Relative Atomic Mass, Relative Molecular Mass & Mass Spectrometry | A-Level Chemistry Revision Notes (2023)

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Introduction to relative atomic mass

An atom is too small and measurement of its mass by an instrument is not a practical solution for day-to-day science. As a result, we need to understand the mass of an atom with respect to a standard. The masses of proton, neutron and electron have been determined experimentally. Through various experiment, the isotope of carbon - C-12 has been found to be the most suitable standard to express the masses of all other elements (including all the isotopes).

Out of this need, the concept of Relative Atomic Mass has been born. It is defined as:

Relative Atomic Mass, Relative Molecular Mass & Mass Spectrometry | A-Level Chemistry Revision Notes (1)

In other words, it is a mass of an atom, relative to the mass of C-12. Since it is a ratio, it is unitless.

For example, let’s consider Magnesium-24. Its RAM will be:

Relative Atomic Mass, Relative Molecular Mass & Mass Spectrometry | A-Level Chemistry Revision Notes (2)

We can clearly see that is is numerically equal to the atomic mass. So, the obvious question is why do we need it at all?

The answer lies in the natural abundance of isotopes. The isotopes of an element have different masses, due to the presence of the different number of neutrons. Whenever we take a sample of an element, the ratio of the natural abundance is reflected in the sample. As we know, one mole of any element will have 6.02214179 x 1023 number of atoms. This is a huge number and there will be billions of atoms for each of the isotopes, according to the natural abundance. Since the practicality of measuring the mass of each and every atom is out of the question, we have to resort to the RAM. This is also known as the atomic weight. It is the weighted average of all the atoms as per the natural abundance.

Let’s take the example of Chlorine. It has two stable isotopes namely 35Cl and 37Cl. In the periodic table, however, the mass of a chlorine atom is given as 35.45 u. This results from the relative abundance of 75.76% of chlorine-35 and 24.24% of chlorine-37.

(Video) Relative Atomic & Molecular Mass | A-level Chemistry | OCR, AQA, Edexcel

A similar method is followed in case of molecules and the mass of a molecule relative to that of C-12 is called the Relative Molecular Mass (RMM).

Relative Atomic Mass, Relative Molecular Mass & Mass Spectrometry | A-Level Chemistry Revision Notes (3)

Relevance of RAM and RMN in Mass spectrometry

Mass Spectrometry is founded on the principle of separation of the charged particles in vacuum through the force exerted by the magnetic and electric fields. This separation is a result of the mass-to-charge ratio (m/z OR m/e) of individual ionic fragments. Often, the ions produced during mass spectrometry are singly charged (z=1), which effectively shortens the mass-to-charge ratio to the more easily calculated term mass.

The technique of mass spectrometry can distinguish any number of isotopes an element may have. The output is presented in a xy plot where the x-axis represents the mass-to-charge ratio and the y-axis represents the relative abundance. When applied to a sample of a pure element, the mass spectrometry reveals each isotope as a separate peak. This technique can be used not only for identifying the relative abundance of isotopes in a sample of an element, but also to identify the presence and percentage of each element in a compound. Since the relative abundance of isotopes is used in this technique, the y-axis does not have any unit and hence, the highest among the peaks obtained in a mass spectrum is assigned as the “base peak” and taken as 100% (not to be confused with the total abundance of each element). All the other peaks are assigned as a percentage of the base peak.

Read more about high-resolution mass spectroscopy

Identifying the RAM and RMN from mass spectra and its usefulness in structural analysis.

Let’s take an example of the mass spectrum of a sample of Boron. It shows two peaks having relative isotopic masses of 10 and 11 on the C-12 scale. The y-axis is designated as “Relative abundance” where the isotope with mass is assigned as the “base peak” with a relative abundance of 100.

Relative Atomic Mass, Relative Molecular Mass & Mass Spectrometry | A-Level Chemistry Revision Notes (4)

In this case, the relative abundances of the two isotopes are:

boron-10 23

boron-11 100

In this case, we would consider the total number of atoms to be 100 + 23 = 123.

(Video) GCSE Science Revision Chemistry "Relative Atomic Mass"

Hence, the total mass of the isotopes = (23 x 10) + (100 x 11) = 1330.

