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CHAPTER 2:: ATOMS, MOLECULES AND IONS
Atom: Basic unit of matter. Smallest particle of the element which retain the
properties of that element.
Eg: Oxygen, Hydrogen
Molecules: Two or more atoms (same or different type of atoms) join
together by chemical force to form molecules.
Avogadro’s Hypothesis
At the same temperature and pressure, equal volumes
volum
of different gases
contain the same number of particles.
Example: 5 liters of oxygen and 5 liters of nitrogen contain same number of
particles! - But it is not true (because different atoms have different size and
properties).
To understand about the atoms, we need to know about what atoms made
of, and how do the various atoms differ?
Eg: H2, H2O
Ions: Atoms or group of atoms (molecules) with positive or negative charge.
+
-
+
Eg: Na , Cl , NH4
Fundamental Chemical Laws
Law of Conversion of mass: In a chemical reaction, mass (weight) is neither
created nor destroyed.
Early experiments to characterize the atom
1) Thomson model
He postulated that “an atom consisted of a
diffuse cloud of positive charge with the
negative electrons embedded randomly in it”.
This model is called as palm-pudding model.
Mass of reactant is equal to mass of product.
Law of Definite Proportion: A given compound always contains exactly the
same proportion of elements by mass.
Law of Multiple Proportions: When chemical elements combine to form a
compound, they do so in a ratio of small whole numbers.
2) Millikan experiment
Millikan found out that the mass of electron is 9.11 X 10
-31
kg.
3) Rutherford experiment
Dalton’s atomic theory
1) Each element is made up of atoms.
2) Element are formed when same type of atoms join tog
together.
He explained that atom has
•
A center of positive charge (nucleus)
•
Electrons moving around the nucleus.
•
It is called as nuclear model.
3) Compounds are formed when atoms of different elements combine.
4) Chemical reactions Involve reorganization of atoms.
During reaction, only the way atoms bound together changes
changes, but
there is no change in atom..
General Chemistry – Handout_Ch-2 (Page: 1)
ATOMIC STRUCTURE
Element Symbol: One or two letter abbreviation. It is based on current name
or original (old) name.
Current Name
Original Name
Symbol
Hydrogen
-
H
Carbon
-
C
Calcium
-
Ca
Oxygen
-
O
Iron
Ferrum
Fe
Copper
Cuprum
Cu
1) Protons present in the nucleus. It has positive chare.
Silver
Argentum
Ag
2) Neutrons present in the nucleus. It has no charge.. It also has almost
the same mass as a proton.
Sodium
Natrium
Na
Atom consist of
3) Electrons move around the nucleus. It has negative charge.
Protons determine the identity of an atom (atoms identified depending on
the number of protons present in it).
Protons and neutrons are present in the tiny nucleus at high density. Both
of them accounts for almost all the atom’s mass (atomic mass)
mass).
Atomic number (Z) = Number of protons (or electrons)
Mass number (A) = Number of protons + Number of neutrons
The atomic number (Z, number of protons) is written as a subscript to the left
of the abbreviation,, while the Mass number is written as a superscript on the
left.
Electrons occupy most of atom’s volume.
Mass number (A)
Electrons form the outer shell of the atom, so that it allows one atom to
interact with another. Thus, it is responsible for the chemical properties of the
atom.
Atomic number (Z)
Atoms are neutral in charge, because the number of protons is equal to
number of electrons.
Example:
Element Symbol
Carbon
Abbreviation: C
Atomic number is in subscript: 6C
ATOMIC SYMBOLS
Mass number is in superscript:
To identify the different elements (atoms), we have a standard way of
representing each element.
The element (carbon) is written as:
12
C
12
6C
Element symbol with atomic number and mass number
number.
General Chemistry – Handout_Ch-2 (Page: 2)
Surprisingly, in periodic table only atomic number (Z) is mentioned along with
the abbreviation of the element (no mass number).
Example: Carbon is written as
6
C
Uses of atomic symbol
You can find out atomic number and mass number from atomic symbol
symbol.
Example.
23
11Na
Atomic number (Z) = 11
Mass number (A) = 23
From the atomic symbol, you can also calculate the number of electron,
proton and neutron.
Example.
23
11Na
Number of proton
Example:
2
He
= 11 (because Z = number of proton)
Atomic number (Z) = 2
Number of electron = 11 (because it is equal to number of protons)
Number of protons = 2
Number of neutrons = Mass number – atomic number
Number of electrons = 2
= 23 - 11 = 12
Uses of periodic table
If the element name (abbreviation) and mass number is given, you can write
the atomic symbol.
