The document discusses atoms, their size, visibility, and existence, stating that atoms are the smallest particles of elements and may exist independently or in combined states as molecules. It explains atomic mass and units, detailing how atomic masses are measured relative to carbon-12 and providing examples with chlorine isotopes. Symbols for elements are also addressed, highlighting Dalton's original symbols and the modern representation based on the element's name.

1. LEARNING TARGETS
• WHAT IS ATOM ?
• HOW BIG ATOMS ARE ? CAN WE SEE THEM ?
• DALTON’S SYMBOL OF ELEMENTS
• MODERN SYMBOLS OF ELEMENTS
• HOW DO ATOM EXIST ?

2. WHAT IS ATOM ?
• Smallest particle of an element which may or may
not be capable of free existence and particle that
takes part in a chemical reaction is called Atom .
• For example Helium , Argon , Neon , Krypton ,
Xenon are unreactive gases and hence exist in free
state .
• Atoms of Hydrogen , Oxygen , Nitrogen are highly
reactive and do not exist in free state . They exist
in combined state with atoms of same element or
atoms of other element . For example , they exist
as Dihydrogen (H2) , Dioxygen (O2) , Dinitrogen
(N2) etc or combined with other elements , they
may exist as Hydrogen Chloride (HCl) ,
Water(H2O) , Ammonia (NH3) etc .

3. HOW BIG ATOMS ARE ? CAN WE SEE THEM ?
• Atoms are very small particles . They are too small that they are
hardly visible under microscope . Recently a highly sophisticated
microscope known as Scanning Tunneling Microscope (STM)
has been devised which has made possible to take photographs of
Atoms .
• As atoms are considered to be spherical , their sizes are
expressed in terms of their radii called as Atomic Radii.
• Radii of atom is usually expressed in Nanometres (nm) .
• Their radii are of order of 10 raise to power -10m (1nm = 10
raise to power -9 or 1m = 10 raise to power 9) .
• Hydrogen atom which is smallest of all atom has radius of
0.037nm .

4. DALTON’S SYMBOLS OF ELEMENTS

5. MODERN SYMBOLS OF ELEMENTS
• SOME GUIDELINES FOR WRITING SYMBOLS ARE :
(1) The symbol of an element is first letter or first letter and another letter of English name or Latin name of an
element . In all the cases first letter is capital and second letter is small .
For example : Iron = Fe , Hydrogen = H , Copper = Cu , Carbon = C , Gold = Au
(2) The necessity of adding another letter arises only in case of elements whose names starts with same letter .
Another letter added is not always second letter of name . Further the another letter added may be a letter from
the Latin name of element .
For example : Copper = Cu (Cuprum) , Lithium = Li , Barium = Ba , Gold = Au (Aurum) ,
Mercury = Hg (Hygrum) , Tin = Sn (Stannum) , Sodium = Na (Natrium) .
Potassium = K (Kalium) , Antimony = Sb (Stibium)

6. MODERN SYMBOLS OF ELEMENT

7. HOW DOES ATOM EXIST ?
• Atoms exist in combined state with other atom of same element or with atom of another element . These are called
Molecules .
• They may also exist in aqueous solution as Ions .

8. ATOMIC MASS AND ATOMIC MASS UNIT
• Atomic mass is the mass of
an atom measured in amu .
• Amu is the unit of atomic
mass.
• Atomic mass of element is
the relative mass of its atom
as compared with the mass
of an atom of carbon-12
isotope taken as 12 units .
Scale on which masses are
expressed is called Atomic
Mass Scale .
• Atomic mass unit may be
defined as 1/12th of mass of
an atom of carbon 12
isotope on the atomic scale.
• 1amu – 1/12th the mass of
carbon 12 isotope .

9. • Chlorine has two isotopes chlorine 35 and chlorine 37.
• Found in the ratio of 3:1.
• Average relative atomic mass of chlorine =3*35+1*37 =35.5 amu
3+1
1amu =1.66*10^-27 kg.
1amu =mass of the C -12
12
ATOMIC MASS OF ISOTOPES

10. SUMMARY
• WHAT IS ATOM ?
• HOW BIG ATOMS ARE ? CAN WE SEE THEM ?
• DALTON’S SYMBOL OF ELEMENTS
• MODERN SYMBOLS OF ELEMENTS
• HOW DO ATOM EXIST ?