What is the average atomic mass of bromine? One isotope of bromine has an atomic mass of 78.92amu and a relative abundance of 50.69%. The other major isotope of bromine has an atomic mass of 80.92amu and a relative abundance of 49.31%.

1 Answer

Bromine is a chemical element with atomic number 35 which means there are 35 protons and 35 electrons in the atomic structure. The chemical symbol for Bromine is Br. Bromine is the third-lightest halogen, and is a fuming red-brown liquid at room temperature that. Atomic mass of Bromine is 79.904 u. Note that, each element may contain more isotopes, therefore this resulting atomic mass is calculated from naturally-occuring isotopes and their abundance. The unit of measure for mass is the atomic mass unit (amu). One atomic mass unit is equal to 1.66 x 10 -24 grams. The atomic mass is useful in chemistry when it is paired with the mole concept: the atomic mass of an element, measured in amu, is the same as the mass in grams of one mole of an element. Thus, since the atomic mass of iron is 55.847 amu, one mole of iron atoms would weigh 55.847 grams. Origin of Name: From the Greek work bromos, meaning stench: Date and Place of Discovery: In 1825 at the University of Heidelberg in Germany and simultaneously at the Laboratory of Medicine and Chemistry in Montpellier, France. Bromine has two naturally occuring isotopes (Br-79 and Br-81) and has an atomic mass of 79.904 amu. The mass of Br-81 is 80.9163 amu, and it's natural abundance is 49.31%. Calculate the mass and natural abundance of Br-79.

The average atomic mass of Br is 79.91 u.

There are two ways to solve this problem.

Method 1

Let's assume we have 10 000 atoms of Br. Then 5069 atoms will have mass 78.92 u, and 4931 atoms will have mass 80.92 u.

Mass of the 5069 atoms = 5069 × 78.92 u = 400 000 u
Mass of the 4931 atoms = 4931 × 80.92 u = 399 000 u
Mass of 10 000 atoms = 799 100 u

Average mass of an atom = #(799 100' u')/(10 000' atoms')# = 79.91 u/atom

Method 2

The average mass is the mass of each isotope multiplied by its percentage. Thus,

Average mass = 50.69 % × 78.92 u + 49.31 % × 80.92 u = 40.00 u + 39.90 u = 79.91 u

The second method is easier, but it doesn't explain intuitively why the answer is the average atomic mass.

Here is a video which summarizes the steps needed to calculate average atomic mass.

Related questions

Atomic Mass For Br

Dobereiners’s Triads

Johann Wolfgang Dobereiner was a German chemist. His effort is considered as one of the earliest attempts to classify the elements into groups.

The earliest classification categorized elements into metals and non-metals. It was difficult to classify the elements, such as boron, which exhibited the properties of both metals as well as non-metals. After further research a German scientist, Dobereiner arrived at a hypothesis in the year 1829

He found that when elements are arranged into groups of three in the order of their increasing atomic mass, the atomic mass of the element; which comes in the middle; is the arithmetic mean of rest of the two. On this basis, he arranged three elements in one group which is known as ‘Triad’. This arrangement of elements is known as Dobereiner’s Triads.

Number Of Subatomic Particles In Bromine

In this table, atomic mass of sodium is equal to arithmetic mean of atomic masses of lithium and potassium. Similarly, atomic mass of strontium is equal to arithmetic mean of atomic masses of calcium and barium.

Limitation of Dobereiner’s Triads:

Dobereiner could find only three such triads (group of three elements) and he could not even put all the elements known at that time in his triads.

The rules of Dobereiner’s triads could not be applied to the elements which had very low or high atomic mass. Such as; if F, Cl and Br are put together in a triad, in increasing order of their atomic masses, the atomic mass of Cl is not an arithmetic mean of atomic masses of F and Br.

After the advancement of techniques of measuring atomic mass more correctly Dobereiner’s Law became obsolete.

Newlands’ Law of Octaves:

Newlands found that every eighth element has similar physical and chemical properties when they are arranged in increasing order of their relative masses. This law is known as Newlands’ Law of Octaves which states that “any given element will exhibit analogues behavior to the eighth element following it in the table”. This means every eight element has the similar chemical and physical properties. For example; Sodium is the eighth element from Lithium and both have similar properties.

The arrangement of elements in Newlands’ Octave resembles the musical notes. In musical notes, every eighth note produces similar sound. Because of this; Newland’s classification of elements was popularly known as just Octaves.

Mass Of Bromine

Limitation of Newlands’ Octaves:

Br Atomic Number

• Newlands’ Octaves could be valid upto calcium only; as beyond calcium, elements do not obey the rules of Octaves.
• Newlands’ Octaves was valid for lighter elements only.
• It appears that Newlands did not expect the discovery of more elements than 56 which were discovered till his time.
• More than one element had to be placed in some of the groups; in order to place the elements having similar properties in one group. But in order to do so, he also put some dissimilar elements in same group.
• Iron; which has similar property as cobalt and nickel, was placed far from them.
• Cobalt and nickel were placed in the group with chlorine and fluorine in spite of having different properties.
• In spite of above limitations; Newlands was the first scientist who arranged the elements in order of their increasing relative atomic masses.