How many protons, neutrons, and electrons
are in the following elements – 3-5-B-R?
Br stands for bromine, which is element 35
on the periodic table.
The number 35 is the atomic number which tells
us the number of protons.
So for this element there are 35 protons.
The second number present, the 80, is the
mass number, which tells us the number of
protons plus the number of neutrons.
So how many neutrons are bromine 35?
Well with a mass number of 80, that is protons
plus neutrons, if we subtract the known number
of protons – 35 – we get 45 neutrons,
okay.
And how many electrons?
Well since this is a neutral element, it should
have the same number of electrons as it has
protons – so 35 electrons.
So for Bromine we have 35 protons, 45 neutrons,
and 35 electrons.
Let’s talk Ca, which stands for calcium.
Calcium has an atomic number of 20, meaning
it has 20 protons and a mass number of 40.
Again, the mass number is the protons plus
the neutrons, so to determine protons, or
rather, to determine neutrons we subtract
the protons.
So 40 minus 20 is 20.
So calcium has 20 protons, 20 neutrons.
Since it is a neutral element and not a charged
ion, it must also have the same number of
electrons – so 20 protons, 20 neutrons,
20 electrons, okay.
Now let’s determine the number of protons,
neutrons, and electrons and name the element
for the following – atomic number 13; mass
number 27; and an ion with a charge of plus
3.
So you may remember from your notes and from
lecture that the atomic number is the defining
characteristic of the element.
It is the number of protons in that element,
and it is the only thing you can use to determine
the element for this particular problem.
So the atomic number of 13 tells us to look
for element 13 on the periodic table, which
is aluminum.
Aluminum has 13 protons.
Again, the atomic number tells you the number
of protons, which is the defining characteristic
of the element.
The mass number tells you the protons plus
the neutrons.
So to determine the neutrons you would subtract
the known number of protons from the mass
number.
So 27 minus 13 is 14.
So we have 13 protons and 14 neutrons, and
this is no longer a neutral element; this
is an ion.
It has a charge of plus 3.
So we are trying to determine the number of
electrons.
If we started with 13 electrons in the neutral
element, and we have a charge of plus 3, do
we lose electrons or gain them?
Well since we’ve gone to be more positive,
we must have lost electrons.
And how many did we lose?
One for each charge.
So with a plus 3, that means we lost 3 electrons.
Again, we started with 13, we lost 3, we now
have 10 electrons – so 13, 14, and 10.
Example B says atomic number 16, mass number
32, and a charge of negative 2.
Again, the atomic number tells you the number
of protons and is the defining characteristic
of the element.
So it tells us what element we’re looking
for.
If we look at our periodic table and we find
element 16, we will see that that is sulfur
– sulfur – so sulfur has 16 protons, okay.
With a mass number of 32, we can determine
the neutrons.
So if we take 32 and subtract 16, we should
get 16.
So we have 16 protons and 16 neutrons.
In a neutral element, we would also have 16
electrons.
However, with a charge of negative 2, we are
not neutral.
Since our charge is negative, we must have
gained some negativity, right?
Electrons are negative; therefore, we gained
electrons.
How many did we gain?
One for each negative charge.
With a charge of negative 2, that means we
must have gained 2 electrons.
Since a neutral element would have 16, we
have gained two more; we must have had 18
electrons.
Thank you for listening.
