A Levels Physics

U** is a doubly-ionised uranium atom. The uranium atom has a nucleon number of 235 and a
proton number of 92.
In a simple model of the atom, how many particles are in this ionised atom?
А 235
в 325
C 327
D 329


Choice B is correct because:

Number of neutrons in the ionised uranium atom

Number of neutrons = Number of nucleons - Number of protons

Number of neutrons = 235 - 92 = 143

Number of protons in the ionised uranium atom 

Number of protons = 92

Number of electrons in the ionised uranium atom

In an unionised atom, the number of electrons = the number of protons = 92

However, the uranium atom undergoes double ionisation. The '++' on U++ denotes that two electrons are removed from the atom.

Therefore, the number of electrons = 92 - 2 = 90

Particles in the ionised atom = Number of neutrons + Number of protons + Number of electrons

Particles in the ionised atom = 143 + 92 + 90 = 325

Choices A, C & D are incorrect because they do not correctly state the number of particles in the ionised atom to be 325.

Particle and Nuclear Physics | 2014 - M/J - 9702/11 | Q39