3.3 Counting Atoms (1/2)

Summary

This video script delves into the complexities of counting atoms, a task not as straightforward as it seems due to their minuscule size. It introduces the concept of the mole and atomic number, explains isotopes and their naming conventions, and discusses how to calculate the average atomic mass of elements using weighted averages. The script also highlights the atomic mass unit (amu) and its standardization with carbon-12, emphasizing the relative mass of protons, neutrons, and the near masslessness of electrons.

Takeaways

• 🔍 Counting atoms individually is impractical due to their small size; chemists use properties like mass and the concept of 'mole' to estimate the number of atoms in a sample.
• 📊 The atomic number above an element in the periodic table represents the number of protons and electrons in a neutral atom of that element.
• 🌐 Elements are arranged in the periodic table by increasing atomic number, indicating their proton count.
• 🏷 Isotopes are variants of an element that have the same number of protons but different numbers of neutrons, resulting in different masses.
• 📝 Isotopes are named based on the number of neutrons; for example, protium (hydrogen-1), deuterium (hydrogen-2), and tritium are isotopes of hydrogen.
• 📈 The mass number of an isotope is the sum of protons and neutrons, which roughly equates to the atomic mass units (amu).
• 🔑 Hyphen notation and nuclear symbols are two methods to designate isotopes, with the latter using the element's symbol, mass number, and atomic number.
• ⚖️ The atomic mass unit (amu) is a relative measure of atomic mass, with carbon-12 as the standard set at exactly 12 amu.
• 🌌 Electrons are almost massless compared to protons and neutrons, with a mass of about 1/2000 of an amu.
• 🧩 Natural elements are often mixtures of different isotopes, and their average atomic mass is calculated as a weighted average based on the abundance and mass of each isotope.
• 📚 The average atomic mass of an element, such as copper's 63.55 amu, is listed on the periodic table below the element's symbol.

Q & A

Why can't we count atoms individually by practical means?

Atoms are so small that they cannot be counted individually by any practical means due to their minuscule size.

What property do chemists use to count the number of atoms in a sample?

Chemists use the mass of atoms and a unit called the mole to count the number of atoms present in a sample.

What is the significance of the atomic number of an element?

The atomic number of an element is the number of protons in the nucleus of an atom and also represents the number of electrons in a neutral atom.

How are elements arranged in the periodic table?

Elements in the periodic table are arranged in order of increasing atomic number.

What is an isotope and how does it differ from the standard element?

An isotope is a variant of an element that has the same number of protons but a different number of neutrons, resulting in a different mass.

What are the names of the three isotopes of hydrogen?

The three isotopes of hydrogen are protium (hydrogen-1), deuterium (hydrogen-2), and tritium.

What is the mass number of an isotope and how is it calculated?

The mass number of an isotope is the sum of the number of protons and neutrons in the nucleus of the atom.

What is hyphen notation and how is it used to represent isotopes?

Hyphen notation is a method of representing isotopes by writing the element symbol followed by a hyphen and the mass number, such as hydrogen-2 for deuterium.

How is the nuclear symbol used to represent isotopes?

The nuclear symbol uses the standard symbol for the element, with the mass number written at the top left and the atomic number at the bottom left of the symbol.

Why do chemists use a relative system for measuring atomic mass?

Chemists use a relative system for measuring atomic mass because the actual masses of individual atoms are extremely small and difficult to work with.

How is the average atomic mass of an element calculated?

The average atomic mass of an element is calculated as a weighted average, taking into account the abundance and mass of each isotope present in the element.

Outlines

00:00

🔍 Introduction to Counting Atoms and the Mole Concept

This paragraph introduces the topic of counting atoms, which is not as straightforward as it seems due to the minuscule size of atoms. It explains that chemists rely on properties like mass and the concept of the mole to quantify atoms in a sample. The paragraph also covers basic vocabulary such as atomic number, which represents the number of protons and electrons in a neutral atom, and how elements are arranged by atomic number in the periodic table. Isotopes are introduced as variants of an element with the same number of protons but different numbers of neutrons, leading to different masses. The paragraph explains the nomenclature of isotopes, such as protium, deuterium, and tritium for hydrogen, and how to denote them using hyphen notation and nuclear symbols, including the concept of mass number.

