Question

Symbol ${}_{35}^{79}Br$and ${}_{{}}^{79}Br$ can be written, where as symbols ${}_{79}^{35}Br$ and ${}_{{}}^{35}Br$ are not acceptable. Answer briefly:

Answer 1

The atomic number of an element should be written in the subscript and mass number (number of protons + number of neutrons) should be written in the superscript of the element when we are going to represent the symbol of an element.

Complete step by step answer:

• Coming to the given element in the question.
• The given element is ${}_{35}^{79}Br$ and we can write as ${}_{{}}^{79}Br$ also.
• The representation of bromine is correct in this form ${}_{35}^{79}Br$, because always atomic number (small number) should be a subscript and mass number (large number) should be written as superscript.
• We can represent ${}_{35}^{79}Br$ in the form of ${}_{{}}^{79}Br$ also. No problem of representing superscript only.
• But coming to ${}_{79}^{35}Br$, it is showing mass number as subscript and atomic number as superscript, meaning smaller number as superscript and bigger number as subscript. This is a right way to represent the atom or element.
• We cannot represent ${}_{35}^{79}Br$ as ${}_{{}}^{35}Br$, because atomic numbers should be written in the subscript only.
• The correct representation of atomic number and mass number of any element is as follows.

${}_{atomic\text{ }number}^{mass\text{ }number}X$

• Here X = any element of the periodic table, atomic number should be at subscript and mass number should be at superscript.

Note:
All isotopes of a particular element have the same number of protons (same atomic number) but different numbers of neutrons (different mass number) in each atom. Examples of isotopes are ${}_{1}^{1}H$, ${}_{1}^{2}H$ and ${}_{1}^{3}H$.
Hydrogen has three isotopes, they have the same atomic number and different mass number.
Isobars are the chemical elements that differ in atomic number but have the same mass number. An examples of isobars are ${}_{18}^{40}Ar$, ${}_{19}^{40}K$ and ${}_{20}^{40}Ca$. All the mentioned elements have the same mass number but different atomic number.
Isotones are the chemical elements that have the same neutron number N (mass number-atomic number), but different proton number Z (atomic number). For example ${}_{6}^{14}C$ and ${}_{8}^{16}O$ are isotones, because ${}_{6}^{14}C$(14-6 = 8 neutrons) and ${}_{8}^{16}O$(16 – 8 = 8 neutrons). Both the elements have different atomic number and different mass number but they have the same number of neutrons.