Question

What is the number of protons, neutrons and electrons in ${}_{18}^{40}Ar$ ?

Answer 1

Hint : We can find the number of any charged subatomic particle from the given in its chemical formula. From the notation we must understand that the number on the left top corner in the symbol is mass number, while the number on the left bottom corner is the atomic number. Carefully remember that the atomic number is able to supply the total number of protons, also keep in mind the number of protons and electrons are most of the time, the same.
$Mass\;number(A) = proton\;count + neutron\;count$

Complete Step By Step Answer:
Let us understand the concepts of protons, neutrons and electrons to then identify or calculate their number from the given chemical formula of isotope of Argon.
Keep in mind that the neutrons have no charge, while the protons have positive charges and at the same time electrons are negatively charged. The charge on the proton and electron is the same size but in the opposite direction. In a neutral atom, the same number of protons and electrons cancel each other out exactly.
Here,
If atomic number and proton number is the same then we can write this;
$\Rightarrow$ $The{\text{ }}atomic{\text{ }}number(Z) =$ $total{\text{ }}proton{\text{ }}count = 18$ ;
since the atomic number here is the number on the bottom left corner, so it is $18$ .
Then this means we have found the electron number also since its equal to proton number;
$\Rightarrow$ $Total{\text{ }}electron{\text{ }}count = 18$
There is another formula to calculate the count of neutrons;
$\Rightarrow neutrons = Mass\;number - Atomic{\text{ }}number$
Here as per the chemical formula given, mass number is $40$ .
So;
$\Rightarrow Neutron\;count = 40 - 18 \\\ \Rightarrow Neutron\;count = 22 \\\$
$\therefore$ We can say that the counts of proton, electron and neutron within ${}_{18}^{40}Ar$ are as follows;
Proton count $= 18$
Electron count $= 18$
Neutron count $= 22$

Note :
Always note that we don’t get confused between mass and atomic number from the chemical notation, the top left is mass number and bottom left is atomic number. The atomic number of a chemical element in the periodic system, it arranges the elements in order of increasing protons in the nucleus. As a result, the atomic number is the number of protons in the neutral atom, which is always equal to the number of electrons.