05:01

📊 Understanding Isotopes and Atomic Mass Units

The second paragraph delves deeper into isotopes, explaining how to calculate the number of neutrons in an isotope by subtracting the atomic number from the mass number. It introduces the concept of atomic mass units (amu) and the use of carbon-12 as the standard for measuring atomic mass, with a defined mass of exactly 12 amu. The paragraph discusses how the atomic masses of other elements are determined relative to carbon-12. It also touches on the negligible mass of electrons compared to protons and neutrons. Furthermore, it explains how the average atomic mass of an element is calculated by considering the weighted average of its isotopes, taking into account their abundance and mass. An example calculation for copper is provided to illustrate this process, showing how the percentages of different isotopes are converted into decimals and used to calculate the average atomic mass listed on the periodic table.

10:02

📚 Conclusion on Average Atomic Mass Representation

The final paragraph serves as a brief conclusion, noting that the average atomic mass of an element, such as copper, is listed on the periodic table beneath its symbol. This average is derived from the weighted contributions of the various isotopes present in the element's natural form.

Keywords

💡Atoms

Atoms are the basic units of matter and the defining structure of elements. They consist of protons, neutrons, and electrons. In the video, the concept of counting atoms is introduced, emphasizing the impracticality of counting them individually due to their minuscule size, which is central to the theme of the video.

💡Mole

The mole is a unit in chemistry used to express amounts of a chemical substance. It provides a way to count atoms by relating their number to their mass. The video mentions the mole as a crucial tool for chemists to determine the number of atoms in a sample, which is integral to understanding atomic quantities.

💡Atomic Number

The atomic number is the number of protons in the nucleus of an atom, which defines the element. It is used to arrange elements in the periodic table. The video script uses the atomic number to explain the identity of elements like hydrogen and helium, highlighting its importance in elemental classification.

💡Isotopes

Isotopes are variants of a particular chemical element that have the same number of protons but different numbers of neutrons. This results in different atomic masses for the isotopes of an element. The video discusses isotopes of hydrogen, such as protium, deuterium, and tritium, to illustrate how elements can have different isotopes with varying numbers of neutrons.

💡Protium

Protium is the most common isotope of hydrogen, with one proton and no neutrons. It is used in the script to exemplify the concept of isotopes and to show that even within a single element, there can be variations in the number of neutrons.

💡Deuterium

Deuterium, also known as hydrogen-2, is an isotope of hydrogen with one proton and one neutron. The video uses deuterium to demonstrate the concept of isotopes and how they differ in mass due to the presence of additional neutrons.

💡Tritium

Tritium is another isotope of hydrogen, with one proton and two neutrons. It is mentioned in the video to further the discussion on isotopes and to show the range of neutron numbers that can exist within the isotopes of a single element.

💡Mass Number

The mass number of an isotope is the total number of protons and neutrons in its nucleus. It is used to distinguish between isotopes of the same element. The video explains the mass number in the context of deuterium and uranium, showing how it helps identify the composition of an atom's nucleus.

💡Hyphen Notation

Hyphen notation is a method of designating isotopes by writing the element symbol followed by a hyphen and the mass number. The video uses this notation to explain how deuterium can be referred to as hydrogen-2, illustrating a practical way to denote different isotopes.

💡Nuclear Symbol

The nuclear symbol is another method of representing isotopes, using the element's standard symbol with the mass number above and the atomic number below. The video script uses the nuclear symbol to explain the composition of deuterium and uranium isotopes, providing a clear visual representation of atomic structure.

💡Atomic Mass Unit (amu)

The atomic mass unit is the standard unit of mass for measuring the mass of atoms. It is defined relative to the carbon-12 isotope. The video discusses the amu to explain how chemists measure atomic mass on a relative scale, using carbon-12 as the reference standard.

