To find the atomic number from the mass number, you need to understand the notation used to represent isotopes of an element. The notation consists of the atomic symbol, which represents the element, along with the mass number and atomic number.
The mass number (A) represents the total number of protons and neutrons in the nucleus of an atom. The atomic number (Z) represents the number of protons in the nucleus of an atom, which determines the element's identity.
Here's how you can find the atomic number (Z) from the mass number (A):
Look up the element in the periodic table. The atomic number is typically listed below the symbol of the element.
Identify the mass number (A) of the isotope you are interested in.
Subtract the number of neutrons (N) from the mass number (A) to find the number of protons (Z). The number of neutrons can be calculated by subtracting the atomic number (Z) from the mass number (A).
N = A - Z
For example, let's consider an isotope with a mass number (A) of 40. The element associated with this isotope is calcium (Ca), which has an atomic number (Z) of 20. Subtracting the atomic number from the mass number, we find:
N = 40 - 20 = 20
Therefore, for this example, the isotope has 20 neutrons.