And there is this question I do not quite understand
What do both an atom and an ion of the isotope of sodium with
nucleon no. 23 and proton no. 11 Na, contain?
A 11 electrons
B 12 neutrons
C 23 protons
D 23 neutrons
Answer is B. But why should it both be 12? after all they
already said the atom and ion of Na are isotope of Na? That's why I
eliminated option B and D initially.
There is a very common misconception among both 'O' and 'A'
level students (and teachers!). Many students write the definition
of isotopes as :
"Isotopes are atoms with the same no. of protons but different
no. of neutrons."
WRONG.
Isotopes are NOT atoms. They are "types" of an element
(and elements are defined by their unique number of
protons).
The correct definition of isotopes is :
"Isotopes of elements have atoms/ions, that have the same
number of protons but different number of neutrons."
Hence, for any given isotope of an element, eg. the isotope of
sodium with nucleon no. 23, the Na atom and the Na+ ion
will BOTH contain :
11 protons
12 neutrons
The only difference between an atom and an ion (of the same
isotope) is of course, the number of electrons (Na atom has 11
electrons, Na+ ion has 10 electrons).