Atomic Number and Mass Number

Here is an example of potassium, one of the elements that can be found on the Periodic Table. This is how it appears on the table. In the middle is the scientific  symbol for the element. Beneath the symbol is the element's name. Above and below the symbol are numbers. These are the atomic number and the mass number.

19 (atomic number)

K (symbol)

Potassium (element name)

39 (mass number)

The atomic number at the top is the number of protons and the number of electrons in an atom. In this example the atomic number is 19. That means there are 19 protons and 19 electrons. If the atomic number was 6, then there are 6 protons and 6 electrons. Get it?

The mass number at the bottom is the number of protons PLUS the number of neutrons in an atom. Since it is the sum of the two numbers added together, we don't know how many of each there are. In other words, since the mass number is 39, does that mean there are 20 protons and 19 neutrons or 12 protons and 27 neutrons??? We don't know yet! This number is used to figure out how many neutrons an atom has.

The number of neutrons is found by subtracting the atomic number from the mass number.

Mass number - atomic number= number of neutrons

In the example, potassium has a mass number of 39 and an atomic number of 19. To find the number of neutrons, subtract the atomic number (19) from the mass number (39).

39-19=20

 

There are 20 neutrons in an atom of potassium. See if you can figure out how I got the number of neutrons in these two elements. Use your periodic table or the one at this web site to get the mass and atomic numbers.

go to Periodic Table

 

	argon	iron
protons	18	26
electrons	18	26
neutrons	22	30

 

Still confused? The answers are below.

argon: atomic number=18, mass number=40

neutrons 40-18= 22

iron: atomic number=26, mass number=56

neutrons 56-26=30

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Questions and comments: lcelaya@amphi.com