Final answer:
An element changing from an atomic mass of 70 to 66 has undergone alpha decay, which involves the emission of an alpha particle consisting of two protons and two neutrons, leading to a decrease of four units in its atomic mass.
Step-by-step explanation:
If an element goes from an atomic mass of 70 to 66, it has undergone alpha decay. During alpha decay, a nucleus emits an alpha particle, which consists of two protons and two neutrons. This process results in a decrease in the atomic number by two and the atomic mass by four. Therefore, a nuclear equation that shows a mass decreasing by four units suggests that an alpha particle has been emitted from the nucleus.
A typical nuclear equation for alpha decay is shown by the uranium-238 to thorium-234 transformation:
¹¹¸8U → ¹¹¸4Th + ¹2⁴He.
In comparison to alpha decay, other types of decay such as beta decay (including beta-minus and positron emission or beta-plus), gamma decay, and electron capture do not result in a change of mass by four units.