💡Average Atomic Mass

The average atomic mass is the weighted average mass of all the isotopes of an element as they naturally occur. The video explains how to calculate this value by considering the abundance and mass of each isotope, using copper as an example to show the calculation process and its significance in chemistry.

Highlights

Counting atoms is not practical due to their small size, necessitating the use of properties like mass and the concept of the mole.

The periodic table organizes elements by increasing atomic number, which is the number of protons and electrons.

Isotopes are variants of an element with the same number of protons but different numbers of neutrons, leading to different masses.

Isotopes of hydrogen, such as protium, deuterium, and tritium, differ in their number of neutrons.

The mass number of an isotope is the sum of protons and neutrons, roughly equivalent to its atomic mass in atomic mass units (amu).

Hyphen notation and nuclear symbols are two methods to designate isotopes, with the latter including the mass number and atomic number.

Chemists use carbon-12 as the standard for atomic mass unit measurement, with a defined mass of exactly 12 amu.

The atomic mass of elements is determined relative to carbon-12, establishing a universal scale for atomic weights.

Electrons have a negligible mass compared to protons and neutrons, approximately 1/2000 of an amu.

Natural elements are often mixtures of isotopes, and their average atomic mass is calculated as a weighted average considering isotope abundance and mass.

The average atomic mass of copper, for example, is calculated by considering the abundance and mass of its isotopes copper-63 and copper-65.

The calculated average atomic mass of an element is listed on the periodic table below its symbol.

Chemists use a mole as a unit to count atoms in a sample, which is based on the number of atoms in 12 grams of carbon-12.

The mole concept allows chemists to work with measurable amounts of atoms, despite the individual atoms being too small to count directly.

Different isotopes of the same element have the same chemical properties but may have different physical properties due to their mass.

Understanding isotopes is crucial for various applications in chemistry, including nuclear reactions and dating techniques.

The concept of isotopes and their notation is fundamental to the study of atomic structure and nuclear chemistry.

