chemist is concerned with those properties

involving similarities and differences among the isotopes of a given

element. It should be stressed that, in addition to stable isotopes, the

nuclear chemist studies radioactive isotopes by the application of the

various techniques of radio- chemistry and radiation chemistry. We often

use the term "radiochemistry" to describe work with radioactive atomic

species, whereas radiation chemistry is now limited to chemical effects

brought about by radiation interacting with a system.

Nuclear chemistry, or radioactivity, differs from other branches of

chemistry in that the important changes occur in the nucleus. These nuclear

changes also are represented by chemical equations. However, because the

isotopes of the same element may, from a nuclear standpoint, be very different in

reactivity, it is necessary that the equations show which isotopes are involved.

Table 1.0. Comparison of Chemical Reactions and Nuclear Reactions

Chemical Reactions Nuclear Reactions

1. Atoms are rearranged by the breaking and forming of

chemical bonds.

Elements (or isotopes of the same elements) are converted from one to another.

2. Only electrons in atomic or molecular orbital are involved in the breaking and forming of

bonds.

Protons, neutrons, electrons, and other elementary particles may be involved.

3. Reactions are accompanied by absorption or release of relatively small amounts of energy.

Reactions are accompanied by absorption or release of tremendous amounts of energy.

4. Rates of reaction are influenced by temperature, pressure, concentration, and catalysts.

Rates of reaction normally are not affected by temperature, pressure, and catalysts.

Both chemical and nuclear reactions are depicted by a complete chemical

equation. However, two major rules guide writing the chemical equation of

nuclear reaction:

(i) The sum of the mass numbers of the reactants must be equal the sum of mass

number of the products

(ii) The sum of atomic number of the reactions must be equal to the atomic

number of the products, this maintain charge balance.

The Structure of the Atom

As is well known, our present concept of the atom includes the notion that

the atom consists of a dense nucleus containing protons and neutrons and

surrounded by electrons occupying certain positions, or following certain

paths of motion.

Each element can be represented by the notation 𝑋𝑍𝐴 , where A, the mass

number, is the sum of the number of protons and the number of neutrons, and Z,

the atomic number, is the number of protons. The protons and neutrons that

make up the nucleus of an atom are called nucleons, and an atom with a

particular number of protons and neutrons is called a nuclide. Nuclides with the

same number of protons but different numbers of neutrons are called isotopes.

Isotopes can also be represented by an alternative notation that uses the name of

the element followed by the mass number, such as carbon-12. The stable isotopes

of oxygen, for example, can be represented in any of the following ways:

Because the number of neutrons is equal to A − Z, we see that the first

isotope of oxygen has 8 neutrons, the second isotope 9 neutrons, and the third

isotope 10 neutrons. Isotopes of all naturally occurring elements on Earth are

present in nearly fixed proportions, with each proportion constituting an isotope’s

natural abundance. For example, in a typical terrestrial sample of oxygen, 99.76%

of the O atoms is oxygen-16, 0.20% is oxygen-18, and 0.04% is oxygen-17.

Any nucleus that is unstable and decays spontaneously is said to be

radioactive, emitting subatomic particles and electromagnetic radiation. The

emissions are collectively called radioactivity and can be measured. Isotopes that

emit radiation are called radioisotopes. Table 1.1 below shows the basic

constituents of an atom.

Table 1.1. Basic constituents of an atom.

The ratio of number of neutron to proton in any nucleus confirms the

stability of the nucleus and also influence nuclear reaction is called neutron –

proton ratio. The neutron – proton ratio of an element dictates which element

will naturally undergoes radioactivity or not.

Neutron - Proton Ratio and Nuclear Stability

The principal factor that determines whether a nucleus is stable is the

neutron-to-proton ratio (n/p). For stable atoms of elements having low atomic

number, the n/p value is close to 1. As the atomic number increases, the neutron-

to-proton ratios of the stable nuclei become greater than 1. This deviation at

higher atomic numbers arises because a larger number of neutrons is needed to

counteract the strong repulsion among the protons and stabilize the nucleus. The

following rules are useful in predicting nuclear stability:

1. Nuclei that contain 2, 8, 20, 50, 82, or 126 protons or neutrons are generally

more stable than nuclei that do not possess these numbers. For example, there

are 10 stable isotopes of tin (Sn) with the atomic number 50 and only 2 stable

isotopes of antimony (Sb) with the atomic number 51. The numbers 2, 8, 20, 50,

82, and 126 are called magic numbers. The significance of these numbers for

nuclear stability is similar to the numbers of electrons associated with the very

stable noble gases (that is, 2, 10, 18, 36, 54, and 86 electrons).