All the elements are arranged in periodic table depending on the atomic
number (Z).
Example:
‘Na’ has mass number 23. Write
W
the atomic symbol.
Atomic number of Na = 11 (find out from the periodic table)
From the periodic table you can find out number of protons and number of
electrons present in an atom.
Mass number = 23
Atomic symbol =
23
11Na
General Chemistry – Handout_Ch-2 (Page: 3)
ISOTOPES
34
Atoms with the same atomic number but different mass numbers are called
isotopes.
Isotopes have same number of proton and electron, but different number of
neutron.
Because the chemical property of the atom depends on electrons, the
isotopes have same chemical property.
23
11Na.
If sodium has 13 neutrons it is written as
24
11Na.
23
11Na
and
24
11Na
-
SIMPLE INTRODUCTION TO PERIODIC TABLE
1. From left to right (row of periodic table) the atomic number (Number
of protons) increases.
2. From top to bottom (vertical column of periodic table), the elements
have similar chemical properties.
are called as isotopes.
IONS
Atoms or group of atoms (molecules) with a net positive or negative charge is
called ions.
+
34
Chlorine ( 17Cl ) has 17 protons (positive charge) and 18 electrons (negative
charge). So it has net negative charge (+17-18
(+1
= -1).
As the different elements were discovered, they are arranged in a table
depending on the atomic number and chemical properties. This is called
periodic table.
Example: Sodium (11Na) has 11 electrons and 11 protons.
If sodium has 12 neutrons it is written as
Chlorine ( 17Cl) has 17 protons (+ve charge) and 17
1 electrons (-ve charge).
So it is neutral (+17-17 = 0).
It is very
ery important to know the metals and non-metals
non
in periodic table (very
helpful in naming simple compounds.
-
Example. Na (net positive charge), Cl (net negative charge)
Cations: Positive ions are called cations.. They are produced by the removal
of one or more electrons.
23
11Na
Example.
---------- Removal of one electron ----
23
+
11Na
23
Sodium ( 11Na) has 11 protons (+ve charge) and 11 electrons ((-ve charge).
So it is neutral (no charge).
23
+
Sodium ion ( 11Na ) has 11 protons (+ve charge) and 10 electrons ((-ve
charge). So it has net positive charge (+11-10 = +1).
Anions: Negative ions are called anions. They are produced by the addition
of one or more electrons.
Example.
34
17Cl
---------- Addition of one electron ----
34
17Cl
General Chemistry – Handout_Ch-2 (Page: 4)
Classification of Ions (depending on number of atoms):
atoms)
Metals
•
Most of the elements are classed as metals because they conduct
heat and electricity.
•
Metals frequently lose electrons to form positive ion
ions.
•
It can form ionic compound with a negatively charged non
non-metal.
•
Metals are in the left side and middle of the periodic table.
Non-metals
•
Non-metals
metals frequently gain electrons to become negatively charged.
•
They can form ionic compound with a metal ion.
•
They also can form covalent compound by bonds with each other.
•
Non-metals are in the upper right side of the periodic table (Diagonal
from B to At) and H.
Ions are classified into monoatomic ions and polyatomic ions depend on the
number of atoms present.
Monoatomic ion: An atom which has net positive or negative
charge.
+
+
Example: H , Na , Cl
Polyatomic ion: Group of atoms (more than 1 atom) which have net
positive or negative charge
+
Example: NH4 , NO3
atoms)
Classification of Ions (depending on number of atoms):
Ions also classified based on the capacity of the metal
met to form one or more
cations,
Type I: Metals forms only one cation. These metals present in the
+
2+
2+
3+
3+
left side of the periodic table (and Ag , Zn , Cd , Al , Ga ).
+
2+
3+
Example. Na form only Na (not Na or Na ).
COMPOUNDS
Two or more different chemical elements join together by chemical force
(chemical bond) to form compounds.
Eg.
+
Na + Cl
-
+
NaCl
-
NH4 + NO3
Type II: Metal forms more than one cation. These metals present in
the middle of the periodic table.
2+
3+
Example. Fe can form Fe and Fe
NH4NO3
Classification of compounds
1. Covalent compounds are formed
ormed by sharing electrons between
non-metals.
Ga
Example:
3+
H + H → H2
2. Ionic compounds Consist of positive (metal cation) and
negative (non-metal anion) charged ions were attracted together
because of opposite charge. Compounds made in this manner are
also called as ionic solids or salts.
