Question

Why do ${}_{17}{}^{35}\mathrm{Cl}$ and ${}_{17}{}^{37}\mathrm{Cl}$ have the same chemical properties? In what respect do these atoms differ? What is the fractional mass of chlorine?

Answer 1

Reason for the same chemical properties:

• A chemical property is determined by the ability of a substance to undergo a chemical change.
• A chemical change occurs during a chemical reaction.
• During a chemical reaction, there is the involvement of electrons. So it is safe to say that chemical properties are determined by the electrons and their electronic configuration.
• ${}_{17}{}^{35}\mathrm{Cl}$ and ${}_{17}{}^{37}\mathrm{Cl}$ have the same number of electrons i.e.; 17 and their electronic configuration is 2, 8, 7. Therefore, they have the same chemical properties.

Differences:

Mass number = Number of protons + Number of neutrons

(The mass number is indicated as a superscript)

Number of neutrons = Mass number - Number of protons

Number of protons = Atomic number (indicated as a subscript)

Isotope	${}_{17}{}^{35}\mathrm{Cl}$	${}_{17}{}^{37}\mathrm{Cl}$
Mass number	35 u	37 u
Number of neutrons	35-17= 18	37- 17= 20

Fractional mass of Chlorine (Cl):

The fractional mass of chlorine is 35.5 u.

• When the value of atomic mass exists as a fraction instead of a whole number, it is called fractional atomic mass.
• Fractional atomic mass occurs for those elements where atoms of the same elements have more than one value of atomic weight. Example: chlorine. Some atoms of chlorine have an atomic mass of 35 u and others have an atomic mass of 37 u.
• Fractional atomic mass = fraction of ${}_{17}{}^{35}\mathrm{Cl}$× atomic weight of ${}_{17}{}^{35}\mathrm{Cl}$+ fraction of ${}_{17}{}^{37}\mathrm{Cl}$× atomic weight of ${}_{17}{}^{37}\mathrm{Cl}$.