OB: what are isotopes and
why are they so important?
You will need a calculator and your
reference tables out.
Take out your 39 atoms handout as
well, it’s going into the in box soon.
John Dalton once said that all atoms of an element are
identical, because at that time he could not imagine that there
were any subatomic particles. He thought all the differences in
atoms were that they had different masses, and that alone
accounted for all the different properties.
He was the father of modern chemistry, and you already are
more educated than him. It turns out he was half right.
He should have said that:
All Atoms Of An Element Are Chemically Identical
They all react alike, & they have the same chemical properties
But, sometimes atoms have a few extra, or a few less neutrons.
Neutrons are neutral and have almost no affect on the chemistry of atoms.
They do affect the mass.
The atoms with a few extra or a few less neutrons are called ISOTOPES.
For example:
Neon – 20
Neon – 21
Neon – 22
10 p+
10 p+
10 p+
10 e―
10 e―
10 e―
10 n°
11 n°
12 n°
These three different ISOTOPES of neon are chemically identical but all
have a different mass. They have the same number of protons + electrons,
but different numbers of neutrons.
Only the mass is affected, not their chemical natures.
Carbon has 2 main isotopes, C-12 and C-14. The carbon 14 is the radioactive kind,
which we will discuss in great detail later in the year.
C-12 makes up 99.45% of all the carbon in the world.
C-14 makes up the rest, just 0.55% of the carbon.
Together they make up 100% of the carbon.
What is the average atomic mass of carbon atoms, the number to put onto the
periodic table in the atomic mass space?
Here’s how we figure that out (do this math):
(12 amu)(.9945) =
(14 amu)(0.0055) = ______________________
(12 amu)(.9945) = 11.934 amu
(14 amu)(0.0055) = 0.077 amu
12.011 amu
Just like the periodic table says.
Decimals come about because the proportions or
percentages of the isotopes are not even steven.
Most of the carbon is C-12, so the weighted average
atomic mass is very close to 12.
The most common isotope is the one that has the mass
closest to the mass on the periodic table.
A new element named Arbuiso is discovered (A). It has two
isotopes, A-44 and A-45. 95.00% of all Arbuiso has mass
of 44 amu, while the rest has mass of 45 amu.
What is the weighted average atomic mass of this cool new
metallic element?
A new element named Arbuiso is discovered (A). It has two
isotopes, A-44 and A-45. 95.00% of all Arbuiso has mass
of 44 amu, while the rest has mass of 45 amu.
What is the weighted average atomic mass of this cool new
metallic element?
(44 amu)(.9500) = 41.80 amu
(45 amu)(.0500) = 2.25 amu
44.05 amu average mass
Does it make any sense that the average mass is so close to 44 amu? Explain
Unknown element X has three isotopes, X-23, X-24, and X-25.
The first isotope makes up 75.00% of all of this element. X-24
makes up 20.0%, while the last isotope X-25 makes up just 5.0%
of all this unknown element.
What is it’s average weighted atomic mass?
(hint, do the same type of math, but there are three lines this time)
Unknown element X has three isotopes, X-23, X-24, and X-25.
The first isotope makes up 75.00% of all of this element. X-24
makes up 20.0%, while the last isotope X-25 makes up just 5.0%
of all this unknown element.
What is it’s average weighted atomic mass?
(23 amu)(.750) = 17.25 amu
(24 amu)(.200) = 4.80 amu
(25 amu)(.050) = 1.25 amu
23.3 amu
Does that make sense, about 23 amu?
Take out your 39 atoms handout. Let’s do 4
quick ones for practice. Check your answers
with the person on your left, unless you have
no left handed person near by, then check
with your righty neighbor.
This goes into the inbox before you leave
Homework tonight: Atomic HW #3, isotopes!
(we’ll do number 2 this week)