Example:
+
-
Na + Cl → NaCl
General Chemistry – Handout_Ch-2 (Page: 5)
MOLE (also written as mol)
Mass number (A)
1 mole of a substance = 6.022 x 10
Mass number = Number of Protons + Number of Neutrons
23
atoms (or) units
23
6.022 x 10 is Avogadro's number.
It is indicated in the top left side of the element symbol.
Example:
Example
12
1 mole of carbon = 6.022 x 10
6C: Mass number of C (carbon) = 12
23
carbon atoms
1 mole of hydrogen = 6.022 x 10
1 mole of CH4 = 6.022 x 10
Atomic mass (atomic weight)
Atomic mass is the mass of an atom in atomic mass units (amu).
Atomic mass = Mass number in amu (unit)
23
23
hydrogen atoms
CH4 molecules
Solution 1: How many moles of oxygen atoms are there in 10 moles of
KClO3?
1 amu = Mass of 1 proton or 1 neutron
1 KClO3 molecule contains 3 oxygen atoms.
Example:
1 mol of KClO3 contains 3 mol O
Mass of
12
C = 12 amu
Therefore,
6
10 mol of KClO3 contains 30 mol O
Average atomic mass
Elements present in nature as mixtures of isotopes (same numbers of
protons but different numbers of neutrons).
Example:
Measuring moles
1 mole of element = Mass number in grams
Carbon
Example:
98.89% present as
1.11% present as
12
C
1 mole of
12
C (carbon) = 12 grams of
12
C
13
C
< 0.01 % present as
14
C
Atomic mass of C = (12 x 98.89) + (13 x 1.11) + (14 x 0.01)
100
= (1186.68 + 14.43 + 0.14)/100 = 12.0125
= 12.01 amu
Thus,
12 gram of
12
C (carbon) = 6.022 x 10
1
23
1 gram of H (hydrogen) = 6.022 x 10
23
carbon atoms
hydrogen atoms
We can find out the relation between atomic mass unit (amu) and gram (g).
6.022 x 10
23
carbon atoms = 12 grams
1 carbon atom = 12 amu
Atomic mass can be determined using an instrument called
‘Mass Spectrometer’.
(6.022 x 10
23
atoms) x (12 amu / 1 atom) = 12 g
= 6.022 x 10
23
amu = 1 g
General Chemistry – Handout_Ch-2 (Page: 6)
MOLAR MASS
Solution 3: How many H atoms are in 72.5 g of C3H8O?
Molar mass = Sum of the masses of 1 mole of constituent atoms (in grams).
1 mol C3H8O = (3 x 12) + (8 x 1) + 16 = 60 g C3H8O
Example:
Convert g to mol (Mol = Mass / Molar mass)
CO2 contain one C and two O atoms (or)
Moles of C3H8O = 72.5 g / 60 g
1 mol of CO2 contains 1 mol C and 2 mol O.
= 1.21 mol
Mass of 1 mol C = 12.01 grams
1 mol C3H8O molecules contain 8 mol H atoms
Mass of 1 mol O = 16.00 grams
1.21 mol C3H8O molecules contain = 1.21 x 8 mol H atoms
= 9.68 mol H atom
Molar mass of CO2 = (1 x 12.01) + (2 x 16.00) grams
1 mol = 6.022 x 10
= 44.01 grams
23
atoms/units
9.68 mol H = 9.68 x 6.022 x 10
= 5.83 x l10
Solution 1: Calculate the molar mass of CaCO3
1 mol of CaCO3 contains 1 mol Ca, 1 mol C and 3 mole O.
24
23
H atoms
H atoms
Solution 4: How many grams do 8.50 x 10
1 mol Ca = 40.08 g
6.022 x 10
1 mol C = 12.01 g
23
22
molecules of NH3 represent?
NH3 atoms/units = 1 mol
Convert number of molecules to moles
3 mol O = 3 x 16 = 48 g
1 mol of NH3 = 14.01 + (3 x 1.008) = 17.034 g
1 mol of CaCO3 = 40.08 + 12.01 + 48 = 100.09 g
Solution 2: Calculate the molar mass of (NH4)2Cr2O7
1 mol NH3 = 6.022 x 10
8.50x10
22
NH3 molecules x
1 mol of (NH4)2Cr2O7 contains 2 mol N, 8 mol H, 2 mol Cr, and 7 mole O.