2. Nuclei with even numbers of both protons and neutrons are generally more

stable than those with odd numbers of these particles (Table 1.2).

3. All isotopes of the elements with atomic numbers higher than 83 are

radioactive. All isotopes of technetium (Tc, Z = 43) and promethium (Pm, Z = 61)

are radioactive.

Table 1.2. Number of stable isotopes with even and odd numbers of protons & neutrons

Fig. 1.2 shows a plot of the number of neutrons versus the number of

protons in various isotopes. The stable nuclei are located in an area of the graph

known as the belt of stability. Most radioactive nuclei lie outside this belt. Above

the stability belt, the nuclei have higher neutron-to-proton ratios than those

within the belt (for the same number of protons). To lower this ratio (and hence

move down toward the belt of stability), these nuclei undergo the following

process, called b-particle emission:

Below the stability belt, the nuclei have lower neutron-to-proton ratios than those

in the belt (for the same number of protons). To increase this ratio (and hence

move up toward the belt of stability), these nuclei either emit a positron

or undergo electron capture. An example of positron emission is

Electron capture is the capture of an electron—usually a 1s electron—by the

nucleus. The captured electron combines with a proton to form a neutron so that

the atomic number decreases by one while the mass number remains the same.

This process has the same net effect as positron emission:

Fig. 1.2. Plot of neutrons versus protons for various stable isotopes, represented by dots. The straight line represents the points at which the neutron-to-proton ratio equals 1. The shaded area represents the belt of stability.

Radioactivity

Radioactivity is a phenomenon in which some elements emit small particles

called radiation, to form another element. This is in contrast to the accepted

Dalton postulate of indestructibility of an atom. However, only elements with

unstable nuclei known as radioactive are capable of undergoing natural

radioactivity while stable nuclei do not. All elements having atomic number

greater than 83 are radioactive and they undergo nuclear transmutation (nuclear

reaction) which differ significantly from ordinary chemical reactions.

Natural Radioactivity (spontaneous disintegration)

This is natural radioactivity caused by the instability of an atom. The

phenomenon of radioactivity was discovered by Antoine Henri Becquerel in 1896.

He discovered that photographic plates develop bright spots when exposed to

uranium minerals, and he concluded that the minerals give off some sort of

radiation.

Fig. 1.3. Separation of the radiation from a radioactive material (uranium mineral). The radiation

separates into alpha (α), beta (β), and gamma (𝛾) rays when it passes through an electric field.

The radiation from uranium minerals was later shown to be separable by electric

(and magnetic) fields into three types, alpha (α), beta (β), and gamma (γ) rays

(Figure 1.3).

Alpha Particles

Alpha rays bend away from a positive plate and toward a negative plate,

indicating that they have a positive charge. The alpha particle is the heaviest. It is

produced when the heaviest elements decay. Alpha and beta rays are not waves.

They are high-energy particles that are expelled from unstable nuclei. In the case

of alpha radiation, the high energy particles leave the nucleus. The alpha particle

is a helium atom and contains two neutrons and two protons. It leaves the

nucleus of an unstable atom at a speed of 16,000 kilometers per second, around a

tenth the speed of light. The alpha particles are relatively large and heavy. As a

result, alpha rays are not very penetrating and are easily absorbed. A sheet of

paper or a 3-cm layer of air is sufficient to stop them. Its energy is transferred

within a short distance to the surrounding media. However, its short flight

knocks about 450,000 electrons out of the surrounding atoms. The alpha particle

emitter will not penetrate the outer layer of our skin, but is dangerous if inhaled

or swallowed. The delicate internal workings of the living cell forming the lining

of the lungs or internal organs, most certainly will be changed (mutated) or killed

outright by the energetic alpha particle. The number of lung cancer cases among

uranium miners from inhaled and ingested alpha sources is much higher than

those of the public at large. Radon, the gas produced by the decay of radium-226,

also emits alpha particles, which poses a hazard to lungs and airways when

inhaled. Homes built in areas with high ground radioactivity should be tested for

radon buildup in enclosed basement spaces.

