What is the difference between A relative atomic weight of an element and the mass of its atom Class 9?

Atomic weight and atomic mass are two important concepts in chemistry and physics. Many people use the terms interchangeably, but they don't actually mean the same thing. Take a look at the difference between atomic weight and atomic mass and understand why most people are confused or don't care about the distinction. (If you're taking a chemistry class, it could show up on a test, so pay attention!)

Atomic mass (ma) is the mass of an atom. A single atom has a set number of protons and neutrons, so the mass is unequivocal (won't change) and is the sum of the number of protons and neutrons in the atom. Electrons contribute so little mass that they aren't counted.

Atomic weight is a weighted average of the mass of all the atoms of an element, based on the abundance of isotopes. The atomic weight can change because it depends on our understanding of how much of each isotope of an element exists.

Both atomic mass and atomic weight rely on the atomic mass unit (amu), which is 1/12th the mass of an atom of carbon-12 in its ​ground state.

If you find an element that exists as only one isotope, then the atomic mass and the atomic weight will be the same. Atomic mass and atomic weight may equal each other whenever you are working with a single isotope of an element, too. In this case, you use the atomic mass in calculations rather than the atomic weight of the element from the periodic table.

Mass is a measure of the quantity of a substance, while weight is a measure of how a mass acts in a gravitational field. On Earth, where we are exposed to a fairly constant acceleration due to gravity, we don't pay much attention to the difference between the terms. After all, our definitions of mass were pretty much made with Earth gravity in mind, so if you say a weight has a mass of 1 kilogram and a 1 weight of 1 kilogram, you're right. Now, if you take that 1 kg mass to the Moon, it's weight will be less.

So, when the term atomic weight was coined back in 1808, isotopes were unknown and Earth gravity was the norm. The difference between atomic weight and atomic mass became known when F.W. Aston, the inventor of the mass spectrometer (1927) used his new device to study neon. At that time, the atomic weight of neon was believed to be 20.2 amu, yet Aston observed two peaks in the mass spectrum of neon, at relative masses 20.0 amu and 22.0 amu. Aston suggested there two actually two types of neon atoms in his sample: 90% of the atoms having a mass of 20 amu and 10% with a mass of 22 amu. This ratio gave a weighted average mass of 20.2 amu. He called the different forms of the neon atoms "isotopes." Frederick Soddy had proposed the term isotopes in 1911 to describe atoms that occupy the same position in the periodic table, yet are different.

Even though "atomic weight" is not a good description, the phrase has stuck around for historical reasons. The correct term today is "relative atomic mass" — the only "weight" part of the atomic weight is that it is based on a weighted average of isotope abundance.

When it comes to Chemistry, there are many things that students have to learn. One important Chemistry topic that all students should learn is the topic of atomic mass and atomic weight.

Students always seem to have thousands of questions about atomic weight vs atomic mass. This is why today we are going to look into all important aspects related to the topic of atomic weight vs atomic mass.

In this article, students will be able to learn about atomic mass and relative atomic mass. We will also talk about the difference between atomic mass and atomic mass numbers, along with the difference between atomic weight and molecular weight.

So, let’s begin by first looking at what atomic mass means. According to experts, atomic mass can be defined as the total number of protons and neutrons present in an atom. It is important to remember that, ideally, the mass of electrons is also counted. However, since the size of the electrons is very small, their mass is often neglected.

The unit for atomic mass is Dalton. This unit is represented by ‘Da.’ Also, students should know that Dalton is the standard unit that is used for indicating the mass of an atom. The atomic mass of different elements varies as the number of protons and neutrons is different for different elements.

Relative Isotopic Mass

Now, let’s look at the meaning of relative isotopic mass. While atomic mass is an absolute mass, relative isotopic mass is a dimensionless number that has no units.

This loss of units is the result of using a scaling ratio concerning the carbon-12 standard. Further, it is interesting to note that the word ‘relative’ in the term relative isotopic mass refers to this scaling of the carbon-12 standard.

Based on this information, it can be said that the relative isotopic mass is the mass of any given isotope when the final value is scaled by the mass of carbon-12. It is important to remember that the mass of carbon-12 should always be determined experimentally in this case.

On a similar note, it can also be stated that the relative isotopic mass of a nuclide or isotope is the mass of the isotope relative to 1/12th of the mass of a carbon-12 atom.

Learn to View the Atomic Mass and Other Details in the Periodic Table

Students must now have a basic idea of the meaning of atomic mass and relative atomic mass. The next important step is to learn about atomic weight and the difference between atomic mass and atomic mass number. However, before that, students need to know how to spot some details in the periodic table.

What we mean by this is that all students must know how to spot the atomic number, atomic symbol, and atomic mass of an element on the periodic table. To help students succeed in this task, we have attached a labelled image below. Students can refer to this image to learn how to spot the atomic number, atomic symbol, and atomic mass of an element on the periodic table.

(image will be uploaded soon)

What is Atomic Weight?

It is a commonly known fact that a single element can have several isotopes. This means that if one wants to calculate the atomic weight, then it is important to consider the masses of different isotopes. The percentage of different isotopes might also vary.

It is also important to remember the values of isotope relative abundance and isotope mass while calculating the atomic weight. Also, atomic weight is known as the relative atomic mass. It can be defined as the ratio of the mean mass of the atoms of a chemical element to a certain standard.

Further, the normal unit of atomic weight has been 1/12th of the atomic mass of the carbon-12 isotope since the year 1961. An isotope can also be referred to as one of two or more species of atoms with different atomic mass numbers of the same chemical element.

