In this video, we are going to talk more about
atoms, and specifically how to differentiate an atom
of one element from an atom of a different element.
There are several ways we can do that, the first way is by using the atomic
number.
The atomic number equals the number of protons that are in the nucleus
of that specific atom, OK. The atomic number is listed on the periodic table and I
have a few elements of the periodic table
listed here. The atomic number is the whole number that is
listed right above the symbol for the element; so for carbon, the atomic number
is 6, for nitrogen, the atomic number is 7, and for oxygen the atomic number
is
8. That tells you how many protons are in the nucleus of that atom.
Every atom of carbon has 6 protons. Every atom of
nitrogen has 7, and every atom of oxygen has
8; they will always be the same; however,
different atoms have different numbers of protons in the nucleus.
Now, the atoms are also
neutral, so that means there's no charge.
Protons, as you recall, are positively charged sub-atomic particles,
so that means we have to have the same number of negatively charged particles
to balance it out.
So the electrons are the negatively charged
subatomic particle, so we're going to have the same number of protons and neutrons
in every neutral atom.
So our atomic number, not only tells us the number of protons; it also tells us
the number of electrons
in that atom as well. So carbon has 6 protons and 6 electrons while oxygen has
6 protons,
I'm sorry, 8 protons and 8 electrons.
Another number we can use to differentiate between
atoms is the mass number. The mass number is not found on the periodic
table,
it is a counted number and it is the sum of the protons and the neutrons that are in
the nucleus.
OK, so you would have to be given the number of neutrons for that specific
atom to determine its mass number. The number of protons, you would find on the
periodic table.
Another way that you, another question you may be asked, is to
determine how many neutrons are in an atom;
in any case, if you knew the mass number, you could subtract the atomic number
(or the number of protons) and that would leave you with the number of neutrons.
Again, number of neutrons, and the mass number, cannot be found on the periodic
table;
one of those numbers would have to be provided for you in the question.
An atomic symbol
is used to represent a specific atom,
it gives you both the mass number and the atomic number.
It is set up differently than the symbols on the periodic table;
for the atomic symbol, the symbol of the element is written,
then the mass number is written as a superscript,
and the atomic numbers written as a subscript.
OK, generally the mass number is larger than the atomic number; however, there are
cases when they are the same.
So, if you were given an atomic symbol
like this, you can determine the number of protons, electrons
and neutrons that are in that specific atom. So,
the atomic number represents a number of protons, so there are six protons.
The atomic number also represents the number of electrons.
The number of neutrons, again, can be determined by subtracting
the atomic number from the mass number, so 13 minus 6
gives us 7 neutrons. This is a more descriptive way to express
the symbol
of the element because it provides mass number and atomic number, so you can
get
the numbers of all the protons, neutrons and electrons for that specific
atom, OK. Another way
you might see these atomic symbols represented
are abbreviated ways; you may just see them written with only
the mass number because if it is 13 C,
then the atomic number has to be 6 because carbon
always has atomic number 6, and then you may also see it written this way:
as C-13. If you see
a symbol of an element followed by a number, that number is always
the mass number, OK.
Then, isotopes: isotopes are
atoms of the same element that have different
mass numbers. As we saw on the periodic table,
the atomic number is always the same for carbon, always 6,
OK. However, the mass number for carbon can be different, and I've put 4 different
isotopes of carbon on the board: mass number of 11,
12, 13 and 14; these are all different isotopes.
Notice the atomic numbers are the same for all 4 of them.
From these atomic symbols, we can determine the number of protons,
electrons and neutrons.
Protons and electrons are going to be the same for every single one of these
isotopes of carbon because the atomic number is the same, so it's 6.
The number of neutrons is what differs, so if you subtract
6 from 11, you have 5 neutrons.
In this isotope, you have 6 neutrons; in this one 7,
and this one 8. So, the only thing that is different
in different isotopes of the same element, is the mass number or number of
neutrons.
The next number we're going to talk about is the atomic mass.
Atomic mass is listed on the periodic table,
underneath the symbol of each element; as you can see, it's a decimal number. For
carbon, the atomic mass
is 12.01; for nitrogen, it's 14.01;
for oxygen, it's 16.00.
This number represents a weighted average
of the mass numbers of all the naturally occurring isotopes for that specific
element; since it is an average, it's a decimal number,
OK. So, you will find this on the periodic table
and it represents that weighted average of all the mass numbers of the isotopes.
So, just a quick review: on the periodic table
you will find the atomic number, which represents the number of protons and
number of electrons in an atom.
On the periodic table, you'll find the atomic mass which is the weighted
average;
the mass number represents the number of protons
and neutrons in an element, OK.
So, with those different numbers, you can identify or differentiate between
atoms of the same element and atoms of different elements.