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• MOLECULES OF ELEMENT : The molecules of an element are constituted by the same type of atoms.
• MOLECULES OF COMPOUND: Atoms of different elements join together in definite proportions to form molecules of compounds.(hetero atomic molecules)
• ATOMICITY : The number of atoms contained in a molecule of a substance (element or compound) is called its atomicity.
• Based upon atomicity molecules can be classified as follows.
Monoatomic molecules: Noble gases helium, neon and argon exist as He Ne and Ar respectively.
Diatomic molecules: H2 , O2, N2,Cl2, CO , HCl .
Triatomic molecules: O3 ,CO2 , NO2.
• The abbreviation used to represent an element is generally the first letter in capital of the English name of element.
capital of the English name of element.
Oxygen • O Nitrogen • N
• When the names of two or more elements begin with the same initial letter, the initial letter followed by the letter appearing later in the name is used to symbolize the element Barium
• Ba Bismuth • Bi
Q.1 Give one example each of molecule of element & molecule of compound.
Q.2 How does an atom differ from molecule ?
Q.3 Name a triatomic gas.
Q.4 Name the element represented by Hg , Pb, Au.
Q.5 What is the difference between an atom of hydrogen and a molecule of hydrogen?
Polyatomic Ion : A group of atoms carrying a charge is as polyatomic ion.
eg: NH4+ - Ammonium Ion ; CO32- Carbonate ion
Valency :The number of electrons which an atom can lose , gain or share to form a bond.
It is the combining capacity of an atom of the element.
[ for valency of various cations anions ,refer NCERT text book table 3.6 , page no. 37 ]
• Chemical Formula: A chemical formula is a short method of representing chemical elements and compounds.
Writing a Chemical Formula -CRISS-CROSS rule
Q.1 What is the difference between an anion & cation ?
Q.2 Write down chemical formula of
Q.3 Write chemical names of
4. Mole Concept
The mole (mol) is the amount of a substance that contains as many elementary entities as there are atoms in exactly 12.00 grams of 12C
The Avogadro constant is named after the early nineteenth century Italian scientist Amedeo Avogadro.
GRAM MOLECULAR MASS
Gram molecular mass is the mass in grams of one mole of a molecular substance. Ex: The molecular mass of N2 is 28, so the gram molecular mass of N2 is 28 g.
ATOMIC MASS UNIT
An atomic mass unit or amu is one twelfth of the mass of an unbound atom of carbon-12. It is a unit of mass used to express atomic masses and molecular masses.
Also Known As: Unified Atomic Mass Unit
MOLECULAR MASS : A number equal to the sum of the atomic masses of the atoms in a molecule.
The molecular mass gives the mass of a molecule relative to that of the 12C atom, which is taken to have a mass of 12. Examples: The molecular mass of C2H6 is approximately 30 or [(2 x 12) + (6 x 1)] . Therefore the molecule is about 2.5 times as heavy as the 12C atom or about the same mass as the NO atom with a molecular mass of 30 or (14+16) .
Q.1 What term is used to represent the mass of 1 mole molecules of a substance?
Q.2 What is the gram atomic mass of i) Hydrogen ii) oxygen ?
Q.3 Calculate molar mass of C2MH2.
5.Molar Mass & Avogadro Constant
QUESTION BANK [ *HOTS ]
1 Mark Questions:
1. Who gave law of conservation of mass ?
2. What term is used to represent the mass of
1 mole molecules of a substance?
3. What name is given to the number 6.023 x 10 23 ?
4. What is molecular mass?
5. Give Latin names for sodium & mercury.
6.How many atoms are there in exactly 12 g of carbon ?
7. Define mole.
8. Calculate formula unit mass of CaCl2. [ At. Mass : Ca = 40 u , Cl = 35.5 u ]
9. Name a diatomic gas.
10. How many atoms are present in H2SO4.
2 Marks Questions:
1. Give the chemical symbols for the following elements: Gold, Copper , Potassium & Iron.
2. What do the following symbols represent - i) 1 H & ii) H2
3. Neon gas consists if single atoms , what mass of neon contain 6.022 x 1023 atoms.
4. What elements do the following compounds contain ?
5. State the differences between an atom or a molecule.
6. Molar Mass of water is 18 g mol-1 , what is the mass of 1 mole of water? .
7. *The number of atoms in 1 mole of hydrogen is twice the number of atoms in one mole of helium. Why?
8. Write the chemical formulas for the following:
9. Calculate molar mass of H2O2 & HNO3.
10. What is the mass of 0.2 moles of oxygen molecules?
3 Marks Questions:
1. State the main postulates of John Dalton’s atomic theory.
2. What are polyatomic ions ? Give two examples.
3. State the following
4. What is the mass of :
5. What is meant by the term atomicity ? State the atomicity of
5. Marks Questions:
1.i) What is molecular formula ? State with example what information can be derived from a molecular formula .
ii) Write the names of the compounds represented by the following formulas:
2. i) What is gram molecular mass?
ii) Write the formulas & names of the compounds formed between :
3. i) Calculate the number of moles for the following:
ii) How many molecules are present in 34 g of ammonia ?
iii) Calculate the mass of 0.5 mole of sugar (C12H22O11)
You are expected to know………
Chemistry Class 9 Notes Chapter 2 IS MATTER AROUND US PURE DOWNLOAD IN PDF
Notes of Chemistry for Class 9, CHAPTER 3 ATOMS AND MOLECULES pdf
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