Take note that the nucleus of an atom is composed of protons and neutrons. And the number of particles present in the nucleus is referred as mass number (Also, called as atomic mass).

mass number = Number of Protons + Number of Neutrons

So, to determine the number of neutrons in atom, we only have to subtract the number of protons from the mass number.

# of neutrons = Mass Number - Number of Protons

To be able to apply this formula, refer to the atomic number and atomic weight of the elements. These two are listed in the Periodic Table of Elements.

Take note that atomic number refers to the number of protons in an atom of an element.

So,

# of protons = atomic number

While, atomic weight is weighted average of the atomic masses of all the natural isotopes of an element. Its values in the Periodic Table are in decimal form. So to get the atomic mass, round off the atomic weight to its nearest whole number.

mass number = atomic weight rounded off to nearest whole number

For example, sodium's atomic number is 11 and its atomic weight is 22.989. So, sodium contains 11 protons and its mass number is 23.

Applying the formula above, the number of neutrons will be:

# number of neutrons = Mass Number - Number of Protons

# number of neutrons = 23 - 11

# number of neutrons = 12

Hence, there are are 12 neutrons in an atom of sodium.

How do I get the number of neutrons?

The periodic table of elements will give you an atom's atomic number.  That is the number of protons (and electrons for neutral atoms) in an atom.  In order to figure out the number of protons in the atom, you need to know what the atom's mass number is.  The mass number is the sum of the protons and neutrons within an atom.  The simplest and most direct way of finding an atom's mass number is to round off the atomic mass listed in the periodic table.  Take the rounded off atomic mass (which is the mass number) and subtract the atomic number from it.  The difference is the number of neutrons.  

Let's take carbon for example.  It's atomic number is 6.  That means it has 6 protons.  It's atomic mass is listed as 12.011.  That would round down to 12.  12 - 6 = 6.  Carbon has six neutrons.  

Now let's look at boron.  It's atomic number is 5.  It's atomic mass is 10.81.  That rounds up to 11.  11 - 5 = 6.  Boron has 6 neutrons.  

How can I find the number of neutrons in an atom?

Knowing some basic information about the sub-atomic particles that make up an atom will help you determine the number of neutrons in different situations:

1. Protons have a positive charge, and have a mass of approximately 1 amu and are found in the nucleus.

2. Neutrons are electrically neutral, and have a mass of approximately 1 amu and are found in the nucleus.

3. Electrons have a negative charge, and have a mass of approximately 0 amu and are found outside the nucleus.

4. The atomic number of an atom is determined by its number of protons.

5. A neutral atom has the same number of protons as electrons.

Here are some ways to determine the number of neutrons in an atom, depending on the data you already have:

• (mass number) - (number of protons) = (number of neutrons)
• if neutral, (mass number) - (number of electrons) = (number of neutrons)
• If you know the element name or symbol and its mass, such as carbon-14, find the atomic number on the periodic table and subtract it from the mass.