It's an example of a molecule
Coefficients, you can’t change the subscripts but you can balance the equation by adding coefficients to both sides
Explanation:
The chemical formula of an ionic compound can be written by using the symbols of the respective cations and anions.
The overall charge on the molecule should be zero.
Hence, the total charge of cations=total charge of anions.
The symbols of the given molecules are shown below:
sodium chloride ---- NaCl
magnesium chloride ---
calcium oxide ---- CaO
lithium phosphide----
aluminum sulfide ----- 
calcium nitride---- 
Density is the ratio of mass to the volume.
The mathematical expression is given as:

Now, density of isooctane = 
Volume = 
Since, 1 gallon = 3.78 L
So, 3.8 gal = 
= 
As, 1 L = 1000 mL
Therefore,
= 
Volume in mL = 
Put the values,


= 
Hence, mass of 3.8 gal of the gasoline is
.
The mass number of this particular element is 37
An element with 17 protons will ALWAYS be chlorine.
The mass number is found by adding the element’s protons and neutrons.
protons+neutrons=mass number
Here is the math if needed,
17+20=37
Therefore the mass number is 37
I hope this helped!