>> Hey, it's
Professor Dave. Let's talk about
nuclide symbols. >> ♪ He knows a lot about
the science stuff ♪ ♪ Professor Dave
Explains ♪ We know that atoms
have a tiny nucleus with positively charged
protons and neutrons, which have
no charge, as well as negatively
charged electrons that are some distance
away from the nucleus. Every element is
a combination of some number of
these three particles. The one that determines
which element an atom belongs to
is the proton. One proton means hydrogen,
two is helium, three is lithium,
and so forth. So an element has
an atomic number, which refers to the number
of protons in the nucleus. Each atom also
has a mass number. While only protons count
for atomic number, both protons and neutrons
count for mass number because protons and
neutrons are each about 1 atomic
mass unit. Electrons are so
much less massive than protons
and neutrons that we ignore them
when we look at mass. So mass number is really
just the number of particles in the nucleus. If an atom has 6 protons
and 6 neutrons, it is an atom
of carbon-12. Here, the number is
referring to the mass. Carbon atoms always
have 6 protons. If they didn't, they
wouldn't be carbon. When we represent an atom,
we use a nuclide symbol. This will consist of
one or two letters that abbreviate
the element. If one letter,
it's capitalized. If two, the first is capitalized
and the second is not. To the bottom left, we
sometimes put the atomic number, which, as we said, is the number
of protons in the nucleus. This is kind
of redundant because the type of element
implies the number of protons, but sometimes
we do it anyway. To the upper left
is the mass number. This is not redundant
because atoms of a given element can
have different masses due to differing
numbers of neutrons. These are called "isotopes"
of a given element. Remember that the mass number
is the number of protons plus the number
of neutrons, which means that the number
of neutrons in an atom is the mass number
minus the atomic number. And to the upper right,
if the atom is an ion, meaning a particle
with electrical charge, the charge will
be listed here. If the number of
protons and electrons in an atom
are the same, the positive and negative
charges cancel out, and it will be
a neutral atom. But if it gains
an electron, there will be one more
negatively charged electron than there
are protons, so the atom will have
an overall -1 charge. If instead it loses
an electron, it will lose a negatively
charged particle, and there will now be one more
positively charged proton than there
are electrons, so it will have an
overall +1 charge. So particles with
differing numbers of protons are
different elements. Particles of a
given element with differing numbers of
neutrons are different isotopes. And particles of
a given element with differing numbers of
electrons are different ions. If we look at
the periodic table, all the elements are
arranged by atomic number. One proton in
the nucleus, all the way to a
hundred and more. Then, we can look
at the atomic masses. But if the atomic mass
is the sum of the particles
in the nucleus, shouldn't they all
whole numbers? There aren't any fractions
of a neutron after all. The reason they
have decimals is because they are
average atomic masses for all the isotopes
of that element. Remember that isotopes have
different numbers of neutrons. The average has
to reflect the relative abundance
of each isotope. For example, chlorine can be
chlorine-35 or chlorine-37. All chlorine atoms
have 17 protons, but they can have either
18 or 20 neutrons. However, about 75% of chlorine
atoms are chlorine-35, and only 25% are
chlorine-37. So the average
is not 36, because there is much
more chlorine-35 than 37. So we have to
do some math. Multiplying each mass
number by a fraction of one that represents
its abundance, and then adding these up, gives
us a more accurate number for the average mass
of all chlorine atoms. So on the periodic table, we
have average atomic masses, but any individual atom will
have its own mass number which must be a
whole number. Let's check
comprehension. (light music) ♪ ♪ Thanks for
watching, guys. Subscribe to my channel
for more tutorials, and, as always, feel
free to email me,
[email protected]. ♪ ♪