Atomic number, mass number, and isotopes | Chemistry | Khan Academy
Summary
TLDRThis educational video script explores the concept of atomic number, isotopes, and the structure of atoms. It uses hydrogen as an example to explain that isotopes of an element have the same number of protons but differ in neutron count, affecting their mass. The script introduces protium, deuterium, and tritium, illustrating how to represent them with symbols that denote atomic and mass numbers. It further explains how to calculate the number of protons, neutrons, and electrons in atoms, using carbon and uranium as additional examples, and introduces the hyphen notation for representing isotopes.
Takeaways
- 🔬 The atomic number (Z) represents the number of protons in an atom's nucleus and can be found on the periodic table.
- 💧 Hydrogen, with an atomic number of one, has one proton in its nucleus and, in a neutral state, one electron to balance the charge.
- 🌐 The Bohr model, though an oversimplification, is a helpful starting point for visualizing the structure of an atom like hydrogen.
- 🔍 Isotopes are variants of a single element that differ in the number of neutrons within their nuclei.
- 🚀 Protium, deuterium, and tritium are isotopes of hydrogen, distinguished by having zero, one, and two neutrons respectively.
- ⚖️ Isotopes have different masses due to the varying number of neutrons, but share the same atomic number.
- 📊 The mass number (A) is the sum of protons and neutrons in an atom's nucleus and is symbolized by A = Z + N, where Z is the atomic number and N is the number of neutrons.
- 📝 Chemical symbols for isotopes include the element symbol, a subscript for the atomic number, and a superscript for the mass number.
- 📚 For example, protium is represented as ^1H, deuterium as ^2H, and tritium as ^3H, using the mass number in the superscript.
- 📉 To find the number of neutrons in an atom, subtract the atomic number from the mass number.
- 📘 The script provides examples using carbon and uranium isotopes to illustrate how to determine the number of protons, electrons, and neutrons.
Q & A
What is the atomic number symbolized by?
-The atomic number is symbolized by 'Z' and it refers to the number of protons in a nucleus.
Where can you find the atomic number for an element?
-You can find the atomic number on the periodic table.
What is the atomic number of hydrogen?
-The atomic number of hydrogen is one, indicating there is one proton in the nucleus of a hydrogen atom.
How does the number of protons relate to the number of electrons in a neutral atom?
-In a neutral atom, the number of protons is equal to the number of electrons, as the positive charges of the protons balance the negative charges of the electrons, resulting in no overall charge.
What is the name of the most common isotope of hydrogen?
-The most common isotope of hydrogen is called protium.
What is the difference between protium, deuterium, and tritium?
-Protium, deuterium, and tritium are isotopes of hydrogen that differ in the number of neutrons in their nuclei: protium has zero neutrons, deuterium has one, and tritium has two.
What is the term used to describe atoms of a single element that differ in the number of neutrons?
-Isotopes are atoms of a single element that differ in the number of neutrons in their nuclei.
What is the mass number and how is it represented in atomic symbols?
-The mass number is the combined number of protons and neutrons in a nucleus, represented by the symbol 'A'. It is shown as a superscript in atomic symbols.
How can you determine the number of neutrons in an atom?
-The number of neutrons in an atom can be determined by subtracting the atomic number (number of protons) from the mass number.
What is the difference between the isotopes of carbon with mass numbers 12 and 13?
-The isotopes of carbon with mass numbers 12 and 13 differ in the number of neutrons: the former has six neutrons while the latter has seven.
What is the hyphen notation used for representing isotopes?
-The hyphen notation is used to represent isotopes by writing the name of the element followed by a hyphen and the mass number, e.g., carbon-13.
How many protons, electrons, and neutrons are in an atom of uranium with an atomic number of 92 and a mass number of 235?
-An atom of uranium with an atomic number of 92 and a mass number of 235 has 92 protons and 92 electrons in a neutral state, and 143 neutrons (calculated by subtracting the atomic number from the mass number).
Outlines
🔬 Atomic Structure and Isotopes of Hydrogen
This paragraph introduces the concept of atomic number, symbolized by Z, which represents the number of protons in an atom's nucleus. It uses hydrogen as an example, explaining that its atomic number is one, meaning there is one proton in the nucleus of a neutral hydrogen atom. The paragraph then delves into the concept of isotopes, which are atoms of the same element that differ in the number of neutrons. Hydrogen has three isotopes: protium with no neutrons, deuterium with one neutron, and tritium with two neutrons. The paragraph explains how isotopes are represented using element symbols with subscripts for the atomic number and superscripts for the mass number, which is the sum of protons and neutrons.
