My answer -
In general, there are two defining numbers for all elements and their
isotopes: the mass number, and the atomic number. These are usually
denoted as follows when an element is written:
AZ X
where A is the mass number, Z is the atomic number, and X is the symbol
for the element in question (note that the A and Z would normally be
aligned, not offset). The mass number, A, is the sum of the protons and
neutrons in the element. The atomic number, Z, is the number of protons
only. On the periodic table, elements are arranged by atomic number. In
this case we see that Carbon is element 6, and so it had 6 protons. The
number of neutrons alone can be easily calculated by subtracting the
atomic number from the mass number, so that
N=A-Z
where N is the number of neutrons. The number of electrons in a given
element, isotope, or ion, can be easily determined as well. In neutral
isotopes, the number of negatively-charged electrons must balance the
number of positively-charged protons. Since the number of protons is
given to us by the atomic number Z, this must also be the number of
electrons. In ions, the plus or minus charge denoted tells us the
overall charge of the species in question, and thus the lack or excess
of electrons. A species with a +2 charge would have two less electrons
than expected, while a -1 charge would mean one excess electron.
In the specific question at hand, we can look up Carbon in the periodic
table and find that it is element number 6. It thus has 6 protons. The
13 to which the question refers is the mass number, or protons plus
neutrons. Since we know N=A-Z, and that A and Z are 13 and 6
respectively, there are 13-6=7 neutrons. The species has no charge, so
electrons must balance protons: this isotope has 6 electrons.
p.s
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