All of the processes discussed in this section are examples of nuclear reactions, which are different from ordinary chemical reactions. Ordinary chemical reactions involve the exchange and sharing of electrons, while nuclear reactions involve alterations in the very core of an atom; that dense nucleus made up of protons and neutrons.
Radioactivity
You will need to be familiar with several types of nuclear reactions and terms related to them to be fully prepared for the SAT II Chemistry test, and in this section we’ll review everything you’ll need to know. The first concept we discuss is radioactivity. Strictly speaking, radioactivity is the spontaneous disintegration of an unstable atomic nucleus and the subsequent emission of radiation. But what makes atoms radioactive to begin with, and what makes them undergo radioactive decay? It turns out that there is a stable ratio of protons to neutrons for each element; for the first 20 elements on the periodic table (hydrogen through calcium), this ratio is 1 proton to 1 neutron, for example. Protons and neutrons in excess of this stable number can be emitted radioactively. Below we have listed examples of the important types of radioactive decay.
Alpha decay occurs when the nucleus emits an alpha particle. Alpha particles have a positive charge and are equivalent in size to a helium nucleus, and so they are symbolized as
. Alpha particles are the largest radioactive particle emitted. This type of radioactivity results in a decrease in the atomic number by 2 and a decrease in the atomic mass by 4. The equation below shows uranium-234 undergoing alpha decay:
Beta decay occurs when the nucleus emits a beta particle. Beta particles have a negative charge and are much smaller than alpha particles. They’re equivalent to high-speed electrons and are symbolized by
or
. This type of radioactivity causes an increase in the atomic number by 1 but no change in mass number. The equation below represents uranium-233 undergoing beta decay.
How does a nucleus, which is composed of only protons and neutrons, eject an electron? A neutron is composed of a proton and an electron fused together. In beta emission, the electron is emitted from the nucleus, while the proton part remains behind, thus increasing the atomic number by 1.
Example
Complete the balanced equation by determining the missing term.
Explanation
Remember, the sum of the atomic numbers and the mass numbers must be equal on both sides of the equation. We are looking for a component that has mass number of 80 and an atomic number of 34 (34 protons). Using this
information and the periodic table, we can identity the element produced by this beta decay as Se, or selenium. The missing term is
Se. And the completed equation is:
