S1.4.2 Relative atomic mass and molecular mass

Summary

This video by MSJ Chem explores the concepts of relative atomic mass (RAM) and relative molecular mass (RMM). RAM is the weighted average mass of an element's isotopes, referenced to carbon-12, and is dimensionless. Elements' RAMs, like hydrogen (1.01) and iron (55.85), are compared to carbon-12 to understand their mass relationships. The video demonstrates calculating RAM using isotopes' abundances, such as for magnesium, yielding 24.32. RMM is the sum of an element's RAMs in a molecule, like H2O's 18.02. The script also introduces relative formula mass, used for ionic compounds like NaCl, calculated at 58.44, emphasizing the importance of these concepts in chemistry.

Takeaways

• 🌟 The relative atomic mass is the weighted average mass of an element's naturally occurring isotopes, compared to 1/12 the mass of a carbon-12 atom.
• 📊 Relative atomic masses are dimensionless and are based on the carbon-12 isotope, which is exactly 12 atomic mass units (amu).
• 🔍 Elements' relative atomic masses can be found on the periodic table, with examples given such as hydrogen (1.01), magnesium (24.31), chlorine (35.45), and iron (55.85).
• 🌐 Relative atomic masses are not whole numbers due to being a weighted average of isotopes, reflecting the natural isotopic distribution of elements.
• ⚖️ Relative atomic mass values allow for the comparison of atomic masses to carbon-12 and to each other, as demonstrated with hydrogen, helium, magnesium, phosphorus, and chlorine.
• 🧪 The calculation of an element's relative atomic mass involves multiplying each isotope's mass by its relative abundance, then summing and normalizing these values.
• 🔢 For magnesium, the relative atomic mass is calculated using the abundances of its isotopes, resulting in a value of 24.32 amu.
• 💧 Relative molecular mass is the sum of the relative atomic masses of the atoms within a molecule, such as H2 (2.02 amu), H2O (18.02 amu), and C2H6 (30.08 amu).
• 🌐 Similar to relative atomic mass, relative molecular mass is also a dimensionless quantity and does not have units.
• 🧩 The term 'relative formula mass' is used for ionic compounds and other substances that do not form discrete molecules, such as sodium chloride (NaCl) with a relative formula mass of 58.44 amu.
• 📚 The video concludes with an introduction to the concept of molar mass, which will be the focus of the next video.

Q & A

What is the definition of relative atomic mass?

Relative atomic mass is the weighted average mass of the naturally occurring isotopes of an element, relative to 1/12 the mass of an atom of carbon-12.

Why is carbon-12 used as the reference for relative atomic mass?

Carbon-12 is used as the reference because it has a mass of exactly 12 atomic mass units (amu), providing a convenient and precise standard for comparison.

What is the relative atomic mass of hydrogen?

The relative atomic mass of hydrogen is 1.01.

How do relative atomic masses help in comparing the masses of different atoms?

Relative atomic masses allow us to compare the masses of different atoms by providing a dimensionless scale where each element's mass is relative to 1/12 the mass of a carbon-12 atom.

Why are relative atomic masses not usually whole numbers?

Relative atomic masses are not usually whole numbers because they represent a weighted average of the naturally occurring isotopes of an element.

How can you calculate the relative atomic mass of an element like magnesium?

You calculate the relative atomic mass of an element by multiplying the mass of each isotope by its relative abundance, summing these values, and then dividing by 100.

What is the relative atomic mass of chlorine?

The relative atomic mass of chlorine is 35.45.

What is the concept of relative molecular mass?

Relative molecular mass is the weighted average mass of a molecule, relative to 1/12 the mass of an atom of carbon-12, and is calculated by summing the relative atomic masses of the atoms in the molecule.

How is the relative molecular mass of a molecule like H2O calculated?

The relative molecular mass of H2O is calculated by adding the relative atomic masses of two hydrogen atoms (1.01 each) and one oxygen atom (approximately 16.00), resulting in a total of 18.02.

What is the difference between relative molecular mass and relative formula mass?

Relative molecular mass is used for molecules, while relative formula mass is used for compounds that do not form molecules, such as ionic compounds.

Can you provide an example of calculating the relative formula mass of an ionic compound?

Yes, the relative formula mass of sodium chloride (NaCl) is calculated by adding the relative atomic masses of sodium (approximately 22.99) and chlorine (35.45), resulting in 58.44.

Outlines

00:00

🔍 Introduction to Relative Atomic and Molecular Mass

This paragraph introduces the concepts of relative atomic mass (RAM) and relative molecular mass (RMM). RAM is defined as the weighted average mass of an element's naturally occurring isotopes, referenced to 1/12th the mass of a carbon-12 atom, which is exactly 12 atomic mass units (amu). The RAM values of elements like hydrogen, magnesium, chlorine, and iron are provided as examples, showing they are not whole numbers due to being a weighted average. The paragraph also explains that RAM values are dimensionless and can be used to compare the masses of different atoms to carbon-12 and to each other. The process of calculating RAM using the isotopes of magnesium is briefly mentioned, and the concept of RMM is introduced as the sum of RAMs of atoms in a molecule, with examples of H2, H2O, and C2H6.

