Considering the definiton of atomic number and atomic mass, the number of neutrons of the potassium is 21.
All atoms are composed of a central nucleus in which there are particles with a positive electric charge, the protons, and a neutral charge, the neutrons, around which other particles with a negative electric charge, which are electrons, move.
Atoms of different elements have different amounts of protons. The atomic number (which is identified by the letter Z) indicates the amount of protons that is present in the nucleus of an atom. This number is then responsible for defining the electronic configuration of the atom and the chemical properties of the element.
On the other side, the atomic mass (A) is defined as the sum of number of protons and number of neutrons:
Atomic mass = Number of protons + Number of neutrons
In this case, the potassium (K) elements has atomic number 19. That means, the number of of protons is equal to 19. And the atomic mass is 40. So, the number of neutrons is calculated as:
40= 19 + Number of neutrons
Solving:
Number of neutrons= 40 - 19
Number of neutrons= 21
Hence, the number of neutrons of the potassium is 21.
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