Question

The number of electrons plus neutrons in $_{19}^{40}{K^ + }$ is.
(A) $38$
(B) $59$
(C) $39$
(D) $40$

Answer 1

Number of electrons in a neutral atom I equal to the number of protons. But in charged atoms if an atom is positively charged then the number of protons are greater than the number of electrons while for negatively charged atoms the number of electrons are greater than the number of protons.

Complete step by step solution:
First of all let us talk about the atom, protons, electrons and neutrons.
Atoms: It is the smallest unit of an element. The elements are made of the same kind of atoms. In the atoms there are three particles: electrons, protons and neutrons.
Electrons: They are the negatively charged particle having negligible mass. They move in the atoms outside the nucleus in a fixed orbit.
Protons: They are positively charged particles. They have mass. They are present in the nucleus (a special room in the atoms, where neutrons and protons are present).
Neutrons: They have no charge but they have mass. They are also present in the nucleus along with protons.
Atomic mass: It is defined as the total number of protons and neutrons present in the nucleus of an atom. It is represented by symbol A.
Atomic number: It is defined as the number of protons in the nucleus of an atom. It is represented by symbol Z.
Here we are given with the atom as $_{19}^{40}{K^ + }$ Here the atomic number is $19$ and we know that the number of protons is equal to the atomic number. An atomic mass is $40$ and atomic mass is the sum of the number of protons and number of neutrons so the number of neutrons will be $40 - 19 = 21$ . And here we can see that potassium has one positive charge so the number of electrons will be one less than the number of protons so the number of electrons will be $19 - 1 = 18$ .
Now, we have to write the sum of electrons i.e. $18$ and neutrons i.e. $21$ which is equal to $18 + 21 = 39$ .
So option C is correct.

Note:
Isotopes: Those elements which have the same atomic number but different atomic mass. For example: $C - 12$ and $C - 13$ . They both have the same atomic number i.e. $6$ but different atomic mass i.e. in $C - 12$ mass is $12$ and in $C - 13$ mass is $13$ .
Isobars: Those elements which have the same atomic mass but different atomic numbers. For example: argon and calcium. The atomic number of argon is $18$ and that of calcium is $20$ but the atomic mass of both the elements is the same i.e. $40$ .