5 Steps to a 5 AP Chemistry

262  Step 4. Review the Knowledge You Need to Score High

Nickel-63 will undergo beta decay according to the following equation:

Notice that the atomic number has increased by 1 in going from Ni-63 to Cu-63, but

the mass number has remained unchanged.

Gamma Emission

Gamma emission is the giving off of high-energy, short-wavelength photons similar to

X-rays. This radiation is commonly represented as γ. Gamma emission commonly accom-

panies most other types of radioactive decay, but is often not shown in the balanced nuclear

equation because it has neither appreciable mass nor charge.

Alpha, beta, and gamma emissions are the most common types of natural decay mode,

but positron emission and electron capture are also observed occasionally.

Positron Emission

A positron is essentially an electron that has a positive charge instead of a negative one.

It is represented as Positron emission results from the conversion of a proton to

a neutron and a positron: It is observed in the decay of some natural

radioactive isotopes, such as K-40:

Electron Capture

The four decay modes described above all involve the emission or giving off of a particle;

electron capture is the capturing of an electron from the energy level closest to the nucleus

(1s) by a proton in the nucleus. This creates a neutron: Electron capture

leaves a vacancy in the 1s energy level, and an electron from a higher energy level drops

down to fill this vacancy. A cascading effect occurs as the electrons shift downward and, as

they do so, energy is released. This energy falls in the X-ray part of the electromagnetic spec-

trum. These X-rays give scientists a clue that electron capture has taken place.

Polonium-204 undergoes electron capture: X-rays. Notice that

the atomic number has decreased by 1, but the mass number has remained the same.

Remember that electron capture is the only decay mode that involves adding a particle to

the left side of the reaction arrow.

Nuclear Stability

Predicting whether a particular isotope is stable and what type of decay mode it might

undergo can be tricky. All isotopes containing 84 or more protons are unstable and will

undergo nuclear decay. For these large, massive isotopes, alpha decay is observed most com-

monly. Alpha decay gets rid of four units of mass and two units of charge, thus helping to

relieve the repulsive stress found in these nuclei. For other isotopes, with atomic numbers

less than 84, stability is best predicted by the use of the neutron-to-proton (n/p) ratio.

If one plots the number of neutrons versus the number of protons for the known stable

isotopes, the nuclear belt of stability is formed. At the low end of this belt of stability

(Z < 20), the n/p ratio is 1. At the high end (Z ≈80), the n/p ratio is about 1.5. One can then

use the n/p ratio of the isotope under question to predict whether or not it will be stable. If it is

unstable, the isotope will utilize a decay mode that will bring it back onto the belt of stability.

For example, consider Ne-18. It has 10 p and 8 n, giving a n/p ratio of 0.8. That is less

than 1, so the isotope is unstable. This isotope is neutron-poor, meaning it doesn’t have

84

204

1

0

83

Po+→ +e^204 Bi

−

− 1 +→

0

1

1

0

ep n.^1

19

40

18

40

1

KAre.→+^0

+

++ 1 →+

1

0

1

1

(^1) pne. 0
0
1
βor e.^0
28
63
29
63
1
Ni→+Cu^0 e.
−
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