Class 9 Atom, atomic mass and Symbols of atoms of different elements

Topics to be covered

`=>` What is an atom?
`=>` Modern day symbols of atoms of different elements
`=>` Atomic mass
`=>` How do atoms exist?


`color{green}(•)` Atoms are the building blocks of all matter.

`color{green}(•)` Atoms are very small, they are smaller than anything that we can imagine or compare with.

`color{green}(•)` Our entire world is made up of atoms. Through modern techniques, we can now produce magnified images of surfaces of elements showing atoms.


`color{green}(•)` Dalton was the first scientist to use the symbols for elements in a very specific sense and for this he used one atom of that element.

`color{green}(•)` Berzilius suggested that the symbols of elements be made from one or two letters of the name of the element.

`color{green}(•)` In the beginning, the names of elements were derived from the name of the place where they were found for the first time.

`color{green}(•)` For example, the name copper was taken from Cyprus.

`color{green}(•)` Some names were taken from specific colours. For example, gold was taken from the English word meaning yellow.

`color{green}(•)` Now-a-days, IUPAC (International Union of Pure and Applied Chemistry) approves names of elements.

`color{green}(•)` According to IUPAC, symbols are the first one or two letters of the element’s name in English.


`color{green}(•)` The first letter of a symbol is always written as a capital letter (uppercase) and the second letter as a small letter (lowercase).

`color{green}(•)` For example (i) hydrogen, `color{red}(H)` (ii) aluminium, `color{red}(Al)` and not `color{red}(AL)` (iii) cobalt, `color{red}(Co)` and not `color{red}(CO)`.

`color{green}(•)` Symbols of some elements are formed from the first letter of the name and a letter, appearing later in the name. Examples are: (i) chlorine, `color{red}(Cl)`, (ii) zinc, `color{red}(Zn)` etc.

`color{green}(•)` Other symbols have been taken from the names of elements in Latin, German or Greek. For example, the symbol of iron is `color{red}(Fe)` from its Latin name ferrum, sodium is `color{red}(Na)` from natrium, potassium is `color{red}(K)` from kalium.


The most remarkable concept that Dalton’s atomic theory proposed was that of the atomic mass. According to him, each element had a characteristic atomic mass. The theory could explain the law of constant proportions so well that scientists were prompted to measure the atomic mass of an atom. Since determining the mass of an individual atom was a relatively difficult task, relative atomic masses were determined using the laws of chemical combinations and the compounds formed.

While searching for various atomic mass units, scientists initially took 1/16 of the mass of an atom of naturally occurring oxygen as the unit. This was considered relevant due to two reasons:

• oxygen reacted with a large number of elements and formed compounds.

• this atomic mass unit gave masses of most of the elements as whole numbers.

But latter carbon-12 isotope was chosen as the standard reference for measuring atomic masses.
One atomic mass unit is a mass unit equal to exactly onetwelfth (1/12th) the mass of one atom of carbon-12. The relative atomic masses of all elements have been found with respect to an atom of carbon-12..For eg: Imagine a fruit seller selling fruits without any standard weight with him. He takes a watermelon and says, “this has a mass equal to 12 units” (12 watermelon units or 12 fruit mass units). He makes twelve equal pieces of the watermelon and finds the mass of each fruit he is selling, relative to the mass of one piece of the watermelon. Now he sells his fruits by relative fruit mass unit (fmu), as in Fig. 3.4

Similarly, the relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to 1/12th the
mass of one carbon-12 atom.


`color{green}(•)` Atoms of most elements are not able to exist independently.

`color{green}(•)` Atoms form molecules and ions and these molecules or ions aggregate in large numbers to form the matter that we can see, feel or touch.