Students should also be aware of the fact that atomic weight is also measured in the units of atomic mass. This is usually abbreviated to AMU. It is also known as Daltons. Before we proceed to discuss the difference between atomic weight and molecular weight, some students might also be interested to learn about the history of this concept.

A Brief History of Atomic Weight

According to various sources, John Dalton was the first scientist who considered that the value of one should be allotted to the hydrogen atom. This was considered as the base of his scale of atomic weights.

After that, for several decades, nuclidic masses were explained on a scale based on 1/18th the mass of neutral atom oxygen. The chemical atomic weight unit at that time was 1.000272 times the physical atomic mass unit.

This period of confusion was eventually brought to an end in 1961. This was done after the acceptance of the 1/12th mass of carbon as the unit for both atomic weights and nuclidic masses.

Differences between Atomic Mass and Atomic Weight

Till now, we have discussed the meaning of atomic mass and atomic weight. We have also looked at various aspects related to both of these topics. Now, it’s time to look at the difference between both of these concepts.

To make it easier for students, we have created a table that highlights the differences that exist between atomic mass and atomic weight. That table is mentioned below.

The Differences Between Atomic Mass and Atomic Weight

Fun Facts About Atomic Mass and Atomic Weight

Did you know that there were two groups of scientists who discovered the concept of relative atomic masses? John Dalton and Thomas Thomson discovered this concept in 1803 and 1805, respectively, while Jons Jakob Berzelius discovered this concept between 1808 and 1826.

Further, earlier, the relative atomic mass was defined with that of the lightest element. However, Prout’s hypothesis stated that the atomic mass of all elements would have to be proven to be exact multiples of the lightest element. Many other theories and experiments were carried out before finally arriving after using the 1/12th carbon-12 standard.

Composition of the Atom

Atoms are the smallest particles of any element. But the atom is also composed of 3 particles. These particles are neutrons, protons, and electrons. Every atom has 1 nucleus that binds 1 or many electrons around it and the nucleus typically has the same number of protons and neutrons. Protons and neutrons together are known as nucleons.

The protons are positively charged, neutrons are neutral and electrons are negatively charged.

Different Kinds of Atoms

The different kinds of atoms are-

• Stable Atoms- Most atoms on the earth are stable and their protons, neutrons, and electrons are balanced. If there are no external forces to a stable atom, it will always stay the same.

• Isotopes-  Isotopes contain a different number of neutrons but the same number of protons and electrons. These are also called atom's cousins. Having extra neutrons might make isotopes radioactive.

• Ions- Atoms with additional or absent electrons are called ions and they have a positive or negative electric charge. Ions are responsible for many chemical reactions in elements.

• Radioactive- Radioactive Atoms are atoms that have multiple neutrons in the nucleus, which makes them very unstable. Radioactive atoms give off particles to eventually become stable.

• Antimatter- All atomic particles have a twin anti-particle, with an opposite electric charge, they are antimatter. Antimatter is very infrequent and delicate.

Types of Subatomic Particles

The 3 types of subatomic particles are-

Electrons

• Electrons have a negative charge. This charge is -1e which approximates \[ - 1.602 \times 10 ^ {-19} \].

• The mass of an electron is nearly \[ 9.1 \times 10 ^ {-31} \].

• Electrons have relatively negligible mass and that's why they are not counted when calculating the mass of an atom.

Protons

• The positively charged subatomic particles are called protons. The charge on a proton is 1e, which is approximately  \[ 1.602 \times 10 ^ {-19} \].

• The mass of this subatomic particle is approximately  \[ 1.672 \times 10 ^ {-24} \]. 

• Protons are 1800 times heavier than electrons.

• The atomic number of the element is always equal to the number of protons in the atoms of a given element.

Neutrons

• The electrically neutral particles which carry no charge at all are called neutrons.

• The mass of a neutron is nearly the same as the mass of a proton that is \[ 1.674 \times 10 ^ {-24} \].

• Isotopes of an element have the same number of protons but differ in the number of neutrons.

Atomic Structure of Isotopes

Each element has a unique number of protons, which is described by its atomic number. But many atomic structures of any element can exist which disagree with the total number of nucleons. These variants are the Isotopes of the element which have different nucleon numbers ( mass number). The isotopes of an element have the exact number of protons but counter in the total number of neutrons. Nucleons are members of the nucleus, present in the atom. Any of the protons or a neutron can be nucleons. The atomic structure of an isotope is described with the help of the 3 things. They are 

• The chemical symbol of the given element 

• The atomic number of the given element

• The mass number of the given isotope. 

Photoelectric Effect

During the photoelectric effect, electrons escape from the surface of a given material. The surface of the material is composed of both positive and negative ions and when light is made to be incident on the given metal surface, some of the electrons which are there near the surface of the metal will absorb sufficient energy from the incident radiation. Thus, they overcome the attraction of the positive ions and when the electrons gain as much sufficient energy as required, they will run out of the metal surface into the surrounding area. This forms the basis for the Photoelectric Effect. Some energy points are- 

• Work Function - Work Function is the minimum energy that is required to remove an electron from the surface of the given metal.

• Threshold Frequency - Threshold frequency is the minimum frequency of light that can force an electron to radiate from the surface of a given metal.

• Threshold Wavelength - Threshold wavelength is the maximum wavelength of light that can displace a photoelectron from the surface of a given metal.

Conclusion

The study notes on the Differences between Atomic Mass and Atomic Weight provides a comprehensive overview of the topic including definitions of both Atomic Mass and Atomic Weight, the differences between them, composition of the atom, atomic structure of isotopes and the photoelectric effect.