Class 9 ppt 2 atoms n molecules
- 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 ?