The answer to why chlorine has an atomic weight
of 35.5 not 35 is because of something called
Isotopes are atoms with the same number of
protons (in the case of chlorine that means 17
protons) but different numbers of neutrons.
Chlorine has two common isotopes, Chlorine-35 and
Chlorine-35 has 17 protons and 18
neutrons and occurs in nature about 75% of the
time. Chlorine-37 has 17 protons and 20 neutrons
and occurs in nature about 25% of the time. That
means that in any mixture of pure chlorine that
can be isolated from all other elements there is
75% Chlorine-35 and 25% Chlorine-37. So, when
the mass of chlorine is measured using an
instrument called a mass spectrometer the
result is a mass of 35.5. The atomic weight, or as
it is properly called, the RELATIVE atomic
mass, is a weighted average of the masses of all
the isotopes of chlorine.
Relative Atomic Mass = (75/100 x 35) +
(25/100 x 37) = 35.5
Isotopes of any element behave the same
chemically because neutrons do not have any
charge. Both isotopes of chlorine have 17
positive charges from the 17 protons in the
nucleus and 17 negative charges from the electrons
in the surrounding orbitals, and so it does not
matter which isotope is in a chemical reaction.
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