Download Module1 for YIC CHEM

Attention: CHEM-101 Students!
The major points on which lectures for CHEM-101 course were focused are given below.
The students are expected to refer the Text book prescribed for the course side by side.
Instructions to be followed by the students while reading the points given below are given
in Red, examples are given in blue and legends are given in pink. Have a nice learning
experience through the summary given below!
With best wishes.
Dr.E.P.Jagadeesh
CHET Dept.
Module-1
Atoms, Molecules and Ions:
In the early 19th century John Dalton proposed Atomic theory
According to the Dalton’s atomic theory matter is made up of indivisible particles called
Atoms.
Today we know that atom is divisible. Also we know that atom is made up of still smaller
particles called subatomic particles
Discharge tube experiments (Fig 2.3a, b, c page 39 of the TB*) led to the discovery of
Cathode rays. Cathode rays consist of negatively charged particles of negligible mass.
They are called electrons.
J.J. Thomson’s experiments on cathode rays in applied electric and magnetic fields (Fig
2.4, Page 40) and Millikkan’s oil-drop experiment led to the discovery of charge to mass
ratio and charge of an electron respectively. From these values mass of the electron was
calculated. (See page 41)
Charge of an electron = -1.6 x 10-19C (C= Coulomb)
Mass of an electron = 9.1 x 10-28g
Since atom as a whole is electrically neutral scientists guessed that there will be
positively charged particles also in atom. Goldstien’s experiments with perforated (with
holes) cathode led to the discovery of Protons. Charge of a Proton is equal to that of an
electron but of opposite sign.
Charge of a Proton
Mass of a Proton
= + 1.6 x 10-19C
= 1836 times that of the mass of an electron.
The arrangement of the subatomic particles within in the atom is called the atomic
structure. First model of the atomic structure was proposed by J.J.Thomson in early1900.
He suggested atom as a uniform positive sphere in which negatively charged electrons
are embedded. The model was called as “plum-pudding “model. (See fig.2.9 page
No.42).
In 1910, Rutherford conducted the gold foil experiment (Fig.2.10 (p-42) and 2.11(p-43))
with alpha-particles and disproved Thomson’s plum-pudding model. He proposed the
nuclear model atom. In 1932 James Chadwick discovered neutrons. Neutron has no
charge and its mass is approximately equal to that of proton. (See table 2.1 p-43.).
Atomic Structure (Modern view)
In an atom the protons and neutron are together in the very small nucleus and electrons
are placed in the vast majority of the volume of an atom around the nucleus.
*TB (Text Book): Chemistry: The Central Science by Brown, LeMay and Bursten
Atomic number, mass number and Isotopes.
The number of protons in an atom is the atomic number. It is equal to the number of
electrons in that atom.
Example: Element Carbon has its atomic number 6. So the atom of Carbon contains 6
protons. It will also contain 6 electrons.
Mass number of is the sum of the number of protons and neutrons in the nucleus of an
atom.
Example: Carbon has 6 protons and 6 neutrons in its nucleus. So the mass number is
6+6=12.
The atom is represented as given below. (See page 45)
12
C
6
Mass number is written as the superscript and atomic number is written as the
subscript of the symbol of the element.
Atoms with same atomic number but with different mass numbers are called isotopes.
Example:Carbon-12 and Carbon-14 are isotopes of carbon.
12
C
6
14
C
6
They differ in their number of neutrons in the nucleus. But they are the same element.
Do sample exercises and practice exercises on page 46
Periodic Table
The arrangements of elements in the increasing order of their atomic number with
elements having similar properties placed in the same vertical columns (called groups) is
known as periodic table. There are 18 groups (vertical columns) and 7 periods (horizontal
rows)
Elements belonging to the same group show similar chemical and physical properties.
See the table 2.3 page 51. (Alkali metals, Alkaline earth metals, Chalcogens, Halogens,
Noble gases are names of some such groups)
All the elements on the left side and the middle of the periodic table are metals.
Elements on the right hand side of the periodic table are Non-metals.
The metals and non-metals are separated by diagonal step like line along which lie the
metalloids
Do sample exercise and practice exercise on page 51.
Molecules and Molecular compounds.
A molecule is an assembly of two or more atoms strongly held together.
Examples: O2, N2, Cl2, H2, I2 etc.
There are two atoms in each of the above molecules. Hence they are called diatomic
molecules. In all the above examples of molecules the atoms held together were of the
same type. There are also molecules which contains atoms of different elements. They
are called molecular compounds.
Examples: CO, CH4, H2O, H2O2,CO2 etc.
Most molecular compounds are formed by non-metals.
Molecular formula: The chemical formulae ( formulas-American English) that indicate
actual number and types of atoms in molecules are defined as molecular formulae.
Example: H2O2, C2H4, N2O4 etc.
Empirical formulae: The chemical formulae that indicate the ratio of atoms of each type
present in molecules are defined as empirical formulae.
Example: HO, CH2, NO2 are the empirical formula for the compounds given above with
molecular formulae H2O2, C2H4 and N2O4 respectively
Do sample exercise No.2.6 and practice exercise on page 54.
Ions and Ionic compounds:
During chemical reactions nucleus remain unchanged in atoms while electrons are lost or
gained by them. The electron loss from an atom will result in a positive ion called cation.
