(ATPT) Atomic Number and Mass Number Lesson

Atomic Number and Mass Number

Every element has a unique number of protons. In other words, an atom can be classified as a particular element based solely on how many protons it has. The number of protons that an element has is called the atomic number. Therefore, you can use atomic number and number of protons interchangeably because they mean the exact same thing. You can find an element's atomic number by looking at the periodic table of elements. The atomic number is a whole number ranging from 1 to 118. The atomic number tells us the number of protons in the nucleus.  

Look at the periodic table below. The periodic table displays all of the known elements and is arranged in order of increasing atomic number. On the periodic table, an element's atomic number is indicated above the elemental symbol. Hydrogen, at the upper left of the table, has an atomic number of 1. Every hydrogen atom has one proton in its nucleus. Next on the table is helium, whose atoms have two protons in the nucleus. Lithium atoms have three protons, beryllium atoms have four, and so on. As you can see, any atom with an atomic number of 2 is a helium atom, and any atom with a different number of protons would be a different element.

If it is difficult to see the information in the individual boxes on the Periodic Table shown above, here is a close up of the information you would find for each element:

Hydrogen description Links to an external site.

Rutherford's experiment showed that the vast majority of the mass of an atom is concentrated in its nucleus, which is composed of protons and neutrons. The mass of an electron is very small compared to the mass of a neutron or proton, so the electrons in an element do not contribute much to the total mass. The mass number is defined as the total number of protons and neutrons in an atom. Remember that both protons and neutrons have a mass of approximately 1 amu. Knowing the mass number and the atomic number of an atom, which is given on the periodic table, allows you to determine the number of neutrons present in that atom by subtraction:

Number of neutrons = mass number - atomic number

The composition of any atom can be illustrated with a shorthand notation, sometimes called A-Z notation, using the atomic number (Z) and the mass number (A). The general form for this notation is as follows:

For example, carbon will look like this :$\frac{12}{6}$ C

Another example is an iron nucleus is composed of 26 protons and 30 neutrons, is denoted as$\frac{56}{26}$ Fe where the atomic number is Z = 26 and the mass number A = 56 . Therefore, the number of neutrons is simply the difference N = A − Z = 30.

You need to be able to determine how many protons, neutrons, and electrons an atom has from the information provided on the periodic table. Look at the filled-in portion of the table below and then see if you can fill out the blanks in the table underneath. Remember, you need to use a periodic table! There are also a number of free periodic table apps that you can download to your smartphone/tablet for additional easy access.  

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