Question

uranium - 238 decays into thorium - 234 by emitting......
gamma rays
a beta particle
an alpha particle
visible light

Explanation:

Uranium - 238 has an atomic number of 92 and a mass number of 238. Thorium - 234 has an atomic number of 90 and a mass number of 234.
When Uranium - 238 decays to Thorium - 234, the mass number decreases by \(238 - 234=4\) and the atomic number decreases by \(92 - 90 = 2\).
An alpha particle (\(\alpha\) - particle) has a mass number of 4 and an atomic number of 2. When a nucleus emits an alpha particle, its mass number decreases by 4 and its atomic number decreases by 2.
Gamma rays (\(\gamma\) - rays) are electromagnetic waves and do not change the mass number or atomic number of the nucleus. A beta particle (\(\beta\) - particle) has a mass number of 0 and an atomic number of - 1 (in terms of nuclear charge). When a nucleus emits a beta particle, its mass number remains the same and its atomic number increases by 1. Visible light is also an electromagnetic wave and has no effect on the nuclear composition.

Answer:

C. an alpha particle