The electron gain by an atom will result in a negative ion called anion.
Example: Sodium atom contains 11protons and 11 electrons. When it loses one electron it
becomes Na+ ion ( a cation with +1 charge) .
When a Chlorine atom with 17protons and 17 electrons gains one electron it
becomes a Cl-ion ( an anion with a -1 charge)
Generally metal atoms lose electrons to become cations and non-metal atoms gain
electrons to become anions.
Do sample exercise 2.7 and practice exercise on page 56.
In addition to the simple ions like Na+ and Cl- ions , groups of atoms joined together
carrying net charge are also present. They are called polyatomic ions.
Examples: NO3-, SO42-, CO32- etc.
The cations and anions combine to form compounds as they have opposite charges and
attract each other. They are called ionic compounds
Example: Na+ ion and Cl- ion combine to form the compound NaCl
Ionic compounds are formed by the combination of metals and non-metals, while
molecular compounds are formed by non-metals only.
Do sample exercise and practice exercise on p-58
Example:
Ionic compound –NaCl (Sodium ( a metal) and Chlorine ( a non-metal)
Molecular compound –H2O (Both Hydrogen and Oxygen are non-metals)
Ionic compound is formed by the combination of many ions (cation and anion) arranged
in a three-dimensional structure. So it is impossible to write chemical formulae for them
with actual number and types. Only empirical formulae can be written for Ionic
compounds.
See fig.2.23 b on page57.
From the above figure it is clear that in Sodium Chloride individual molecules of the type
“NaCl” do not exist. But we write the formula as we know that compound id electrically
neutral and for neutrality equal charges of opposite signs must be available. Thus, when
Barium chloride is formed from Ba2+ ion and Cl- ion there should be two chloride ions for
each Ba2+ atom. Hence the empirical formula will be BaCl2.
When both cation and anion have equal charge the subscript for both atoms will be one
and the formula for the compound will be A1B1 ie AB ( where A and B are cation and
anion with equal charge).
If the cation “A” has X+ charge and anion “B” has Y- charge, the compound formed will
have the formula AYBX. . The charge of one ion becomes the subscript of the other ion
in the formula of their compound.
Mg2+ +N3-
Mg3N2
Do sample exercise 2.10 and practice exercise on p-59
Naming the inorganic compounds
Inorganic compounds are formed consist of ionic compounds and molecular compounds.
We know that ionic compounds are formed when metal ions (positive) and non-metal
ions (negative) are combined.
Positive Ions (cations) are having the same name of the metal (example: Sodium ion).
If a metal forms more than one ion, the charge is indicated by a roman numeral in
parenthesis following the name of the metal.
Fe2+ iron (II) ion
Fe3+
iron (III) ion
An older method used to differentiate the two differently charged ions of a metal was to
add –ous and –ic endings to the root of their Latin name.
Fe2+ Ferrous ion.
Fe3+
Ferric ion.
Cations formed from the non-metal atoms have their name ending as –ium
Example: NH4+ Ammonium ion, H3O+ Hydronium ion
Negative ions (Anions) : Monatomic anions derive their name from the corresponding
elements name with an ending –ide.
Example: H- Hydride ion, O2- Oxide ion, N3- Nitride ion etc. Some polyatomic
anions also have -ide ending
OH- Hydroxide ion, CN- Cyanide ion, O22- Peroxide ion etc
Poly atomic anions with Oxygen have their ending with –ate or –ite.
Example: NO3- Nitrate
NO2- Nitrite
Sulphate, Sulphite, Perchlorate, Chlorate, Chlorite, Hypochlorite etc are other examples
of the Oxyanions.
Anions derived by adding hydrogen ion to oxyanions are given a prefix hydrogen or
dihydrogen to the name of the respective oxyanion.
CO32- + H+
= HCO3- Hydrogen carbonate ion
Ionic compounds: The name is given with the cation name followed by the anion name.
Ba(NO3)2
CrCl3
Barium Nitrate
Chromium (III) Chloride
Do sample exercise 2.12, 2.13, 2.14, 2.15 and practice exercises on p-65, 66 & 67
Naming the acids
An acid is a compound which yield H+ ions. The name of the acid is related to the anion
it is attached.
Anion with –ide is present the name of the acid ends with –ic with prefix hydro-
Example: In H2S, Sulphide is the anion. The name of the acid will be Hydrosulphuric
scid.
Anions ending with –ate named with ending –ic
Example: HNO3
Nitric acid
Anions ending with –ite named with ending –ous
Example: HNO2
Nitrous acid
Do sample exercises 2.14 and practice exercise on p-66
Naming the Binary molecular compounds
The element in the left of the periodic table written first excepting oxygen
Carbon dioxide, Oxygen difluoride, Chlorine dioxide
If elements are in the same group the element with the higher atomic number written first
Sulphur dioxide, Silicon Carbide
Name of the second element with ends with “-ide “
All the above examples
Greek prefixes used to indicate the number of atoms of each element
Prefix mono never used for the first element.
When prefix ends with –a or –o and the second element’s name starts with a vowel, the
-a or –o the is dropped from prefix
dichlorine monoxide
dinitrogen tetroxide