The following problems are taken from old exams .
1) How many neutrons are there in the nucleus of a
Explanation: The number superscript left of the element symbol ‘Th’ is the mass
number, which is the total number of protons and neutrons in the nucleus of the
atom. We can find the number of protons present by looking up Th (Thorium) on
the periodic table (which lists the elements in order of increasing atomic number
(number of protons)). Thorium is element number 90, hence we find the number
of neutrons by subtracting the atomic number (protons) from the mass number
(protons + neutrons).
After all that:
232 – 90 = 142 neutrons
Which of the following pairs of atoms represent isotopes?
Explanation: Isotopes are atoms of the same element (note that therefore they
have the same number of protons in their nuclei, as it is the number of protons
(atomic number) which gives an atom it’s identity !!!) which have different mass
numbers, meaning that they have different numbers of neutrons in their nuclei.
Isotopes will hence have the same element symbols (they’re atoms of the same
element), so we immediately can spot the answer as D.
3) Naturally occurring chlorine consists of the two isotopes: chlorine-35 and chlorine-37.
The atomic mass for chlorine is approximately 35.5. What are reasonable estimates of
the relative percentages of chlorine-35 and chlorine-37, respectively?
A) 75, 25
B) 50, 50
C) 67, 33
D) 25, 75
E) 33, 67
Explanation: The atomic mass for an element, as stated on the periodic table, is
a weighted average of all the isotopes present. In this question we have two
isotopes, namely chlorine-35 and chlorine-37. We are not told the atomic masses
of each isotope, though we know that atomic masses are measured relative to
carbon-12. Pure isotope carbon-12 has an atomic mass of 12.0000 u, meaning
that 1 nucleon (a particle which resides in the nucleus of an atom) has a mass of
around 1 u (12 nucleons in carbon-12, which has a mass of 12.0000 u). Note that
each nucleon is not 1 u
, as neutrons and protons have different
Our isotopes are around 35 u and 37 u. The average is 35.5 u,
so we must have a lot more Cl-35 than Cl-37 since the average is closer to 35 u.
The value 35.5 is actually ¼ of the way between the numerical values 35 and 37,