📘 Understanding Isotopes and Atomic Composition
The second paragraph builds upon the first by teaching how to determine the number of protons, electrons, and neutrons in an atom using its atomic number and mass number. It uses carbon and uranium as examples to illustrate the process. For carbon with a mass number of 12, there are six protons, six electrons, and six neutrons. For carbon-13, there are seven neutrons, maintaining the same number of protons and electrons. The paragraph also introduces hyphen notation as an alternative way to represent isotopes, such as carbon-13. Finally, it applies the same principles to uranium-235, which has 92 protons, 92 electrons, and 143 neutrons, reinforcing the method of calculating atomic composition.
Mindmap
Keywords
💡Atomic Number
💡Protons
💡Electrons
💡Isotopes
💡Neutrons
💡Protium
💡Deuterium
💡Tritium
💡Mass Number
💡Hyphen Notation
💡Uranium
Highlights
The atomic number, symbolized by Z, refers to the number of protons in an atom's nucleus and can be found on the periodic table.
Hydrogen, with an atomic number of one, has one proton in its nucleus and one electron in a neutral state.
The Bohr model is an oversimplified representation of an atom, useful for beginners to understand atomic structure.
Protium is the most common isotope of hydrogen, with one proton and no neutrons.
Isotopes are atoms of the same element that differ in the number of neutrons in their nuclei.
Deuterium is an isotope of hydrogen with one neutron, giving it more mass than protium.
Tritium is another hydrogen isotope with two neutrons, further increasing its mass compared to protium and deuterium.
The mass number (A) is the sum of protons and neutrons in an atom's nucleus and is distinct for each isotope.
The formula A = Z + N is used to calculate the number of neutrons in an atom.
Different isotopes of the same element have the same atomic number but different mass numbers.
Isotopes are represented by element symbols with subscripts for atomic number and superscripts for mass number.
The hyphen notation, such as carbon-13, is another way to represent isotopes, indicating the mass number after the element name.
The number of neutrons in an atom can be determined by subtracting the atomic number from the mass number.
Carbon-12 has six protons, six electrons, and six neutrons, maintaining electrical neutrality.
Carbon-13 is an isotope with seven neutrons, differing from carbon-12 by one neutron.
Uranium-235 has 92 protons, 92 electrons, and 143 neutrons, making it a heavy and highly fissile isotope.
The atomic number remains constant across isotopes of an element, while the mass number varies with the number of neutrons.
Transcripts
So the atomic number is symbolized by Z
and it refers to the number of protons in a nucleus.
And you can find the atomic number on the periodic table.
So we're going to talk about hydrogen in this video.
So for hydrogen, hydrogen's atomic number is one.
So it's right here, so there's one proton
in the nucleus of a hydrogen atom.
In a neutral atom, the number of protons
is equal to the number of electrons,
because in a neutral atom there's
no overall charge and the positive charges
of the protons completely balance
with the negative charges of the electrons.
So let's go ahead and draw an atom of hydrogen.
We know the atomic number of hydrogen is one,
so there's one proton in the nucleus.
So there's my one proton in the nucleus, and we're talking
about a neutral hydrogen atom, so there's one electron.
I'm going to draw that one electron
somewhere outside the nucleus and I'm going
to use the oversimplified Bohr model.
So this isn't actually what an atom looks like,
but it's a very simply view that helps you get started.
So this is one, this one version of hydrogen.
This is one isotope of hydrogen.
So this is called protium.
Let me go ahead and write that here.
So this is protium and let's talk about isotopes.
An isotope, isotopes are atoms of a single element.
So we're talking about hydrogen here.
That differ in the number of neutrons in their nuclei.
So let's talk about the next isotope of hydrogen.
So this is called deuteriums.
Let me go ahead and write deuterium here.
Deuterium is hydrogen, so it must have one proton
in the nucleus and it must have one electron outside
the nucleus, but if you look at the definition
for isotopes, atoms of a single element that differ
in the number of neutrons, protium has zero
neutrons in the nucleus.
Deuterium has one.
So let me go ahead and draw in deuterium's one neutron.
I'll use blue here, so neutrons are going to be blue.
So deuterium has one neutron and since neutrons have mass,
deuterium has more mass than protium.
So isotopes have different masses because they differ
in terms of number of neutrons.
Notice though, that they have the same atomic number,
they have the same number of protons in the nucleus.
Right, it's one proton in the nucleus.
And that's important because if you change
the number of protons, you're changing the element,
and that's not what we're doing here.
We're talking about atoms of a single element.
Deuterium is still hydrogen, it's an isotope.
Finally, our last isotope, which is tritium.
