# Question #4b778

Aug 1, 2017

$Z = 4$

$A = 9$

#### Explanation:

You know that your atom contains $\textcolor{red}{4}$ protons and $\textcolor{b l u e}{5}$ neutrons inside its nuclues and $4$ electrons surrounding its nucleus.

Now, the atomic number of an element, $Z$, is simply the number of protons that it contains inside its nucleus.

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

In your case, you know that the atom contains $\textcolor{red}{4}$ protons inside its nucleus, so you can say that

$Z = \textcolor{red}{4}$

The mass number of an atom, $A$, tells you the number of protons and neutrons, or, in other words, of nucleons, present inside its nucleus.

$\text{mass number" = A = "no. of protons" + "no. of neutrons}$

$Z = \text{no. of protons}$

you can say that you have

$A = Z + \text{no. of neutrons}$

This tells you that if you add the number of neutrons present inside the nucleus and the atomic number of the element, you get the mass number of the atom.

In your case, you will have

$A = \textcolor{red}{4} \textcolor{w h i t e}{.} \text{protons" + color(blue)(5)color(white)(.)"neutrons}$

$A = 9$

Therefore, your atom will have an atomic number equal to $4$ and a mass number equal to $9$.