An example of a transmutation takes place when uranium decays into the

element thorium (Th) by emitting an alpha particle, as depicted in the following

equation:

𝑈 → 92238 𝐻𝑒 + 𝑇ℎ 90

234 24

More examples of reactions involving emission of alpha particles are shown below

𝑅𝑎 → 88226 𝐻𝑒 + 𝑅𝑛 86

222 24

𝑃𝑜 → 84210 𝐻𝑒 + 𝑃𝑏 82

206 24

In general, alpha decay process is summarized below

𝑋 → (𝑍𝐴 )𝑌 + 𝐻𝑒 + 𝑄 2

4 𝑍−2𝐴−4

Where:

A is the mass number

Z is the atomic number

Q is the kinetic energy of the decay product

Beta Particles

Beta rays bend in the opposite direction to alpha particles, indicating that

they have a negative charge; they are now known to consist of high-speed

electrons. Beta rays are much lighter energy particles. The beta particle is an

energetic electron given off by the nucleus of unstable isotopes to restore an

energy balance. They leave the nucleus at a speed of 270,000 kilometers per

second. They can be stopped, for instance, by an aluminum sheet a few

millimeters thick or by 3 meters of air. The RS-500 can detect most energetic

beta particles through the case. Weaker beta particles can be detected through the

tube window. Although the beta particle is around 8000 times smaller than the

alpha particle, it is capable of penetrating much deeper into living matter. Each

encounter with a living cell, and there may be many before the beta energy is

dissipated, is likely to dam age some of the chemical links between the living

molecules of the cell or cause some permanent genetic change in the cell nucleus.

If the damage occurs within the generative cells of the ovaries or testes, the

damage may be passed to new generations. The normal background radiation

level must contribute to the mutation of the gene pool. Most mutations are

undesirable with a very few leading to "improvements". Any increase in the

background level of radiation should be considered harmful. Shown below are

some examples of beta emission:

The beta decay process is summarized as follows

𝑋 → (𝑍𝐴 )𝑌 + 𝛽 + 𝑄 −1

0 𝑍+1

𝐴

Where:

A is the mass number

Z is the atomic number

Q is the kinetic energy of the decay product

Note that beta emission leads to an increase in the atomic number by 1, the mass

number remain unchanged.

Gamma rays

Gamma rays are unaffected by electric and magnetic fields: they have been shown

to be a form of electromagnetic radiation that is similar to x- rays, except they are

higher in energy with shorter wavelengths (about 1pm). It is capable of damaging

living cells as it slows down by transferring its energy to surrounding cell

components. Gamma ray sources are used to find flaws in pipes and vessels and to

check the integrity of welds in steel.

Uranium minerals contain a number of radioactive elements, each emitting

one or more of these radiations. Uranium-238, the main uranium isotope in

uranium minerals, emits alpha rays and thereby decays, or disintegrates, to

thorium-234 nuclei. A sample of uranium-238 decays, or disintegrates,

spontaneously over a period of billions of years. After about 30 billion years, the

sample would be nearly gone. Strontium-90, formed by nuclear reactions that

occur in nuclear weapons testing and nuclear power reactors, decays more

rapidly. A sample of strontium-90 would be nearly gone after several hundred

years. In either case, it is impossible to know when a particular nucleus will decay,

except if precise information about the rate of decay of any radioactive sample is

given.

In gamma emission, there is no change in either the mass number or atomic

number. The gamma symbol (𝛾) is indicated in the nuclear equation by adding to

the products as shown in the equations below.

