In this question, it says write the
symbol for every chemical element that
has an atomic number greater than 55 and
an atomic mass less than 143.1u .
So on a periodic table, the atomic number
is generally written above the element,
and it can be a little bit difficult to
see my periodic table here, but the one
you pull up on Aleks,
you know, is very easy to see. So 19 is
the atomic number for potassium. This
tells us that for potassium, there
are 19 protons in the nucleus. The atomic
number defines the element so all
potassiums have 19 protons in the nucleus; anything we call potassium has 19 protons
in the nucleus. So every way you,
whichever way you want to think about it.
In this case you'll see (and in all cases)
that they're in order, so helium has one
Pro - excuse me - hydrogen has one proton, helium has two, lithium has three. So we go
to 55, which is cesium, but we want an
atomic number greater than 55, so we want
to start with 56. And if you look at the
mass of barium, it's 137.33 which
is less than 143.1 . The number
below the element is the average atomic
mass. So we want every element with an
average atomic mass less than 143.1,
but with an atomic number greater than
55, starting with number 56. So the first
one is barium. You'll notice that the next
element on the periodic table is number
71. That's because what breaks out right
here are what's called the lanthanides and
actinides. We don't talk about these a
tremendous amount in general chemistry,
but since they're in this question, we can
certainly discuss them. So 56 is here, 57
is here, which is lanthanum, which also
has a molar mass, an average molar mass,
less than 143.1 . Followed
by cerium Ce, followed by Pr. When you get
to neodymium, what you find is that
the molar mass is 144.24,
which is greater than 143.1, so
we do not want to include that.