23
NH3 molecules
1 mol NH3
x 17.037 grams
23
1 mol NH3
6.022x10 molecules of NH3
= 2.40475 grams
2 mol N = 2 x 14.01 = 28.02 g
8 mol H = 8 x 1.008 = 8.064 g
8.50 x 10
22
molecules of NH3 = 2.40 grams
2 mol Cr = 2 x 52 = 104 g
7 mol O = 7 x 16 = 112 g
1 mol of (NH4)2Cr2O7 = 28.02 + 8.064+ 104 + 112 = 252.084 g
General Chemistry – Handout_Ch-2 (Page: 7)
Sample Questions:
6) Which one of the following is highly responsible for the
chemical properties of an atom?
1) Which of the following is true?
a) An individual atom is a solid
a) Electron
b) Proton
b) An individual atom is a liquid
c) Neutron
d) None of the above
c) An individual atom is a gas
d) An individual atom cannot be considered to be a solid,
liquid or gas
2) The weight of a container with some chemicals is 250 g. If the
chemicals are burned in a closed container, which one of the
following is true?
a) Weight decrease less than 250 g
change (250 g)
b) Weight do not
c) Weight increase more than 250 g
depends on the chemical
d) Weight change
7) How is an ion formed from a neutral atom?
a) Adding or subtracting protons
subtracting neutrons
c) Adding or subtracting electrons
a) Atomic mass
b) Number of neutrons
c) Alphabetical order
d) Atomic number
9) How many protons, neutrons and electrons are in
Atomic mass = 235
Number of protons = 92
Number of electrons = 92
46 g + 96 g = 54 g + Mass of carbon dioxide
Number of neutrons = 235 – 92 = 143
Calculate the number of protons, electrons and neutrons for the
following atoms.
4) Which one of the following has no charge?
b) Proton
c) Neutron
d) All the above
5) Which one of the following determines the identity of an
atom?
a) Electron
b) Proton
c) Neutron
d) All the above
23592U?
Atomic number = 92
Mass of Ethanol + Oxygen = Mass of Water + Carbon dioxide
a) Electron
d) All the above
8) Elements were arranged in periodic table depending on ………
3) In a combustion reaction, 46 g of ethanol reacts with 96 g of
oxygen to produce water and carbon dioxide. If 54 g of water is
produced, what mass of carbon dioxide is produced?
Mass of carbon dioxide = (46+96) – 54g = 88 g
b) Adding or
Atom
208 Pb
82
No. of Protons
No. of Electrons
No. of Neutrons
82
82
208-82 = 126
13
6C
6
6
13-6 = 7
19
9F
9
9
19-9 = 10
41
20Ca
20
20
41-20 = 21
235 U
92
92
92
235-92 = 143
General Chemistry – Handout_Ch-2 (Page: 8)
10) How many protons and electrons in ‘He’?
13) What is the atomic symbol (AZX) for Z=8, and number of
neutron = 9.
Atomic number of He (see the periodic table) = 2
If Z = 8, Element name = O (see the periodic table)
Number of protons = 2
Number of protons = 8 (from Z, atomic number)
Number of electrons = 2
Number of neutrons = 9
Calculate the number of protons and electrons for the following
atoms.
Atom
No. of
Protons
Atomic number
(from the periodic
table)
Mass number (A) = 8 + 9 = 17
Atomic symbol =
No. of
Electrons
17 O
8
14) How many numbers of protons and electrons present in Ba2+
ion.
Mg
12
12
12
Zn
30
30
30
11) How many protons, neutrons and electrons are in
Atomic number of Ba (see the periodic table) (Z) = 56
Number of protons in Ba2+= 56
Number of electrons in Ba2+ = 56-2 = 54
79Br?
For each of the following ions, indicate the number of protons and
electrons.
Atomic mass (A) = 79
Atomic number (see the periodic table) (Z) = 35
Atom
Z
No. of Protons
30
30
30-2 = 28
7
7
7+3 = 10
35
35
35+1 = 36
Number of protons = 35
Number of electrons = 35
Zn 2+
Number of neutrons = 79 – 35 = 44
N3-
Calculate the number of protons, electrons and neutrons for the
following atoms.
Atom
A
Z
No. of
Protons
No. of
Electrons
133
55
55
55
133-55 = 78
81Br
81
35
35
35
81-35 = 46
56Fe
56
26
26
26
56-26 = 30
133Cs
Br
-
No. of
Electrons
No. of
Neutrons
12) What is the atomic symbol (AZX) for Z=27 and A = 60.
If Z = 27, Element name = Co (see the periodic table)
Atomic symbol =
60 Co
27
General Chemistry – Handout_Ch-2 (Page: 9)