𝑇ℎ →90234 𝑃 + 𝛽 + 𝛾 −1

0 91

234

𝑈 →92238 𝑇ℎ + 𝐻𝑒 + 𝛾 2

4 90

234

Table 1.3. Products of natural radioactivity

Artificial Radioactivity or Nuclear Transmutation

The scope of nuclear chemistry would be rather narrow if study were

limited to natural radioactive elements. An experiment performed by Rutherford

in 1919, however, suggested the possibility of producing radioactivity artificially.

When he bombarded a sample of nitrogen with 𝛼 particles, the following reaction

took place:

𝑁 + 714 𝛼 → 𝑂8

17 + 𝑃 11 2

4

An oxygen-17 isotope was produced with the emission of a proton. This reaction

demonstrated for the first time the feasibility of converting one element into

another, by the process of nuclear transmutation. Nuclear transmutation differs

from radioactive decay in that the former is brought about by the collision of two

particles.

The preceding reaction can be abbreviated as 𝑁 𝛼, 𝑝 𝑂817 . 7

14 Note that in

the parentheses the bombarding particle is written first, followed by the ejected

particle.

The reactions below further illustrates artificial radioactivity.

𝐿𝑖 + 36 𝑛 → 𝐻𝑒2

4 + 𝐻 13 𝑜

1

𝑈 + 92238 𝑛 → 𝑃𝑢94

239 + 𝐻 + 2 𝛽 −10 1

3 𝑜1

𝑆 + 1632 𝑛 → 𝑃15

32 + 𝐻 11 𝑂

1

Example 1.1.

Write the balanced equation for the nuclear reaction 𝐹𝑒 𝑑, 𝛼 𝑀𝑛2554

2656 , where d

represents the deuterium nucleus (that is, 𝐻 12 ).

Strategy

To write the balanced nuclear equation, remember that the first isotope 𝐹𝑒 2656 is

the reactant and the second isotope 𝑀𝑛2554 is the product. The first symbol in

parentheses (d) is the bombarding particle and the second symbol in parentheses

(a) is the particle emitted as a result of nuclear transmutation.

Solution

The abbreviation tells us that when iron-56 is bombarded with a deuterium

nucleus, it produces the manganese-54 nucleus plus an 𝛼 particle. Thus, the

equation for this reaction is

𝐹𝑒 2656 + 𝐻 1

2 → 𝛼 24 + 𝑀𝑛25

54

Although light elements are generally not radioactive, they can be made so

by bombarding their nuclei with appropriate particles. As we saw earlier, the

radioactive carbon-14 isotope can be prepared by bombarding nitrogen-14 with

neutrons. Tritium 𝐻,13 is prepared according to the following bombardment:

Many synthetic isotopes are prepared by using neutrons as projectiles. This

approach is particularly convenient because neutrons carry no charges and

therefore are not repelled by the targets—the nuclei. In contrast, when the

projectiles are positively charged particles (for example, protons or 𝛼 particles),

they must have considerable kinetic energy to overcome the electrostatic

repulsion between themselves and the target nuclei. The synthesis of phosphorus

from aluminum is one example:

A particle accelerator uses electric and magnetic fields to increase the kinetic

energy of charged species so that a reaction will occur (Figure 1.4). Alternating

the polarity (that is, + and -) on specially constructed plates causes the particles

to accelerate along a spiral path. When they have the energy necessary to initiate

the desired nuclear reaction, they are guided out of the accelerator into a collision

with a target substance.

Figure 1.4. Schematic diagram of a cyclotron particle accelerator. The particle (an ion) to be accelerated starts at the center and is forced to move in a spiral path through the infl uence of electric and magnetic fi elds until it emerges at a high velocity. The magnetic fi elds are perpendicular to the plane of the dees (so-called because of their shape), which are hollow and serve as electrodes.

The Transuranium Elements

Particle accelerators made it possible to synthesize the so-called transuranium

elements, elements with atomic numbers greater than 92. Neptunium (Z = 93) was

first prepared in 1940. Since then, 24 other transuranium elements have been

synthesized. All isotopes of these elements are radioactive. Table 1.4 lists the

transuranium elements up to Z = 111 and the reactions through which they are

formed.

Table 1.4. The transuranium elements

Exercise 1.