The average mass would therefore be, the total mass divided by the total number of atoms.

i.e. 1330/123 = 10.8 which is the relative atomic mass (RAM) of Boron.

The other method is to express the y-axis as the “Relative abundance %”. The mass spectrum of Zirconium is shown below.

Relative Atomic Mass, Relative Molecular Mass & Mass Spectrometry | A-Level Chemistry Revision Notes (5)

The relative abundances are given as percentages. The 5 peaks are of relative isotopic masses of 90, 91, 92, 94 and 96 on the C-12 scale.

In this case, the relative abundance % of the 5 isotopes are:

zirconium-90 51.5

zirconium-91 11.2

zirconium-92 17.1

zirconium-94 17.4

zirconium-96 2.8

(Video) A Level Chemistry Revision "The Mass Spectrometer"

In this case, we would consider the total number of atoms to be 100.

The total mass would then be (51.5 x 90) + (11.2 x 91) + (17.1 x 92) + (17.4 x 94) + (2.8 x 96) = 9131.8

Hence, the RAM = 9131.8 / 100 = 91.3 for Zr.

A similar calculation can be done for compounds. Since these comprise of molecules, the term we should find is the Relative Molecular Mass (RMM).

An example of chlorobenzene molecule can help in understanding the utility of mass spectrometry in differentiating the subtleties in the above-mentioned terms and its effectiveness in dealing with isotopes.

Relative Atomic Mass, Relative Molecular Mass & Mass Spectrometry | A-Level Chemistry Revision Notes (6)

This is the mass spectrum of chlorobenzene with the natural abundance of all isotopes.

Relative Atomic Mass, Relative Molecular Mass & Mass Spectrometry | A-Level Chemistry Revision Notes (7)

It can be clearly seen that the chlorobenzene with a natural abundance of both isotopes of chlorine has a MW of 112.56 which is also called the RMM.

Key Points:

1. RAM is a mass of an atom, relative to the mass of C-12. Since it is a ratio, it is unitless.

2. RMM is a mass of a molecule, relative to the mass of C-12. Since it is a ratio, it is unitless.

3. The mass spectrum is obtained as a xy plot where the x-axis represents the mass-to-charge ratio and the y-axis represents the relative abundance or relative abundance %.

(Video) GCSE Science Revision Chemistry "Relative Formula Mass"

4. The highest peak is taken as the “base peak” and in the relative abundance method, it is assigned as 100% and all other peaks are assigned as a percentage of that.

5. A mass spectrum can identify the number and percentage of isotopes present in a compound.

Frequently Asked Questions

What is relative atomic mass?

Relative atomic mass is the ratio of the mass of an element to 1/12th of the mass of a carbon atom. Relative atomic mass depends on the relative abundance of different isotopes of an element.

What is mass spectrometry?

Mass spectrometry is a technique which is used to separate different isotopes of an element and to calculate their relative abundances. It works on the principle; that when charged particles move through electric and magnetic fields in a vacuum, they get separated on the basis of their mass-to-charge ratio.

What is relative abundance?

Different isotopes of an element are present in different proportions in a naturally occurring sample of an element. Relative abundance means the percentage of a certain isotope of an element in a naturally occurring sample.

Is relative atomic mass same as mass number?

No. mass number is the sum of protons and neutrons present in the nucleus of an atom and it is a whole number value while relative atomic mass is the ratio of the mass of an element to the 1/12th of the mass of a carbon atom. RAM can be in decimal form.

Frequently Asked Questions

What is relative atomic mass?

Relative atomic mass is the ratio of the mass of an element to 1/12th of the mass of a carbon atom. Relative atomic mass depends on the relative abundance of different isotopes of an element.

What is mass spectrometry?

Mass spectrometry is a technique which is used to separate different isotopes of an element and to calculate their relative abundances. It works on the principle; that when charged particles move through electric and magnetic fields in a vacuum, they get separated on the basis of their mass-to-charge ratio.

What is relative abundance?

Different isotopes of an element are present in different proportions in a naturally occurring sample of an element. Relative abundance means the percentage of a certain isotope of an element in a naturally occurring sample.

