RAM VII ISOTOPES

Lithium has a Relative Atomic Mass of 6.9, so it seems like its nuclei must contain a fraction of a neutron. Because this is impossible, this video explains that this is due the presence of isotopes. Isotopes are atoms with the same number of protons, but with a different number of neutrons.

In the case of Lithium, all its isotopes have 3 protons. That’s what makes it lithium. However, some atoms have 3 neutrons, giving it an Atomic Mass of 6, and some have 4 neutrons, giving it an Atomic Mass of 7. The lithium isotope with an Atomic Mass of 6 is called lithium-6, and the isotope with an Atomic Mass of 7 is called lithium-7. The notations 6Li and 7Li can also be used. This means that the average Atomic Masses of the lithium is somewhere between 6 and 7. But why is the actual average Atomic Mass 6.9, which is much closer to 6 than to 7? This is because there are more lithium-7 atoms than lithium-6 atoms.

The video then explores the isotopes of other kinds of atoms, and how some are unstable, causing them to be radioactive. These radioisotopes disintegrate after a period of time, and change into another species, perhaps even a different kind of atom. The stable isotopes, as well as the radioisotopes, for all the known atoms in the Universe are also explored.

Transcript:
Lithium has 3 protons in its nucleus, weighing 3, but its Atomic Mass is 6.9. Does that mean it has 3.9 neutrons in its nucleus? But neutrons can’t come in fractions- it’s got to be either 3 neutrons or 4 neutrons. What’s going on here? Can you think of anything?
In nature, a real lump of lithium would be made of many, many lithium atoms. All with 3 protons. Some of them have 3 neutrons. But other lithium atoms have 4. The atom with 3 protons and 3 neutrons weighs 6 on the Atomic Weight scale. The atom with 3 protons and 4 neutrons weighs 7. This means that the average Atomic Mass is somewhere between 6 and 7. That’s why it’s 6.9.
When atoms of the same kind have different numbers of neutrons, we call them isotopes. So we can say that Isotopes are atoms with the same number of protons, but a different number of neutrons.
To tell them apart, scientists write the left isotope Lithium-6 and the right isotope Lithium-7. The number is called the Mass Number, and is just the count of the atom’s protons plus neutrons.
But aren’t these 2 lithium's different kinds of atoms if they have a different number of neutrons? Maybe one’s a gas and the other’s a solid? No. Both nuclei have 3 protons, so both have an Atomic Number of 3, making them both lithium. If we separate the isotopes, into one bit all made of lithium-6 atoms, and the other bit all made of lithium-7 atoms, we could hardly tell them apart. They’d both be soft and shiny like lithium, and both explode in water like lithium, because they both are lithium. The lithium-6 lump will just be a bit lighter than the lithium-7 lump, and explode in water a bit faster than lithium-7 because its lighter atoms can move quicker. But overall, they’re Like almost-identical twins.
But why is the actual Atomic Mass of Lithium 6.9? Why closer to 7 than to 6? There are more lithium-7 than lithium-6 atoms. 92 out of every 100 lithium atoms are lithium-7, and only 8 are lithium-6, so its average mass is closer to 7 than to 6. And because about 9/10ths of lithium atoms are lithium-7, it leans 9/10ths of the way between 6 and 7. That’s why it’s 6.9.
The Relative Atomic Mass is the average mass of all the isotopes found in nature expressed as a ratio to the mass of a carbon atom taken as 12.
But most of lithium’s isotopes fall apart quickly, throwing out subatomic particles and radiation, and then they change into another isotope, maybe a different kind of atom. Scientists call these unstable ones radioactive isotopes, or radioisotopes. Some only last for a billion thrillionth of a second. The time taken for radioisotopes to disintegrate is measured by their half-lives, and we’ll explain that in another video. Only 2- lithium-6 and 7 - are counted as stable isotopes of lithium.
Scientists have detected over 3 thousand radioisotopes of all the different kinds of atoms in the world, most of which have been made artificially in the lab. Of these, there are only 252 stable isotopes.
There are 26 monoisotopic elements, meaning they have only one stable isotope. And there are 2 elements with no stable isotopes at all- technetium and promethium.

[MarioDallaRiva]

Highly informative!
Sad to hear of the loss of a great educator.

[f.d.english]

How do you know the % of isotopes of ALL lithium, do you go around pulling down the pants of every lithium atom?
-Dr. Ian Malcolm, Jurassic Park-😂

[RetrieverMapping]

11:12 There are 38 elements with no stable isotopes and you didn’t include natural elements like Uranium and Thorium.

[theskinimin]

Wow! I truly understood this! Great way of teaching!

[taniya]

Really really good. if I have any doubts I will ask you sir can I ?. .very helpful thankyou so much sir.i watched somany videos but your videos helping me.i am on 10th std I missed my 9th Classes but your class helping me to understand all missed classes

[kisho]

How is the exact mass be measured technically?