Element 118, known currently by its IUPAC systematic name ununoctium

(symbol: Uuo), was first created in 2006 in Dubna, Russia. The nuclear reaction

used to produce this element was 𝐶𝑓 𝐶𝑎2048 ,𝑋 𝑈𝑢𝑜118

294 . 98249 Determine the product

X and write the balanced equation for this nuclear reaction.

EQUATIONS INVOLVING NUCLEAR REACTIONS

To discuss nuclear reactions in any depth, we need to understand how to write

and balance the equations. Writing a nuclear equation differs somewhat from

writing equations for chemical reactions. In addition to writing the symbols for

various chemical elements, we must also explicitly indicate protons, neutrons, and

electrons. In fact, we must show the numbers of protons and neutrons present in

every species in such an equation.

The symbols for elementary particles are as follows:

As earlier mentioned, the superscript in each case denotes the mass number (the

total number of neutrons and protons present) and the subscript is the atomic

number (the number of protons). Thus, the ―atomic number‖ of a proton is 1,

because there is one proton present, and the ―mass number‖ is also 1, because

there is one proton but no neutrons present. On the other hand, the ―mass

number‖ of a neutron is 1, but its ―atomic number‖ is zero, because there are no

protons present. For the electron, the ―mass number‖ is zero (there are neither

protons nor neutrons present), but the ―atomic number‖ is -1, because the electron

possesses a unit negative charge.

The symbol 𝑒 −10 represents an electron in or from an atomic orbital. The symbol

𝛽 −10 represents an electron that, although physically identical to any other

electron, comes from a nucleus (in a decay process in which a neutron is

converted to a proton and an electron) and not from an atomic orbital. The

positron has the same mass as the electron, but bears a +1 charge. The 𝛼 particle has

two protons and two neutrons, so its atomic number is 2 and its mass number is

4.

In balancing any nuclear equation, we observe these rules:

• The total number of protons plus neutrons in the products and in the reactants

must be the same (conservation of mass number).

• The total number of nuclear charges in the products and in the reactants must

be the same (conservation of atomic number).

For example, in

1. 𝐴 + 𝑏𝑎 𝐵 = 𝐶𝑦

𝑥 𝑑𝑐

The algebraic sum:

a + c must be equal to x

b + d must be equal to y

and in

2. 𝑈 + 𝑏𝑎 𝑊 = 𝑋𝑓

𝑒 + 𝑌𝑛𝑚

𝑑𝑐

The algebraic sum:

a + c must be equal to e + m

b + d must be equal to f + n

Example 1.2

Find the missing number (x, y) in the following nuclear reactions.

i. 𝐹 + 919 𝐻 → 𝑁𝑒𝑥

20 1𝑦

ii. 𝑁 + 7𝑥 𝑛 → 𝐶𝑦

14 01 + 𝐻1

1

iii. 𝐶 → 614 𝑒 + 𝑋7

14 𝑥𝑦

Solution

i. 19 + y = 20

y = 20 -19

= 1

9 + 1 = x

x = 10

∴ 𝐻 = 𝐻11 𝑎𝑛𝑑 𝑁𝑒 = 𝑁𝑒10

20 𝑥20 1

𝑦

ii. x + 1 = 14 + 1

x+ 1 = 15

x = 15-1

x =14

7 + 0 = y + 1

y = 7-1

y= 6

∴ 𝑁 = 𝑁714 𝑎𝑛𝑑 𝐶 = 𝑦

14 𝐶 614

7𝑥

iii. 14 = 14 + y

y = 14 – 14

= 0

6 = x + 7

x = 6 – 7

= -1

∴ 𝑒 = 𝑒−10 𝑥

𝑦

When a heavy nucleus breaks down or disintegrate into simpler and lighter

nuclei, they release at the same time enormous amount of energy. Alpha particle

( 𝐻𝑒24 ) and beta particle ( 𝑒−1

0 ) earlier discussed are the two main products of such

nuclei disintegration and are used in balancing nuclear equations.

Q. Show that the mass number and the total charge are both conserved in the

natural disintegration of 𝑈: 92238

238U

Solution

The equation may be rewritten including all atomic numbers and mass numbers: