Nuclear Chemistry

The Discovery of Radiation

• Nuclear reactions are different from other types of reactions.

• Nuclear chemistry is concerned with the structure of atomic nuclei and the changes they undergo.

• Wilhelm Roentgen discovered x-rays in 1895.

• Marie Curie and her husband Pierre isolated the first radioactive materials.

Nuclear Reactions Nuclear reactions are different

from other types of reactions. Involve changes in the nucleus

of the atom protons (p+) and neutrons (nº)

The location of the atom (which compound it is in) has no effect

Differences from Chemical reactions

Unaffected by changes in conditions temperature and pressure

Speed of reaction cannot be changed “Actuarial tables”

Mass is not conserved

Radioactivity The process where the nucleus of an atom undergoes changes and releases energy, particles, or both

Radioisotopes Isotopes of an element that are

radioactive due to unstable nuclei

Undergo radioactive decay Nucleus falls apart, not grows larger

• Unstable nuclei emit radiation to attain more stable atomic configurations

Why do radioisotopes emit radiation?

A. to balance charges in the nucleus

B. to release energy

C. to attain more stable atomic configurations

D. to gain energy

Radioactivity

Types of radiationAlpha particlesBeta particlesGamma radiation “Positron” emission at times

Types of Radiation

• The ability of radiation to pass through matter is called its penetrating power.

• Gamma rays (like x-rays) are highly penetrating because they have no charge and no mass.

Alpha particles Symbolized as:

42He

4

2 Charge of +2 Mass ~4 amu

Heaviest of the particles

Types of Radiation

• Alpha radiation is not very penetrating—a single sheet of paper will stop an alpha particle.

Beta particles Symbolized as

0-1e-

O

-1 Charge of -1 Mass ~ 0.00055amu

Actually an electron emitted from the nucleus

Types of Radiation

• Beta radiation is a stream of fast moving particles with greater penetrating power—a thin sheet of foil will stop them.

Gamma radiation

Not a particle High energy electromagnetic

radiation (“light”) Also known as a type of “photon” Symbolized as No charge, no mass

Neutrino Symbolized as 00 Have no charge and extremely little

mass Carry energy away from the reaction Hypothesized to exist to explain the

wide variance of energy released in some nuclear reactions

Penetratin

g power~0.05m

m~4mm Very

highShielding Paper,

clothes, dead skin

Metal foil Pb, concrete

(incomplete)

energy ~5 MeV ~1 MeV

~1 MeV

X rays are most similar to what type of nuclear emissions?

A. gamma rays

B. alpha particles

C. beta particles

D. delta waves

Nuclear reactions The total of all the mass numbers

of the reactants equals the total of all the mass numbers of the products

The same holds true for the atomic numbers

Actual mass is not conserved Matter is transformed into energy

Symbols in nuclear reactions

Alpha particle 4

2He or 42

Beta particle 0

-1e or 0-1

Gamma radiation 0

0 Neutrino

00

Proton 1

1p+ or 11H

Neutron 1

0no

Electron 0

-1e

Positrons 0

+1e

Writing Nuclear reactions

1) The nuclide that decays is the one on the left-hand side of the equation.

2) The order of the nuclides on the right-hand side can be in any order.

3) The neutrino symbol is the Greek letter "nu." 00

4) The mass number and atomic number of a neutrino are zero.

Alpha ( ) Decay1) The nucleus of an atom splits into

two parts.2) One of these parts (the alpha

particle) goes zooming off into space.3) The nucleus left behind has its atomic

number reduced by 2 and its mass number reduced by 4 (that is, by 2 protons and 2 neutrons).

Types of Radiation

• Alpha radiation is not very penetrating—a single sheet of paper will stop an alpha particle.

Alpha ( ) Decay

Beta ( ) Decay 1) A neutron inside the nucleus of

an atom breaks down, changing into a proton.

2) It emits an electron and an anti-neutrino which go zooming off into space.

3) The atomic number goes UP by one and mass number remains unchanged.

Types of Radiation

• Beta radiation is a stream of fast moving particles with greater penetrating power—a thin sheet of foil will stop them.

Beta ( ) Decay

Types of Radioactive Decay (cont.)

Positron (01+e)

Emission 1) Something inside the nucleus of an

atom breaks down, which causes a proton to become a neutron.

2) It emits a positron and a neutrino which go zooming off into space.

3) The atomic number goes DOWN by one and mass number remains unchanged.

Positron (01+e)

Emission

Electron Capture Electron capture is not like any

other decay - alpha, beta, or position. All other decays shoot something out of the nucleus. In electron capture, something ENTERS the nucleus

Electron Capture 1) An electron from the closest energy

level falls into the nucleus, which causes a proton to become a neutron.

2) A neutrino is emitted from the nucleus. 3) Another electron falls into the “empty

space” in the electron cloud and so on causing a cascade of electrons falling.

4) The atomic number goes DOWN by one and mass number remains unchanged.

Writing electron capture equations

1) The nuclide that decays is the one on the left-hand side of the equation.

2) The electron must also be written on the left-hand side.

3) A neutrino is involved , It is ejected from the nucleus where the electron reacts, so it is written on the right-hand side.

Electron Capture

Types of Radioactive Decay (cont.)

Nuclear Chemistry

What causes nuclear instability?

The “wrong” number of neutrons

Atoms 1 – 20, p+ to no ratio is ~ 1:1

Atoms 21 and up, the ratio approaches 1:1.5

Nuclear Stability (cont.)

• The strong nuclear force acts on subatomic particles that are extremely close together and overcomes the electrostatic repulsion among protons.

Nuclear Stability (cont.)

• As atomic number increases, more and more neutrons are needed to produce a strong nuclear force that is sufficient to balance the electrostatic repulsion between protons.

• Proton to neutron ratio increases gradually to about 1:1.5

What if there are too many protons and neutrons?

Lose some of both Alpha emission (4

2)

Remember- an alpha particle is two protons and two neutrons

Mass number goes down by 4 atomic number goes down by 2

What if there are too many n0?

Convert a no into a p+

Beta emission Mass number stays the same, but atomic number goes up by 1

What if there are too few no?

Either 1) Positron emission: Convert a proton into a neutron

158O 15

7N + 01+e + 0

0

or 2) e- capture: A proton merges with an electron to become a neutron

Both: #p+ by 1 and #n0 by 1

Credits….

Portions of this presentation were adapted from the ChemTeam website