Transcripts

so in this video we're going to be

covering chapter 3 section 3 which deals

with

counting atoms and you may initially

think that counting atoms is very simple

you just

have a set number of atoms and you look

at them and you count them but the

problem is

atoms are so small that you cannot count

them

individually by any practical means so

chemists have to use other known

properties

such as the mass

and they also use a separate unit that

we'll learn about later called

the mole to count the number of atoms

present in a sample

so just to cover the basic vocab that

we'll be discussing

during chemistry uh if you look at a

periodic table you'll notice that each

element

let's say hydrogen has a number above

and some number below it the number

above it is what is known as the

atomic number and that is the number

of protons and because atoms are

electrically neutral

also the number of electrons present in

any given atom of hydrogen

and you'll notice as you look at the

table

that the elements are arranged by

increasing

atomic number so for example you can

move on

and you'd see that helium is

element number two and therefore has two

protons

and two electrons moving on now we're

going to be discussing isotopes

now while elements let's say all

elements of hydrogen which is element

number one

they all have this one proton they do

not necessarily have the same number of

neutrons for example hydrogen can have

zero neutrons or one neutron or two

neutrons

and each of these are what are known as

isotopes of hydrogen

that is they have the same element they

have the same number of protons

but a different mass and this

mass is caused by the additional mass

of these neutrons now

each of these isotopes also has a name

for example if you have

well they each have the one proton right

but if you have

let's say no neutrons you have what is

known as

protium or hydrogen one

if you have one neutron

you have what is known as deuterium

because due to that is known as hydrogen

2

and if you artificially create some with

two neutrons you have what is known as

tritium and most of the time you won't

refer to

isotopes specifically by name like you

can with hydrogen

usually you'll refer to them as you know

hydrogen

one that is the mass number because

there's one

nucleon in the nucleus whereas with

deuterium you would be

referring to hydrogen two because

there's two

uh nuclear particles in total that is

the neutron and the proton now the mass

number of

an isotope is simply the number of

protons

plus the number of neutrons because for

all intents and purposes

these weigh roughly the same what is

known as one

atomic mass unit with a little bit of

you know some decimal variation way down

the line

so for example protium which has just

the one proton

would have a mass number of one whereas

deuterium which has the proton

plus the extra neutron would have

you know one proton plus one neutron

would give it a mass number of two

so there are two main ways of

designating the differences between

isotopes and the first of those is what

is known as hyphen notation

now for example if you were to have

deuterium that we showed earlier with

the one proton and the one neutron

which has a mass number

of two all you would do is write the

element in this case

hydrogen put a hyphen

and then clarify the mass number in this

case two so deuterium

right here is also known as hydrogen 2

or

if you want to get a bit more complex

you could use nuclear fuel

such as uranium

235 that is it has a mass number of 235

235 total neutrons and protons

alternately you could use each isotopes

nuclear symbol now the nuclear symbol

uses the standard symbol for the element

in the case of deuterium it's hydrogen

so you'd write the h

that you can find in the box on the

periodic table

then you put the mass number at the top

of the symbol

and the element number at the bottom

so this indicates that it has one proton

but two

a mass number of two and therefore if

you subtract

the number of protons which are down

here

from the mass number you can get the

number of neutrons down here

in this case one and the same thing you

could do with uranium which has the

symbol u

and 235 mass number as we have

listed over here now uranium is element

92

i'll save that so you don't have to look

it up but when you subtract

you can find that uranium has a total of

143 neutrons

just by knowing the element's mass

number

and it's atomic number now because the

masses of individual atoms are

minuscule they're something like 10 to

the negative

23 grams chemists have decided to use

a relative system for measuring atomic

mass that is they choose

one atom in this case carbon 12

as the standard to set

the atomic mass unit or amu

what this means is that internationally

chemists have decided that the isotope

carbon 12 has a

mass of exactly 12 atomic mass units

and all other isotopes are measured

relative to this

for example oxygen

16 has a mass of roughly 16 atomic mass

units

because carbon 12 is composed of the six

protons

and the six neutrons a proton

and a neutron weigh roughly

one atomic mass unit so you can see in

oxygen 16

which has eight protons and eight

neutrons

you'll get a total of roughly 16

atomic mass units and this is true for

all elements

now in case you were curious while the

proton and neutron

each weigh about 1 amu

to within about a hundredth of an atomic

mass unit

an electron has a mass of about

1 2 000 of an atomic mass unit

so electrons are almost massless

so as i mentioned earlier most elements

even in their pure form in nature are

composed of a

mixture of different isotopes of that

element

so when you calculate the average atomic

mass

of an elephant of an element rather you

have to take into account

the various isotopes that go into that

sample of the element

now these average atomic masses tend to

be what are known as

weighted averages now that means they

take account

both the abundance

of a certain isotope and the mass of

that certain

isotope when calculating this average

atomic mass

so for example if you were to calculate

the average atomic mass

of let's say copper well copper

is 69.15 percent

copper 63

in nature whereas it is

30.85 percent that is the rest

is copper 65.

so when calculating the average you have

to take into account

both the different masses the 63 and the

65 as well as the abundance

that is there's going to be a heavier

weight towards the copper 63

than there will be towards the copper 65

when you calculate the average

and the easiest way to do this is by

converting these percentages into

decimals

so first you would take the 0.6915 the

converted percentage

and multiply by the atomic mass of the

copper 63

which is 62.93

amu and then you would add that

to the relative abundance of the copper

65 in this case that would be

0.3085 times the

atomic mass of copper 65 which is 64.93

atomic mass units and then once you do

the actual calculation and add both

those

products up you get a final average

atomic mass

of 63.55

amu and if you were to look up copper

on the periodic table and look below the

chemical signal

symbol you would see

63.55 listed

and that's because the average atomic

mass

is listed underneath the symbol

on the periodic table