Nuclear Chemistry

Nucleons vs. Nuclide Nucleons: General name referring to

nucleus made up off Protons + Neutrons

Nuclide: Nuclear chemistry’s way of referring to the atom For example:

• Radium-228 or

The Nucleus

Remember that the nucleus is comprised of two subatomic particles; protons and neutrons.

The number of protons is the atomic number. The number of protons and neutrons together is

effectively the mass of the atom.

The difference between the mass of an atom and the sum of the masses of its protons, neutrons, and electrons is called the mass defect.

Mass Defect and Nuclear Stability

The measured mass of , 4.002 602 amu, is:

0.030 377 amu less than the combined mass, 4.032 979 amu.

Mass Defect Mass Defect: Difference between mass of atom

and sum of masses of it’s p+, n, and e-

For example: Helium-4

• 2 Protons= (2 x 1.007276)• 2 Neutrons= (2 x 1.008665)• 2 Electrons= (2 x .0005486)

______________________________

4.032979

But its actual mass is measured at 4.00260

that’s .03038 less

Where did the .03038 go?

It was converted to energy when formed

Nuclear Binding Energy: energy released when nucleus is formed

E = mc2

Albert Einstein

Studied the mass and energy of atoms and found the relation between the two

Came up with the equation E = mc2

Led to the understanding of binding energy (energy that holds an atom together)

Band of Stability: Most stable nuclei are 1:1

(Proton: Neutrons)

Isotopes

Not all atoms of the same element have the same mass because of different numbers of neutrons in those atoms.

For example, there are three naturally occurring isotopes of uranium:

Uranium-234 Uranium-235 Uranium-238

All isotopes of uranium have 92 protons, but they all have different numbers of neutrons

Stability of Isotopes

The “like” charges of the protons in the nucleus push the particles apart from each other, threatening to push the nucleus apart

Binding energy keeps the nucleus together Stable atoms have a binding energy that is strong

enough to hold the nucleus together Because some isotopes have an extra neutron (or

more), the binding energy cannot hold the nucleus together

This makes the atom unstable These are radioisotopes

What is radioactivity?

Radioactivity is the act of emitting radiation spontaneously with the resulting emission of radiation resulting in the formation of a new nuclei

Does not need a source to travel through space and penetrate another material

Atoms with unstable nuclei are radioactive

Usually the number of neutrons will determine if a nuclei is unstable

Transmutation

Elements with atomic numbers greater than 83 are radioisotopes

Those elements with atomic numbers less than 83 have isotopes and most have at least 1 radioisotope

Radioisotopes try to stabilize They try to transform into a new, stable element This is transmutation The change occurs

due to changes in the

nucleus and results in

radioactive decay

Radioisotope Half-Life

Radioisotopes are unstable. They decay, or change into new elements, over time.

The half-life of an element is the time it takes for half of the material you started with to decay. Remember, it doesn’t matter how much you

start with. After 1 half-life, half of it will have decayed.

Each element has its own half life.

Half-Life Questions What is the half-life of this

element? Half-life is where ½ of the

element remains Go to 50% on the y-axis Then drop down to x-axis and

that is the half-life 1,000,000 years

What percent of the material originally present will remain after 2 million years?

Go to 2 million on x-axis Go over to y-axis 25%

Half-life calculations

mf = mi (.5)n

mf : final mass of sample

mi: initial mass of sample

n: number of half-lives

tf = t1/2 n

tf = total time of decay

t1/2 = half life

n = number of half-lives

Example problem:

Chromium-48 has half-life of 21.6 hours. How long will it take 360.0 g of Cr-48 to decay to 11.25 g?

108 hours If the half-life of tungsten-190 is 30.0

minutes, how much tungsten-90 will remain after 114 minutes if I’m originally given a 400.0 gram sample?

28.7 g

Types of Radioactive Decay Spontaneous breakdown of a nucleus resulting

in release of energy and matter Type of radiation emitted by radioactive

materials Radioactive decay and transmutation occur

simultaneously Constantly releasing energy and matter as they

are transforming into a new, stable isotope Alpha Beta Gamma Positron

Alpha Decay α

Alpha decay results when an unstable nuclei loses a Helium-4 particle (2 protons and 2 neutrons)

The new nucleus will have an atomic

number that is 2 less than the original The new nucleus will have a mass

number that is 4 less than the original

Beta Decay β Beta decay occurs when a neutron in an

unstable nucleus splits to make a proton and a electron

The atomic number increases by 1 because of the extra proton

The mass number does not change since one neutron is subtracted, but one proton is added

Gamma Ray γ Energy is emitted from the nucleus in the form of

gamma rays Electromagnetic waves with very high frequencies

and energy Naturally occurring waves (identical to X-rays) Very dangerous to life Usually accompanies alpha or beta decay

Positron Decay β+

Opposite of beta decay Occurs in nuclei with too few neutrons Proton turns into a neutron Atomic number decreases by 1 but mass

number stays the same

Penetrating Ability

Nuclear Equations

Nuclear Equations show the original radioisotope and also tell you which type of decay that radioisotope underwent

All you have to do is add or subtract to determine which type of decay occurred

Example:

Loss of 2 protons

Loss of 4 from the mass number

That means it had to undergo alpha decay

Example problems

a) Beta decay = extra proton X = 83

b) Alpha decayy = 206; x = 82

c) Increases by one protonx = electron

d) Gamma radiationY = 226; x = 88

e) Beta decayX = 214; y = 84

f) Alpha decayX = 226; y = 88

g) Alpha decayX = helium atom

h) Gamma radiationx = same as on left