Question

How many Protons and Neutrons in these isotopes? (* Note that mass follows the name of the elements)
Chromium-$58$
Chromium- $63$
Carbon- $12$
Carbon- $15$
Nitrogen- $15$
Nitrogen- $20$
Sodium- $12$
Sodium- $20$
Sulfur- $23$
Sulfur- $25$

Answer 1

Hint: As it is given that each element is followed by its name and it’s mass. We know that for writing the element in the symbolic form we used to write it’s atomic number (Z) and atomic mass (A) as ${}_Z^AX$.Number of electrons and the number of protons are always equal to each other and also equal to the atomic number of elements.

Complete step-by-step answer:
We have given the atomic mass of the elements in terms of pairs of isotopes. For writing the number of protons we have to firstly know that the atomic number is equal to the number of protons and electrons. While the sum of protons and neutrons is equal to their atomic mass.
$(1)$ We have two isotopes of chromium- Chromium-$58$ and Chromium- $63$ written as ${}_{24}^{58}Cr$ and ${}_{24}^{63}Cr$ , it is clear from the symbol that atomic number of chromium is $24$ so number of protons in both isotopes are same that is $24$ while number of neutrons are $(atomic{\text{ }}mass\, - \,number\,of\,electrons)$ means for ${}_{24}^{58}Cr$ $(58 - \,24) = \,34$ , so for ${}_{24}^{58}Cr$ \[protons = 24,\,neutrons = \,34\].
Similarly for ${}_{24}^{63}Cr$ \[protons = 24\] and \[neutrons = \,63 - 24 = 39\].

$(2)$ For two isotopes of carbon, ${}_6^{12}C$ and ${}_6^{16}C$ number of protons are equal for both \[protons = 6\] while as they are isotopes they have different atomic masses, so number of neutrons comes out to be different for both, for ${}_6^{12}C$ \[neutrons = \,12 - 6 = 6\] and for ${}_6^{16}C$ \[neutrons = \,16 - 6 = 10\].

$(3)$ For two isotopes of nitrogen ${}_7^{15}N$ and ${}_7^{20}N$ the number of protons and electrons comes out to be same as for all the above \[protons = 7\] and number of neutrons for which we can write $(atomic{\text{ }}mass\, - \,number\,of\,electrons)$ , for ${}_7^{15}N$ \[neutrons = \,15 - 7 = 8\] and for ${}_7^{20}N$ \[neutrons = \,20 - 7 = 13\].
For isotopes of sodium ${}_{11}^{12}Na$ and ${}_{11}^{20}Na$ , the number of protons are same \[protons = 11\] as they are isotopes and number of neutrons for ${}_{11}^{12}Na$ \[neutrons = \,12 - 11 = 1\] while for ${}_{11}^{20}Na$ \[neutrons = \,20 - 11 = 9\].

For isotopes of sulphur, ${}_{16}^{23}S$ and ${}_{16}^{25}S$, the number of protons are same \[protons = 16\] while number of neutrons for ${}_{16}^{23}S$ \[neutrons = \,23 - 16 = 7\] and for ${}_{16}^{25}S$ \[neutrons = \,25 - 16 = 9\].So like this way you can easily find out the number of electrons, protons and neutrons for isotopes.


Note: Here we are given with the isotopes for which atomic number is same, by the definition of isotopes they are atoms of same elements having same atomic number but different mass number. But for isobars atoms of different elements having the same atomic mass and different atomic number, in that case the number of electrons will be different.