05:02

📚 Calculation and Application of Relative Formula Mass

The second paragraph delves into the calculation of relative atomic mass for elements like magnesium, using the abundance and mass of its isotopes. It clarifies that RMM is the sum of the RAMs of all atoms in a molecule, and provides examples of how to calculate the RMM of simple molecules like H2, H2O, and C2H6. The paragraph also introduces the term 'relative formula mass,' which is used for compounds that do not form discrete molecules, such as ionic compounds. The relative formula mass of sodium chloride (NaCl) is given as an example, calculated by adding the RAMs of sodium and chlorine. The video concludes with a mention of the next topic to be covered: molar mass.

Keywords

💡Relative Atomic Mass

Relative Atomic Mass is a dimensionless quantity that represents the weighted average mass of the naturally occurring isotopes of an element, compared to 1/12th of the mass of a carbon-12 atom. It is the fundamental concept of the video, explaining how atomic masses are standardized against carbon-12. For example, hydrogen has a relative atomic mass of 1.01, indicating it is significantly lighter than carbon.

💡Isotopes

Isotopes are variants of a particular chemical element, which have the same number of protons but different numbers of neutrons. The video discusses how the relative atomic mass is calculated based on the weighted average of these isotopes' masses. For instance, magnesium has three naturally occurring isotopes: magnesium-24, magnesium-25, and magnesium-26.

💡Periodic Table

The Periodic Table is a tabular arrangement of the chemical elements, ordered by their atomic number, electron configuration, and recurring chemical properties. The video mentions that relative atomic masses of elements can be found on the periodic table, which is a key tool for chemists to understand element properties.

💡Weighted Average

A Weighted Average takes into account the relative importance of different values in a set. In the context of the video, it is used to calculate the relative atomic mass by considering the abundance of each isotope of an element. For example, the relative atomic mass of magnesium is calculated using the masses of its isotopes and their relative abundances.

💡Dimensionless

Dimensionless refers to quantities that do not have units of measurement. The video emphasizes that relative atomic masses are dimensionless because they are ratios compared to a standard (carbon-12), rather than absolute measurements.

💡Relative Molecular Mass

Relative Molecular Mass is the sum of the relative atomic masses of all the atoms in a molecule. It is a key concept in the video, illustrating how to calculate the mass of a molecule based on its constituent atoms. For example, the relative molecular mass of water (H2O) is calculated by adding the relative atomic masses of two hydrogen atoms and one oxygen atom.

💡Diatomic Molecule

A Diatomic Molecule is a molecule consisting of two atoms, usually of the same element. The video uses hydrogen (H2) as an example of a diatomic molecule, where each hydrogen atom has a relative atomic mass of 1.01, and together they form a molecule with a relative molecular mass of 2.02.

💡Ionic Compound

Ionic Compounds are formed by the electrostatic attraction between oppositely charged ions. The video explains that for ionic compounds, which do not form discrete molecules, the term 'relative formula mass' is used instead of 'relative molecular mass'. An example given is sodium chloride (NaCl), with a relative formula mass of 58.44.

💡Relative Formula Mass

Relative Formula Mass is the term used for the mass of a formula unit of an ionic compound, calculated by summing the relative atomic masses of the constituent atoms. The video clarifies the distinction between 'relative molecular mass' for molecular compounds and 'relative formula mass' for ionic compounds, using sodium chloride as an example.

💡Molar Mass

Although not explicitly defined in the script, Molar Mass is a related concept that is hinted at the end of the video. It is the mass of one mole of a substance and is numerically equal to the relative atomic or molecular mass of the substance in grams. The video suggests that molar mass will be the focus of a subsequent video.

Highlights

The video discusses the concept of relative atomic mass and relative molecular mass.

Relative atomic mass is defined as the weighted average mass of an element's naturally occurring isotopes, relative to 1/12 the mass of a carbon-12 atom.

The relative atomic mass scale is based on carbon-12, which has an exact mass of 12 atomic mass units (amu).

Relative atomic masses of elements are dimensionless and can be found on the periodic table.

Examples of relative atomic masses include hydrogen at 1.01, magnesium at 24.31, chlorine at 35.45, and iron at 55.85.

Relative atomic masses are not whole numbers due to being a weighted average of isotopes.

The concept of comparing atomic masses using their relative atomic masses is introduced.

Relative atomic mass values allow for comparisons of atomic masses to carbon-12 and to each other.

A method to calculate the relative atomic mass of an element using its isotopes and their abundances is explained.

