28.2

Nuclear Transformations

Nuclear Stability and Decay Stable nuclei are in the “band of stability”-

this is part of a neutron vs. proton graph

Too many neutrons relative to protons Ratio is > 1 or 1.5 Results in beta decay Beta decay results in more protons and

fewer neutrons

Pg 846 Cu-29

Too many protons compared to neutrons Or too few neutrons Ratio is <1.5 or 1 Results in a proton being converted to a

neutron -this can happen when an electron is

captured by the nucleus (Ni-28 pg 846) - this can also happen when a positron

is emitted from the nucleus ( caused by a proton changing into a neutron)

Positron same mass as an electron, but positively

charged 0 e +1

Elements above 83 Too many neutrons and protons All nuclei with atomic number >83 are

radioactive and undergo alpha decayPg 846- Pb-82, Ra-88

Half-life Time requires for ½ of a radioactive

isotope to decay ( t1/2) Each isotope has its own half-life ( can be

fraction of a second to billions of yrs)

*Notice the shape of the graph

Half-Life Isotopes with shrot half-lives can be useful

in nuclear medicine because there are no long-term biological radiation hazards

Isotopes, such as U-238 has a long half-life and is useful in dating rocks- very, very old rocks ( like from the start of the solar system)

Problem N-13 emits beta radiation and decays to

C-13 with a half life (t1/2) of 10 minutes.

Assume a starting mass of 2.00g of N-13a) How long is 3 half-lives?b) How many grams will be present at the

end of 3 half-lives?

Problem a)3x 10min=30 min 3 half-lives is 30

minutes b) 1/2n = fraction remaining where n= #

half-lives so… 1/23= 1/8 2.00g x 1/8=.250gDraw the graph for this to double check.

Problem Mn-56 is a beta emitter with a half-life of

2.6 h. What is the mass of Mn-56 in a 1.0g sample at the end of 10.4 h?

Problem A sample of Th-234 has a half-life of 25

days. What fraction of the sample will remain after 50 days?

Transmutation Reactions This is the conversion of an atom of an

element onto another element This is done naturally by radioactivity- N-

14 can eventually decay into C-14; U-238 after many transmutations becomes stable Pb-206

Transmutation This can also be done artificially by bombarding a

high energy particle ( such as a proton, neutron or alpha particle) at the nucleus of an atom

Ernest Rutherford ( remember him?) was the first person to do this. He bombarded N-14

with an alpha particle to produce F-18. Write the nuclear equation for this!

The F-18 was unstable and it decayed into O-17 ( what else was produced?)- write the nuclear eq.!

Transmutation

This led to the discovery of the proton Chadwick discovered the neutron involved the

transmutation of Be-9 to C-12 by the bombarding of Be with a alpha particle (write this eq.)

Elements with atomic numbers above 92 are known as the transuranium elements- none occur in nature, they were artificially made, all are radioactive and decay

They were made in particle accelerators or nuclear reactors

I discovered

the neutron