Question

An atom contains 16 protons, 18 neutrons, and 16 electrons what is its mass number?

Answer 1

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,  

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

+

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.

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,  

Ne

, the noble gas that comes before sulfur in the periodic table, will look like this

S:  

[

Ne

]

3

s

2

3

p

4