So tritium has one proton in the nucleus,
one electron outside the nucleus, and we draw
that in here, and it must differ in terms
of number of neutrons, so tritium has two neutrons.
Let me go ahead and draw the two
neutrons here in the nucleus.
And so those are the isotopes of hydrogen.
How do we distinguish between the different isotopes?
Well we're going to write little symbols
to represent these isotopes.
And so the symbol that we'll draw here
for protium is going to have the element symbol,
which is, of course, hydrogen, and then down here
we're going to write the atomic number.
So the subscript is the atomic number which is one,
because there's one proton in the nucleus,
and then for the superscript, we're going
to write in the mass number.
So let me move down here so we can look
at the definition for the mass number.
The mass number is the combined number
of protons and neutrons in a nucleus,
so it's protons and neutrons, and it's symbolized by A.
So A is the mass number, which is equal
to the number of protons, that's the atomic
number which we symbolized by Z,
plus the number of neutrons.
So A is equal to Z plus N.
And for protium, let's look at protium here.
So in the nucleus there's only one proton
and zero neutrons, so one plus zero
gives us a mass number of one.
And I'll use red here for mass number so we can distinguish.
Alright, so mass number is red and let me use
a different color here for the atomic number.
Let me use magenta here.
So the subscript is the atomic number
and that's Z, and the superscript
is the mass number and that's A.
So this symbol represents the protium isotope.
Let's draw one for deuterium.
So it's hydrogen so we put an H here.
There is still one proton in the nucleus,
right one proton in the nucleus,
so we put an atomic number of one.
The mass number is the superscript,
it's the combined number of protons and neutrons.
So we look in the nucleus here.
There's one proton and one neutron.
So one plus one is equal to two.
So we put a two here for the superscript.
And finally for tritium, it's still hydrogen.
So we put hydrogen here.
There's one proton in the nucleus,
atomic number of one, so we put a one here.
And then the combined numbers of protons
and neutrons, that would be three.
So one proton plus two neutrons gives us three.
So there's the symbol for tritium.
So here are the isotopes of hydrogen
and using these symbols allows us
to differentiate between them.
So let's take what we've learned
and do a few more practice problems here.
So let's look at a symbol for carbon.
So here we have carbon with subscript six, superscript 12.
And if we want to know how many protons,
electrons and neutrons there are.
So let's first think about protons.
Well we know that the subscript is
the atomic number and the atomic number is
equal to the number of protons.
So there are six protons in this atom of carbon.
And if it's a neutral atom of carbon, the number
of electrons must be equal to the number of protons.
So if there are six protons, there
must also be six electrons.
And finally, how do we figure out the number of neutrons?
Well let's go ahead and write down the formula we discussed.
The mass number is equal to the atomic
number plus the number of neutrons.
So the mass number was right here, that's 12.
So we can put in a 12.
The atomic number was six, right here.
So we put in a six.
Plus the number of neutrons.
Plus the number of neutrons.
So the number of neutrons is just equal to
12 minus six, which is, of course, six.
So there are six neutrons.
So just subtract the atomic number
from the mass number and you'll get
the number of neutrons in your atom.
Let's do another one.
This is carbon and this time we have a superscript of 13.
The atomic number doesn't change
when you're talking about an isotope.
If you change the atomic number,
you change the element.
So there's still six protons in the nucleus
of this atom and in a neutral atom,
there must be the equal number of electrons.
So six electrons and then finally,
how many neutrons are there?
Well just like we did before, we subtract
the atomic number from the mass number.
So we just have to 13 minus six
to figure out the number of neutrons.
So 13 minus six is, of course, seven.
So there are seven neutrons in this atom.
Another way to represent isotopes,
let's say we wanted to represent this isotope
in a different way, sometimes you'll see it
where you write the name of the element.
So this is carbon.
And then you put a hyphen here
and then you put the mass number.
So carbon hyphen 13 refers to this isotope
of carbon and this is called hyphen notation.
So let me go ahead and write this hyphen notation.
Alright, let's do one more example here.
Let's do one that looks a little bit scarier.
So let's do uranium.
So U is uranium.
The atomic number of uranium is 92.
The mass number for this isotope is 235.
So how many protons, electrons,
and neutrons in this atom of uranium?
So once again for protons, we look
at the atomic number, that's 92.
So there must be 92 protons.
In a neutral atom, the number of electrons is equal
to the number of protons.
So there are 92 electrons and then finally,
to figure out the number of neutrons,
we subtract this number from the mass number.
So we just need to do 235 minus 92.
And that gives us 143.
So there are 143 neutrons.
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