Atomic number represents the number of protons in an atom. As atoms are electrically neutral, an atom contains as many electrons as it has protons.
Atomic mass, mass number or nucleon number
The total number of protons and neutrons present in one atom of an element is known as its mass number.
Mass number = number of protons + number of neutronsIt can also be said that:
Mass number = Atomic number + number of neutronsAtomic number and mass number can also be represented on the symbol of an element as follows:
Let's take an example:
- Represent the atom of sodium whose atomic mass is 23 and atomic number is 11. Calculate the number of protons, electrons and neutrons.
23Na11
Atomic number Z =11Atomic mass A = 23
No. of protons = Z = 11
No. of electrons = 11
23 - 11 = 12
Study the table carefully
| Lithium - 7 | Silicon - 28 | Copper - 65 | Dysprosium - 164 | Uranium - 238 | |
|---|---|---|---|---|---|
| Proton Number Z | 3 | 14 | 29 | 66 | 92 |
| Nucleon number A | 7 | 28 | 65 | 164 | 238 |
| Number of neutrons N (A-Z) | 4 | 14 | 36 | 98 | 146 |
| Symbol | 7 Li 3 | 28 Si 14 | 65 Cu 29 | 164 Dy 66 | 238 U 92 |
Attempt the following:
- The atomic number of an element is 12. How many protons and electrons are there in the atom?
Solution:
Atomic number = 12
Number of protons = Number of electrons = Atomic number = 12- The nucleus of an atom of an element contains 11 protons and 12 neutrons. Determine the atomic number and mass number of the element.
Solution:
Atomic number = number of protons = 11
Mass number = number of protons + number of neutrons= 11 + 12 = 23
