Question

Boron has two stable isotopes, $10$B (\[19\% \]) and \[11\]B (\[81\% \]). The atomic mass that should appear for boron in the periodic table is
(A) 10.8
(B) 10.2
(C) 11.2
(D) 10.0

Answer 1

Hint: Isotopes are various forms of the same element having the same atomic number but different masses. For example, various forms of hydrogen popularly known as proton, deuterium and tritium are isotopes having the same atomic number that is one but different mass number .The existence of different masses of the same element leads to the existence of atomic masses in decimal. The atomic masses can be considered as average of the masses found in nature. If the element has two masses in 50% then the atomic mass that would appear in the periodic table would be half of the sum of the masses.

Complete Step by Step Solution:
Boron has two stable isotopes having masses \[10\] and\[11\]. But the atomic mass appearing in the periodic table would have been \[10.5\]if both had the same percentage appearance in nature. But the appearance is not the same. Boron with mass number \[11\] is found more in nature than boron with mass number \[10\].
Hence the actual atomic mass is given by the multiplication of the percentage appearance and mass number. Thus, calculating it for boron
$\frac{{19(10)}}{{100}} + \frac{{81(11)}}{{100}}$
$ = 10.81$
Thus, the correct option is A.

Additional information: isobars are the forms of different elements having different atomic number but the same mass number. For example, cobalt with atomic number $27$and nickel with atomic number $28$have the same mass number that is$64$.

Note: the mass number of an element is written in superscript whereas the atomic number is written in subscript of the element.