Is relative atomic mass the same as the mass number?

No, the mass number is the sum of protons and neutrons present in the nucleus of an atom and it is a whole number value while relative atomic mass is the ratio of the mass of an element to the 1/12th of the mass of a carbon atom. RAM can be in decimal form.

References:

1. https://www.youtube.com/watch?v=-xqTwkaiDps

2. http://www.sisweb.com/mstools/isotope.htm

(Video) Calculating Relative Atomic Mass from Mass Spectra

3. http://www.chemguide.co.uk/analysis/masspec/elements.html

4. https://www.khanacademy.org/science/chemistry/atomic-structure-and-properties/mass-spectrometry/a/isotopes-and-mass-spectrometry

FAQs

What is relative molecular mass and relative atomic mass? ›

1. The ratio of the average mass of one molecule of an element or compound to one twelfth of the mass of an atom of carbon-12 is called the relative molecular mass. 2. Relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to [112]th the mass of one carbon-12 atom.

How do you find the relative atomic mass using mass spectrometry? ›

To calculate the relative atomic mass. We multiply the percentage abundance by the master charge

What is relative atomic mass in chemistry class 11? ›

Relative atomic mass is a dimensionless physical quantity. It is the ratio of the average mass of atoms of an element (from a single given sample or source) to 112thof the mass of an atom of carbon-12 (known as the unified atomic mass unit).

What is relative atomic mass AQA a Level chemistry? ›

The relative atomic mass of an element is the average mass of its atoms, compared to 1/12th the mass of a carbon-12 atom.

What is the unit of relative atomic mass? ›

Since relative atomic mass is a ratio so it is unitless.

What is the formula of relative molecular mass? ›

The relative molar mass (symbol Mr) is a dimensionless quantity related to the molar mass M by Mr = M/(103kg·mol1). In the case of an element, this reduces to the Atomic Weight Ar and, rounded to the nearest integer, it gives the number of nucleons in the most abundant isotope.

How is MZ value calculated? ›

m/z represents mass divided by charge number and the horizontal axis in a mass spectrum is expressed in units of m/z. Since z is almost always 1 with GCMS, the m/z value is often considered to be the mass.

What is the formula for atomic mass? ›

How To Calculate The Average Atomic Mass - YouTube

How do you calculate mass spectrometry? ›

Mass calculations in mass spectrometry - YouTube

What is the relative mass means? ›

The relative atomic mass of an element is defined as the weight in grams of the number of atoms of the element contained in 12.00 g of carbon-12. To calculate the relative atomic mass of chlorine, the average mass of one atom of chlorine is found by considering 100 atoms of chlorine.

What is relative atomic mass with example? ›

The relative atomic mass of an element is a weighted average of the masses of the atoms of the isotopes - because if there is much more of one isotope then that will influence the average mass much more than the less abundant isotope will. For example, chlorine has two isotopes: 35Cl and 37Cl.

What is the use of relative atomic mass? ›

In order to perform accurate chemical calculations, relative atomic mass must be used rather than an individual mass number. As a result, relative atomic mass takes into account all of the naturally occurring stable isotopes of an element.

What is the symbol of relative molecular mass? ›

The relative molecular mass is represented by the symbol Mr and defined as MR = mf/mu.

What are relative masses measured in? ›

Relative masses are measured in terms of unified atomic mass units. The relative atomic mass (Ar) is the average mass of an atom of an element relative to one twelfth of the mass of an atom of carbon-12.

Why is carbon-12 used in the definition of relative atomic mass? ›

Each carbon atom has the same number of protons and electrons, 6. 12C has 6 neutrons, 13C has 7 neutrons, and 14C has 8 neutrons and so on. So, we must specify which C atom defines the scale. All the masses of the elements are determined relative to 12C.

What is a symbol of an atom? ›

Atomic Symbols - YouTube

What is the symbol of atomic mass unit? ›

The term “atomic mass unit” (unit symbol: amu) has been used as a unit of mass defined as one-sixteenth the mass of a single atom 16O [m(16O)=16 amu] in physics and as one-sixteenth the isotope-averaged atomic mass (equivalent to the atomic weight) of oxygen [Ar(O)=16 amu] in chemistry.

