Radioactive Decay

TEKS Par 112.35 #12b

What's Causing the Radiation?

Scientists discovered that certain elements give off rays

Alpha (helium ion) Beta (electron) Gamma (high-energy, low-wavelength light)

They wanted to know why that happens, and how. They know that there are different kinds of atoms,

with the same chemical behavior but differing masses, called isotopes.

Stable or Unstable?

Some isotopes are stable. They don't give off radiation An example is the isotope of carbon, C-12

Some isotopes are not stable. They give off radiation Example, “radiocarbon,” C-14 This isotope is used to find the age of objects that

contain carbon As time goes on, there is less and less C-14 in it.

What's Happening to C-14?

Frankswebspace.org.uk

The graph shows how much C-14 remains after a

certain number of years.

Other tests show that N-14, the

next element in the periodic table,

has taken its place.

C-14 has turned into N-14, by giving off a beta particle, e-.

Balancing the Equation

We can write the decay process, using a shorthand everyone has agreed on.

It's called a nuclear equation, because it shows the decay of the nucleus.

Here is the equation for the decay of carbon-14 into nitrogen-14 and an electron:

Cyberphysics.co.uk

How to Write a Nuclear Decay Equation

Start off with what you know decays, and what it decays into.

Write the starting “reactants”, then write a right-facing arrow, and then write the decay products.

Make certain that the numbers on top (the mass) are equal on both sides.

Make certain that the numbers on the bottom (the charge) are equal on both sides.

Manipulate those numbers so they match.

Radium example

A radium isotope, mass 222, gives off an alpha particle. What new isotope is produced?

Start with the only “reactant”, Ra-222

Add the known product, an alpha particle. Determine what the top and bottom numbers on the

other product must be by subtraction. Look up the product on the periodic table (bottom

number)

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You Do One!

Iodine-131 decays by giving off an electron and gamma radiation. What isotope is also given off?

Hint—the gamma radiation has no charge and no mass.

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Beta decay changes the charge and the element identification and

chemical properties, not the mass

Summary

Common Nuclear Decay

Type Symbol Particle given off

Change in charge

Change in mass

Alpha α Heliumnucleus

Down by2

Down by4

Beta β electron Up by 1 None