The calculation example for magnesium's relative atomic mass is provided, resulting in 24.32.

Relative molecular mass is the weighted average mass of a molecule, also relative to 1/12 the mass of a carbon-12 atom.

Relative molecular mass is calculated by summing the relative atomic masses of the atoms in a molecule.

Examples given for calculating relative molecular masses include H2, H2O, and C2H6.

Relative molecular masses, like relative atomic masses, are unitless.

The term 'relative formula mass' is used for compounds that do not form molecules, such as ionic compounds.

The calculation of relative formula mass for sodium chloride (NaCl) is demonstrated as an example.

The video concludes with a look forward to the next topic, molar mass, in the subsequent video.

Transcripts

this is msj chem in this video i'll be

looking at

relative atomic mass and relative

molecular mass

so we'll start with the definition of

relative atomic mass

relative atomic mass is the weighted

average mass of the naturally occurring

isotopes of an element

relative to 1 12 the mass of an atom of

carbon 12

the relative atomic mass scale is based

on the isotope carbon-12

which has a mass of exactly 12 amu

on the left we can see the symbol for

the isotope carbon-12

it is to this isotope that the masses of

other atoms

are compared the relative atomic masses

of

elements can be found on a periodic

table

hydrogen has a relative atomic mass of

1.01

magnesium has a relative atomic mass of

24.31

chlorine has a relative atomic mass of

35.45

and iron has a relative atomic mass of

55.85

as you can see relative atomic masses

are not usually whole numbers

this is because they are a weighted

average of the naturally occurring

isotopes of that element

we'll look at how to calculate relative

atomic mass later

in the video note that because this is a

relative scale

relative atomic masses are dimensionless

in that they have no units

next we look at how the masses of atoms

can be compared

using their relative atomic masses in

this table we have the element

its relative atomic mass and its mass

compared to the isotope carbon-12

the first example is hydrogen which has

a relative atomic mass of 1.01

this means that an atom of hydrogen is

approximately

12 times lighter than an atom of carbon

12.

next is helium which has a relative

atomic mass of 4.00

which means that the helium atom is

approximately three times lighter

than an atom of carbon 12. next is

magnesium

with a relative atomic mass of 24.31

which means that a magnesium atom is

approximately two times heavier than an

atom of carbon 12.

next is phosphorus with a relative

atomic mass of 30.07

and is approximately 2.5 times heavier

than an atom of carbon

12 and finally we have chlorine with a

relative atomic mass of 35.45

which is approximately three times

heavier than an atom of carbon 12.

we can also use relative atomic masses

to compare the masses of different atoms

for example an atom of helium is

approximately four times heavier than an

atom of hydrogen

and an atom of magnesium is

approximately six times heavier

than an atom of helium so from this

table

we can see that relative atomic mass

values can be used to compare the masses

of different atoms

to carbon-12 as well as each other

next we look at how to calculate the

relative atomic mass of an

element in this table we have the three

naturally occurring

isotopes of magnesium they are magnesium

24

magnesium 25 and magnesium 26.

in the second column we have their

relative abundances

so we'll use these values to calculate

the relative atomic mass of magnesium

to do this we multiply the mass of each

isotope by its relative abundance

we then add these together and divide by

100

to get the mass of one atom so using

these values

we get a relative atomic mass for

magnesium

of 24.32 note that because it's a

relative scale

there are no units in this slide we'll

look at relative molecular mass

relative molecular mass is the weighted

average mass of a molecule

relative to 1 12 the mass of an atom of

carbon 12.

the relative molecular mass of a

molecule is the sum of the relative

atomic masses

of the atoms in the molecule so in this

table we have some examples

starting with hydrogen which is a

diatomic molecule consisting of two

hydrogen atoms bonded together

each hydrogen atom has a relative atomic

mass of 1.01

so we multiply this value by 2 to get a

relative molecular mass

of 2.02 the next example is h2o

which is composed of two hydrogen atoms

and one

oxygen atom if we add up the relative

atomic masses of the atoms

we get a relative molecular mass of

18.02

and finally we have c2h6 which is ethane

a molecule of ethane is composed of two

carbon atoms

and six hydrogen atoms if we add up the

relative atomic masses of the atoms

we get a relative molecular mass of

30.08

note that like relative atomic masses

relative molecular masses

do not have units we'll end the video

by looking at relative formula mass the

term

relative formula mass is mostly used for

compounds

that do not for molecules such as ionic

compounds

on the left we can see the structure of

an ionic compound

because ionic compounds do not form

molecules

we use the term relative formula mass

instead of

relative molecular mass for example the

relative formula mass of sodium chloride

which is nacl is 58.44

we calculate this by adding together the

relative atomic masses of

sodium and chlorine so that's all from

this video

in the next video i'll be looking at

molar mass