# Question 4e40c

Dec 31, 2017

Here's what I got.

#### Explanation:

The idea here is that, for any neutral atom, you have

$\textcolor{b l u e}{\underline{\textcolor{b l a c k}{\text{no. of electrons = no. of protons}}}}$

and

$\textcolor{b l u e}{\underline{\textcolor{b l a c k}{\text{mass number = no. of protons + no. of neutrons}}}}$

In your case, the element is said to have an atomic number equal to $50$ and a mass number equal to $120$.

The atomic number tells you the number of protons located inside the nucleus, so you can say that this element has $50$ protons inside its nucleus.

The mass number tells you the number of protons and neutrons located inside the nucleus. This means that you can use the mass number and the atomic number to find the number of neutrons located inside the nucleus.

$\text{no. of protons" = "mass number " - " atomic number}$

In your case, you have

$\text{no. of neutrons} = 120 - 50 = 70$

Since no information is provided about a net charge, you can assume that the atom is neutral and conclude that it contains

$\text{no. of electrons} = 50$

surrounding its nucleus. Therefore, you can say that this element, let's say $\text{X}$, which in isotopic notation is written as $\text{_(color(white)(1)50)^120"X}$, has

""_(color(white)(1)50)^120"X" => {("50 protons"), ("70 neutrons"), ("50 electrons") :}#