Radioactivity

Total mass of nucleus is always less than the sum of its protons and neutrons

Compare the mass of He-4 to that of its nucleus- mass of 2 protons + 2 neutrons = 4.031882 μ- mass of He nucleus = 4.002602 μ

0.02928 μ of mass is lost Mass/energy did not disappear, its gone into

another form

Binding Energy

Lost mass becomes energy

E = mc2

Energy from the mass is being used to hold the nucleus together – Called Binding Energy

The nucleus is composed of protons (+) and neutrons (neutral), so electrostatic forces are pushing the particles apart

Binding energy forms the Strong Nuclear Force holding the nucleus together

Total Binding Energy

Emission of subatomic particles or high energy electromagnetic radiation from the nucleus of an atom

Atoms with a higher neutron to proton ratio will decay spontaneously

Radioactivity

Discovered in 1896 by Henri Becquerel Marie and Pierre Curie discovered

radioactive elements and emissions produced by decay

Discovery

Alpha Decay () Beta Decay () Gamma decay ()

3 Types of Radioactivity

Atom decays into a smaller atom and a positively charged alpha particle ()

Alpha particle consists of a He nucleus (2 protons, 2 neutrons)

Atom undergoing alpha decay is called the parent

When the parent emits the particle, it becomes the daughter

Alpha Decay

Process of forming the new element is called transmutation

226Ra 222Rn + 4He

Has largest ionizing power = ability to ionize molecules & atoms due to largeness of -particle

But has lowest penetrating power = ability to penetrate matter

Skin, even air, protect against -particle radiation

U – 235 undergoes alpha decay. Write the nuclear equation.

Example

Emission of the electron from a nucleus Electron is emitted from the nucleus as a

neutron changes into a proton 1

0n 11p + 0

-1e No nucleons are lost, therefore the atomic

mass is the same for the parent and daughter

Beta Decay

Since the parent loses a charge of –1, the daughter must gain a charge of +1

614C 7

14N + e

Again, the identity of the atom changes – transmutation

Beta Decay

Lower ionizing power than alpha particle But higher penetration power Requires sheet of metal or thick piece of wood

to arrest penetration more damage outside of body, but less in

(alpha particle is opposite)

K – 43 undergoes beta decay, write the nuclear reaction.

Example

Very similar to emission of photons by excited electrons

Nucleus itself is excited and when it falls back to lower energy, emits a chargeless photon which is gamma radiation

Gamma decay does not change the element Happens along with alpha and beta decay

Gamma Decay

Lowest ionizing power, highest penetrating power requires several inches lead shielding

Write a nuclear equation for each of the following:

1. beta decay in Bk-2492. alpha decay of Ra-224

Practice

Time for ½ of sample to decay

Half-Life

P-32 has a half-life of 14.3 days. If you start with 4.0 mg of P-32, how many mg remain after 57.2 days?

The half-life of radon is 3.824 days. After what time will 0ne-fourth of the given amount of radon remain?

A sample begins with 16mg of Po. After 12 min, the sample contains only 1.0mg. What is the half-life of Po?

Example