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Relative Atomic Mass

Relative Atomic Mass

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Have you ever wondered how tiny atoms really are? Their mass is so minuscule that it's hard to measure with a device. Even protons and neutrons, the building blocks of atoms, have a mass that's just a billionth of a billionth of a billionth of a kilogram. That's incredibly small, making it impossible to measure practically! This tiny mass is known as the relative atomic mass of an atom.

Atoms are so tiny that it is difficult to measure them using a device! Olive
Atoms are so tiny that it is difficult to measure them using a device! Olive

 

Scientists have found a better way to measure the mass of an atom. They use the mass of a carbon-12 atom as the basis to measure the masses of all other atoms. This is called the carbon-12 standard. When we say relative mass, we mean the mass of an atom or molecule compared to the mass of a carbon-12 atom.

In this article, we will learn about relative masses. First, we'll talk about the carbon-12 standard and how it works. Then, we'll learn how to calculate relative atomic mass (Ar) and relative formula mass (Mr) using this standard. We'll also explore why a weighted average of masses is used. So, let's dive in and explore the fascinating world of relative atomic mass!

What is relative isotopic mass?

In nature, two of the same atoms can exist but have a different number of neutrons.

When an atom of the same element has a different number of neutrons, it is called an isotope.

The mass of an atom of an isotope compared to  of the mass of carbon-12 is called relative isotopic mass.

To calculate the relative isotopic mass () scientists use this formula: = Scientists measure the mass of an atom of an isotope by comparing it to one unified atomic mass unit or 1u. 1u equals  of the mass of a carbon-12 atom. Note: You won’t use this formula in your exam but it’s good to know!

What is relative atomic mass?

Relative atomic mass (RAM or Ar) is the way we measure the mass of an element's isotopes compared to the mass of a carbon-12 atom. Each element has different isotopes, which are atoms with the same number of protons but different numbers of neutrons. Some isotopes are more common than others, and the relative atomic mass takes this into account.

The number we see on the periodic table for an element's relative atomic mass is the average of the masses of all its isotopes. This average is weighted based on how often each isotope occurs in nature. This is known as percentage abundance.

For example, chlorine has two isotopes: chlorine-35 and chlorine-37. Three-quarters of the chlorine found in nature is chlorine-35, while one-quarter is chlorine-37. We can calculate the relative atomic mass of chlorine using this formula: Ar = sum of isotope mass x isotope abundance / 100. Using this formula, the relative atomic mass of chlorine would be:

(35 × 75) + (37 × 25) ÷ 100 = (2,625 + 925) ÷ 100 = 35.5

On the periodic table, we can see that the relative atomic mass of chlorine is 35.5. This means that the average mass of all the isotopes of chlorine is 35.5 times the mass of a carbon-12 atom.

relative atomic mass

When we say 'weighted average', we mean it takes into account the masses of all the isotopes of that element.

What is relative molecular mass?

The weighted average of the mass of a molecule relative to 112 of the mass of a carbon-12 atom is called the relative molecular mass (Mr or RMM).

We must say 'weighted average' when speaking about RMM. As an example, let's look at the molecule .

An average sample of  molecules will have both chlorine-37 and chlorine-35 atoms. This means that the masses of the molecules will vary, like this:

12 + 1 + (3 x 35) = 118

12 + 1 + (2 x 35) + 37 = 120

12 + 1 + 35 + (2 x 37) = 122

12 + 1 + (3 x 37) = 124

So a weighted average includes how many of each (or the abundance) of these molecules we find in an average sample of a substance. We calculate the abundance of an isotope as a percentage.

Relative molecular mass refers to molecules with a fixed number of atoms joined together by covalent bonding, including noble gases. It does not include things ionically bonded together like sodium chloride (NaCl). We can calculate Mr by adding up the relative masses of the atoms in a molecule. For example, the  (water) molecule has two hydrogen atoms and one oxygen atom.

relative molecular mass
relative molecular mass

Hydrogen atomic mass = 1Oxygen atomic mass = 16Mr of H2O= (2x1) + (1x16) = 2 + 16= 18

has no unit because we measure masses by comparing them to the mass of carbon-12. The value we calculate is not the actual mass of an atom but a comparative measure.

What is relative formula mass?

The relative formula mass (Mr) is another way to measure the mass of a compound. It's also sometimes called the relative molecular mass. This value represents the weighted average of the masses of all the atoms in a formula unit compared to the mass of a carbon-12 atom.

A formula unit is the simplest formula of a chemical compound, also known as the empirical formula. When calculating Mr, we use the empirical formula of the compound.

The relative formula mass can be used to find the mass of both covalently bonded substances and ionically bonded ones. However, it's important to note that scientists use the term "relative formula mass" instead of "relative molecular mass" for ionic compounds and ions. This is because ions are not molecules. An ion is formed when an atom has more or fewer electrons than protons. This causes the ion to have a positive or negative charge. An ion with more electrons than protons has a negative charge, while an ion with fewer electrons than protons has a positive charge. For example, Na+ is a positively charged ion formed when a sodium atom loses an electron. Cl- is a negatively charged ion formed when a chlorine atom gains an electron. In summary, the relative formula mass is a valuable measurement that helps us understand the mass of a compound or formula unit. It's important to use the correct terminology when discussing ionically bonded substances and ions.

Sodium and chlorine ions
Sodium and chlorine ions

How do we calculate relative formula mass? You can find the relative formula mass of a compound by adding up the relative atomic masses of the atoms it contains.

NaCl (sodium chloride)Sodium and chlorine
NaCl (sodium chloride)Sodium and chlorine

Relative Atomic Mass - Key takeaways Relative mass is the mass of an atom or molecule compared to that of 1/12 of a carbon-12 atom. Relative atomic mass (Ar) is the weighted average of the masses of the isotopes of an element compared to 1/12 of the mass of the carbon-12 atom. The formula for relative atomic mass is ∑ isotope mass x isotope abundance / 100.Relative molecular mass (Mr) is the weighted average of the mass of a molecule compared to 1/12 of the mass of the carbon-12 atom. You use it only when talking about covalent compounds. Relative formula mass (Mr) is the same as relative molecular mass. You can use it to talk about covalent compounds and ionic ones.

Relative Atomic Mass

What is an atomic number?

We call the number of protons in an atom of an element the atomic number. Atomic numbers can help you identify an element. For example, if you find an atom has an atomic number of eight, you can conclude it's an oxygen atom. So all the isotopes of an element have the same atomic number!

What is the relative mass of an electron?

An electron weighs 9.11 x10^-28  grams. That’s 1/2000 of the mass of a proton!

What is the relative mass of a neutron?

A neutron weighs 1.68 x 10^-24 grams. We can say neutrons have a mass of one unified atomic mass unit (or 1u), approximately.

How do you calculate relative formula mass?

You can find the relative formula mass of a compound by adding up the relative atomic masses of the atoms it contains. As an example, the formula mass of NaCl is 58.5. (Mr of sodium = 22.99; Mr of chlorine = 35.45.)

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