1

Nuclear Chemistry IntroductionIsotopes

12

6C

mass number (A)

atomic number (Z)

the number of protons+

the number of neutrons

mass number (A)

number of protons

number of neutrons

6

12 – 6 = 6

atomic number (Z) = the number of protons

=

2

Nuclear Chemistry IntroductionIsotopes

Isotopes are atoms of the same element having a different number of neutrons.

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Nuclear Chemistry IntroductionIsotopes

• Of the known isotopes of all elements, 264 are stable and 300 are naturally occurring but unstable.

• A radioactive isotope, called a radioisotope, is unstable and spontaneously emits energy to form a more stable nucleus.

• Radioactivity is the nuclear radiation emitted by a radioactive isotope.

• An even larger number of radioactive isotopes, called artificial isotopes, have been produced in the laboratory.

NUCLEAR RADIATION Used to :

treat medical conditionsdetect and treat tumorsdetect the function of an organproduce scans/images of organs

ORIGIN

Most element have at least one unstable isotopes.

Unstable isotopes arise when the attraction/repulsion between

protons and neutrons in the nucleus is unbalanced.

An unstable nucleus has too few or too many neutrons as

compared to protons.

e.g. 2412Mg 23

12Mg (Mg-23) 2712Mg (Mg-27)

RADIOACTIVITY

* This is emission of small particles of energy or rays

(radiation) by unstable elements (to be stable).

* Elements that emit radiation spontaneously (e.g.

Thorium, Th and Uranium, U) are said to be radioactive and

called radioisotope.

* The radiation is emitted from the nucleus of the atom

TYPES OF RADIATION

* Particles and Rays

• Alpha and Beta Particles and Gamma Rays

1. ALPHA PARTICLES (α particles)

* Attracted towards negative field (positively

charged).

* Made up of (two protons and two neutrons),

helium nuclei, therefore have charge of +2.

* Have low penetrating power and are harmless

when they strike the body.

* Harmful when inhaled or get into body through

open wound and can cause damage to internal

organs.

2. BETA PARTICLES ( β particles)

* Attracted towards positive field (negatively charged)

* Made up of electrons, therefore have a charge of -1

* Produced in the nucleus by transformation of

neutron into a proton and an electron.

* Slightly more power than the alpha particles but can

be stopped by heavy clothing.

* Harmful when inhaled or get into body through open

wound and can cause damage to internal organs.

3. GAMMA RAYS (γ rays)

* Not affected by electrostatic field, therefore have no

charge

* Very penetrating, pass through body and can cause damage

to internal organs and cells as they pass through the body.

* Used to treat inaccessible brain tumors.

PROTECTION

1. DISTANCE

* Increasing the distance between the source of

radiation and the body reduces the effect of the

radiation.

Radiation = 1 (distance)2

2. SHIELDING

* Protective gloves, clothing and tongs must be

used at all times when handling radioactive

material.

3. LIMITING EXPOSURE (TIME)

* Limit the time that one spends next to

radioactive material. The shorter the time, the

lesser the dose of radiation.

NUCLEAR REACTIONS

* Process by which an unstable radioactive nucleus emits

radiation, forming a new nucleus of different composition

* This result in change of either atomic number, mass number or

both on the new nucleus.

* The following can be emitted during nuclear reactions;

Alpha particles, α, (42He)

Beta particles, β, (o-1e)

Gamma ray (γ)

Neutron (10n)

Proton (11p) or (1

1H)

Positron (o1e)

e.g. 1. A thorium atom (23490Th) decays by alpha

emission.

23490Th → α + X

23490

Th)→ 42He + 230

88X

23490Th→ 4

2He + 23088Ra

2. A thorium atom (23490Th) decays by beta and

gamma emission

23490Th → β + γ + X

23490Th → o

-1e + γ + X

23490Th→ o

-1e + γ + 23491X

23490Th → o

-1e + γ + 23491Pa

• Units Of Radiation

• Radiation measured using the Geiger Counter. Curie. This indicates the number of nuclear disintegrations

occurring per second from 1g of a radioactive substance (radium)

1 curie = 3.7 x 1010 disintegrations per second

Becqurel (Bq) = 1 disintegration per second (SI unit)

Rad (radiation absorbed dose) = amount of radiation dosage absorbed by one gram of substance (human tissue)

Unit gray (Gy) Rem (radiation equivalent in man) measures biological damage or

damage to body tissues or organs.

LD50 (lethal dose that can cause death to 50 % of the population).

LD50 (dogs) = 300 rem

LD50 (humans) = 500 rem

LD50 (rats) = 800 rem

• HALF-LIFE (t1/2)

• The amount of time required for half the amount of a radioactive

substance to decay.

• Remianing Mass = Mi (1/2)n

• e.g. A 400 mg sample of iodine-131 is left in a laboratory cupboard. How

much of the sample will be left after 80 days if the half-life of iodine is 8

days.

• NB: For medicinal purposes only radioisotopes with short half-lives are

used to treat patients.

• e.g. Technetium (6 hours), I-131 (8 days), Gold (2.7 days)

• C-14 (5730 yrs) Used for carbon dating.

NUCLEAR ENERGY

Energy generated from the nucleus of radioactive substances.

NUCLEAR FISSION

Splitting of heavy elements into smaller pieces, accompanied

by large amounts of energy.

23690U + 1

0n → 9136Kr + 142

56Ba + 3 10n + energy (E = mc2)

Chain reaction (The three neutron produced can bombard

other nuclei to produce more energy)

e.g. Atomic bomb

NUCLEAR FUSION

Small nuclei combine to form larger nuclei.

21H + 3

1H → 42He + 1

0n, + energy (E = mc2)

e.g. Sun

• Write a balanced equation for the following nuclear reactions;a. When two oxygen-16 atoms collide, one of the

products is an alpha particle.

b. When californium-249 is bombarded by oxygen-18, a new isotope and four neutrons are produced.

c. Silicon-26 decays by positron emission.

d. Rb-77 decays by beta and alpha emission.

• Cesium-137 has a half-life of 30 yearsa. Write a nuclear equation for beta decay of

cesium-137

b. How many grams of a 16 g sample would remain after 90 years.

c. How many years would be needed for 28 g of cesium-137 to decay to 3.5g?