Chapter
notes
Class: IX
Chapter
Name: Atoms and molecules
Chapter
Notes
1.
Laws of chemical combination
§ Law of conservation of mass:
Mass can neither be created nor destroyed in a chemical reaction
§ Law of constant proportion or Law of definite
proportion: In a chemical substance the elements are always
present in definite proportions by mass
2.
Postulates of Dalton’s atomic theory
•
All matter is made of very tiny particles called
atoms
•
Atoms are indivisible particles, which cannot be
created or destroyed in a chemical reaction
•
Atoms of a given element are identical in mass and
chemical properties
•
Atoms of different elements have different masses
and chemical properties
•
Atoms combine in the ratio of small whole numbers to
form compounds
•
The relative number and kinds of atoms are constant
in a given compound
3.
Some Limitations of Dalton’s atomic theory
•
According to Dalton’s atomic theory atoms of a given
element are identical in mass and chemical properties. Atoms of different
elements have different masses and chemical properties. This is wrong because
atoms of some elements vary in their mass and density. Such atoms of the same
element having different masses are called isotopes. For example, chlorine has
two isotopes having mass numbers 35 a.m.u and 37 a.m.u.
•
According to Dalton’s atomic theory atoms of different
elements have different masses and chemical properties. This has been proved
wrong in certain cases like argon and calcium atoms, which have the same atomic
mass of 40. Such atoms of different elements that have the same atomic mass are
called isobar.
•
According to Dalton’s atomic theory atoms combine in
the ratio of
small
whole numbers to form compounds. This is not seen in complex organic compounds
like sugar C12H22O11. Ratio of C, H and O is
12:22:11 which is not simple.
4.
Atoms are building block of all matter
5.
Atomic radius is measured in nanometres (1 m = 109 nm)
6. Individual
atoms can be observed using Scanning Tunnelling Microscopes
7.
Each element has a name and a unique symbol
8.
Rules for assigning symbols for atoms of various
elements
a.
The abbreviation used to represent an element is
generally the first letter in capital of the English name of element
|
English name of element
|
Symbol
|
|
Hydrogen
|
H
|
|
Boron
|
B
|
|
Oxygen
|
O
|
|
Nitrogen
|
N
|
|
Fluorine
|
F
|
b. 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
|
Name of element
|
Symbol
|
|
Barium
|
Ba
|
|
Bismuth
|
Bi
|
|
Bromine
|
Br
|
|
Silicon
|
Si
|
|
Cadmium
|
Cd
|
c. Symbols of some elements are derived from
their Latin names
|
Name
of element
|
Latin
name
|
Symbol
|
|
Sodium
|
Natrium
|
Na
|
|
Potassium
|
Kalium
|
K
|
|
Copper
|
Cuprum
|
Cu
|
|
Iron
|
Ferrum
|
Fe
|
|
Gold
|
Aurum
|
Au
|
|
Silver
|
Argentum
|
Ag
|
9. One
atomic mass unit is a mass unit equal to exactly one twelfth(1/12th) the mass
of one atom of carbon-12
10.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
11.Atoms of most elements are not able to exist
independently. Atoms form molecules and ions
12. A
molecule can be defined as the smallest particle of an element or a compound
that is capable of an independent existence and shows all the properties of
that substance
13.
Molecules of element are formed by the atoms of the
same type
14. Atoms
of different elements join together in definite proportions to form molecules
of compounds
Based upon
atomicity we can classify molecules as follows.
•
Monoatomic molecules: Noble gases like helium, neon,
argon, etc., exist in atomic forms, i.e., they are monoatomic. For example,
helium, neon and argon exist as He Ne and Ar respectively.
•
Diatomic molecules: These molecules consist of two
atoms. For example, hydrogen (H2 ) oxygen (O2), nitrogen (N2), chlorine (Cl2), carbon monoxide (CO),
hydrogen chloride (HCl) and sodium chloride (NaCl).
•
Triatomic molecules: These molecules consist of
three atoms. For example. ozone (O3) is a triatomic molecule.
Similarly, carbon dioxide (CO2), nitrogen dioxide (NO2) and hydrogen sulphide (H2S) are examples of triatomic
molecules.
•
Tetraatomic molecules: These molecules consist of
four atoms. For example, P4
is a tetraatomic molecule of phosphorus.
•
Polyatomic molecules: Any molecule containing more
than four
atoms is called a polyatomic molecule. For example,
sulphur (S8),
methane (CH4),
nitric acid (HNO3),
ethanol (C2H5OH).
16. An
ion is a charged particle and can be negatively or positively charged
17.
A negatively charged ion is called an ‘anion’
17.A
positively charged ion is called ‘cation’
18. Ions
may consist of a single charged atom or a group of atoms that have a net charge
on them
19. Ionic
compounds contain charged species called ions as their smallest unit
20.A group of atoms carrying a fixed charge on them
are called polyatomic ions or radicals
21. The
chemical formula of a compound is a symbolic representation of its composition
22.
Valency is the combining capacity of an element
23. Valency
can be used to find out how the atoms of an element will combine with the
atom(s) of another element to form a chemical compound
24.
Names and symbols of some ions
25.Rules
for writing formula of compound:
•
Formula of compound is given by writing side by side
the symbols of constituent elements
•
Symbol of the more metallic element is written first
in formula
•
Number of atoms of each of the constituent element
present in the molecule is indicated by subscript
•
When either of the ions or both the ions are
polyatomic and their valency is more than 1, we enclose the polyatomic ions in
brackets. No brackets are necessary if the valency(ies) of polyatomic ion(s)
is(are) 1
•
While writing formula of compound if the valency
numbers have a highest common factor (H.C.F), divide the valency numbers by
H.C.F to get the simple ratio between the combining elements
26.The simplest compounds, which are made up of two
different elements are called binary compounds
27.Formula of a binary compound is written by criss
crossing the valencies of elements present in a molecule of the compound
28.A chemical compound is always electrically
neutral, so the positive and negative valencies or charges of the ions in the compound
must add up to zero
29. The molecular mass of a substance is the sum of
the atomic masses of all the atoms in a molecule of the substance. It is
therefore the relative mass of molecule expressed in atomic mass units (u)
30.The formula unit mass of a substance is a sum of
the atomic masses of all atoms in a formula unit of a compound
31.
1mole of any substance = 6.022 x 1023units
32. The
mole is the amount of substance that contains the same number of particles
(atoms/ ions/ molecules/ formula units etc.) as there are atoms in exactly 12 g
of carbon-12
33.Mass
of 1 mole of a substance is called its molar mass
34.Atoms of different elements are of different
sizes and masses. A mole of one type of atoms will have a different mass from a
mole of another type of atoms
35. Molar
mass = Mass of one mole of any substance = Gram atomic mass or gram molecular
mass or gram formula mass of the substance
36.
Number of moles = Given number of
particles/Avogadro’s number 37.Molar mass is expressed in g/mol
38. Atomic mass of an element expressed in grams is
called gram atomic mass
39.Molecular mass of a substance expressed in grams
is called gram molecular mass
40.Mass of one mole of atoms of any element is equal
to gram atomic mass of that element
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