# A certain atom consists of 16 protons, 18 neutrons, and 16 electrons. What is its atomic number, mass number, and electron configuration?

Dec 21, 2015

Here's what I got.

#### Explanation:

So, you know that your atom contains

• $16$ protons
• $18$ neutrons
• $16$ electrons

Right from the start, you know that you're indeed dealing with a neutral atom, since the number of protons it has in its nucleus is equal to the number of electrons it has surrounding its nucleus.

Now, an atom's atomic number, $Z$, tells you how many protons it has in its nucleus. Nothing more, nothing less.

In your case, you are told that the atom contains $16$ protons in its nucleus, which means that $Z$ will be equal to

$Z = 16$

A quick look in the periodic table will reveal that you're dealing with an atom of sulfur, $\text{S}$.

An atom's mass number, $A$, tells you how many protons and neutrons it contains in its nucleus. Since the number of protons is given by $Z$, you can say that

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

In your case, the atom contains $18$ neutrons in its nucleus. This means that $A$ will be equal to

$A = 16 + 18 = 34$

Finally, focus on the atom's electron configuration, which you know that must account for a total of $16$ electrons.

$\text{S: } 1 {s}^{2} 2 {s}^{2} 2 {p}^{6} 3 {s}^{2} 3 {p}^{4}$

Its noble gas shorthand notation, which uses the electron configuration of neon, $\text{Ne}$, the noble gas that comes before sulfur in the periodic table, will look like this

"S: " ["Ne"] 3s^2 3p^4