Atomic number, mass number, and isotopes | Chemistry | Khan Academy

Summary

This 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

00:00

🔬 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.

05:01

📘 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.

Keywords

💡Atomic Number

The atomic number, symbolized by 'Z', is a fundamental property of an element that represents the number of protons in an atom's nucleus. It is a key identifier on the periodic table and is crucial for understanding an element's position and properties. In the video, it is used to distinguish between isotopes of hydrogen, such as protium, deuterium, and tritium, all of which have an atomic number of one.

💡Protons

Protons are subatomic particles with a positive electric charge found in the nucleus of an atom. They are essential in determining the atomic number and thus the identity of an element. The video script explains that hydrogen's atomic number is one, indicating there is one proton in the nucleus of a hydrogen atom, which is a defining characteristic of the element.

💡Electrons

Electrons are negatively charged subatomic particles that orbit the nucleus of an atom. In a neutral atom, the number of electrons equals the number of protons, balancing the positive charge to result in no net charge. The script illustrates this by discussing the neutral hydrogen atom, which has one electron corresponding to its one proton.

💡Isotopes

Isotopes are variants of a particular chemical element that have the same number of protons but different numbers of neutrons in their nuclei. The video discusses three isotopes of hydrogen: protium, deuterium, and tritium, highlighting that isotopes of an element share the same chemical properties but can have different physical properties due to differences in mass.

💡Neutrons

Neutrons are neutral subatomic particles found in the nucleus of an atom alongside protons. They contribute to the mass of an atom but do not affect its chemical properties. The script explains that the number of neutrons differentiates isotopes, such as protium with zero neutrons, deuterium with one, and tritium with two.

💡Protium

Protium is the most common isotope of hydrogen, with one proton and no neutrons in its nucleus. The script uses protium as an example to illustrate the concept of isotopes and how they relate to the atomic number and mass number.

💡Deuterium

Deuterium is an isotope of hydrogen that contains one proton and one neutron in its nucleus. The video script explains that deuterium has a greater mass than protium due to the presence of the additional neutron, while still maintaining the same atomic number.

💡Tritium

Tritium is another isotope of hydrogen, characterized by one proton and two neutrons in its nucleus. The script discusses tritium to further illustrate the concept of isotopes and how varying numbers of neutrons affect the mass of an atom without changing its atomic number.

💡Mass Number

The mass number, symbolized by 'A', is the total number of protons and neutrons in an atom's nucleus. It is used to differentiate between isotopes of the same element. The video script explains the calculation of the mass number and how it is represented in the notation of isotopes, such as protium (^1H), deuterium (^2H), and tritium (^3H).

💡Hyphen Notation

Hyphen notation is an alternative way to represent isotopes, where the element's name is followed by a hyphen and the mass number. The script mentions this notation as a method to denote specific isotopes, such as 'carbon-13', providing a clear and concise way to identify different atomic masses of the same element.

💡Uranium

Uranium is a heavy metal and actinide element with the atomic number 92. The video script uses uranium as an example to demonstrate the calculation of protons, neutrons, and electrons in an atom. It shows that uranium-235 has 92 protons, 92 electrons in a neutral state, and 143 neutrons, providing a practical application of the concepts discussed in the video.

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.