Question

an atom with an atomic number of 50 and a mass number of 120 contains
50 protons, 50 electrons, 20 neutrons
50 protons, 50 electrons, 50 neutrons
50 protons, 50 electrons, 70 neutrons
120 protons, 50 electrons, 70 neutrons

Explanation:

Step1: Recall atomic number concept

The atomic number ($Z$) is equal to the number of protons and the number of electrons in a neutral atom. Given atomic number $Z = 50$, so the number of protons $p=50$ and the number of electrons $e = 50$.

Step2: Calculate number of neutrons

The mass - number ($A$) is the sum of the number of protons and neutrons. The formula is $A=p + n$. Given $A = 120$ and $p=50$, we can solve for the number of neutrons $n$. Rearranging the formula gives $n=A - p$. Substituting the values, we have $n=120 - 50=70$.

Answer:

50 protons, 50 electrons, 70 neutrons