Why atomic mass has no unit? ›

Expert-verified answer

The relative atomic mass of an element has no atomic unit because atoms are invivible. We can neither see tem nor weight them. Hence scientists had to compare their masses with the mass of another atom. Hence atomic masses are also called as relative atomic masses and there is no unit.

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

The molar mass of an element (in terms of atom) is equal to its relative atomic mass (Ar) in grams. The molar mass of a molecular substance is equal to its relative molecular mass (Mr) in grams. The molar mass of an ionic compound is equal to its relative formula mass (Mr) in grams.

What is the difference between atomic mass and molecular mass? ›

The combined atomic masses of the atoms that make up a compound are referred to as molecular mass. The mass of a molecule is measured by its molecular mass. It's also known as atomic mass. It is usually calculated by multiplying the mass of each atom by the number of atoms of the element present in the molecule.

What is the difference between relative atomic mass and atomic mass? ›

The main difference between relative atomic mass and atomic mass is that relative atomic mass is the ratio of the average mass of atoms of an element to one twelfth of the mass of carbon-12 whereas atomic mass is the total mass of nucleons present in the nucleus of an atom.

What's the difference between molar mass and atomic mass? ›

Molar mass is the mass of the one mole of the compound whereas atomic mass is the mass of the individual unit of the compound. Basically, molar mass is the mass of an average of many elements of the compound and atomic mass is the mass of the atom.

What is relative atomic mass in chemistry? ›

The relative atomic mass of an element is defined as the weight in grams of the number of atoms of the element contained in 12.00 g of carbon-12. To calculate the relative atomic mass of chlorine, the average mass of one atom of chlorine is found by considering 100 atoms of chlorine.

What is the formula for atomic mass? ›

How To Calculate The Average Atomic Mass - YouTube

What is atomic mass notes? ›

Atomic mass in an atom or group of an atom is the sum of the masses of protons, neutrons and electrons. The electrons have very less mass in comparison to protons or neutrons so the mass of electrons is not influenced in the calculation.

What is molecular mass with example? ›

The molecular mass of a substance is the sum of atomic masses of all the atoms in a molecule of the substance. It is used for those substances whose constituent particles are molecules. For example: The moleculear mass of CO2=(1×atomic mass of carbon)+(2×atomic mass of oxygen)

What is the use of relative atomic mass? ›

In order to perform accurate chemical calculations, relative atomic mass must be used rather than an individual mass number. As a result, relative atomic mass takes into account all of the naturally occurring stable isotopes of an element.

What is difference between group and period? ›

Groups and periods are two ways of categorizing elements in the periodic table. Periods are horizontal rows (across) the periodic table, while groups are vertical columns (down) the table. Atomic number increases as you move down a group or across a period.

What is the difference between Monoisotopic and average mass? ›

The monoisotopic mass of an ion is calculated using the 'exact' mass of the predominant isotope of each element. The average mass (or chemical mass) is a weighted average of the natural isotopes for the atomic mass of each element.

What is the number of molar mass? ›

The molar mass of a substance is the mass in grams of 1 mole of the substance. As shown in this video, we can obtain a substance's molar mass by summing the molar masses of its component atoms. We can then use the calculated molar mass to convert between mass and number of moles of the substance.

What is meant by molecular mass? ›

The molecular mass of a substance is the relative mass of its molecule as compared to the mass of the Carbon-12 atom taken as 12-units. Molecular mass is the total mass of a compound. It is equal to the sum of the atomic masses or mass numbers of each element present in a compound or molecule.

Is the unit of atomic mass? ›

Atomic weight is measured in atomic mass units (amu), also called daltons. See below for a list of chemical elements and their atomic weights. … measured in terms of the atomic mass unit, which is defined to be 1/12 of the mass of an atom of carbon-12, or 1.660538921 × 1024 gram.

What is the symbol of relative molecular mass? ›

The relative molecular mass is represented by the symbol Mr and defined as MR = mf/mu.

What is a symbol of an atom? ›

Atomic Symbols - YouTube

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