CHEM 1411 - Chapter 2
ATOMS MOLECULES AND IONS
Dalton’s Atomic Theory
1.
2.
3.
4.
Matter is composed of very minute particles called atoms, which are indivisible.
Atoms of an element are identical in all respects.
Atoms of different elements have different mass and properties
Atoms of different elements combine in a fixed ratio by mass to form “compound
atoms”. (Molecules)
5. Atoms can neither be created nor destroyed
Law of definite proportions
The law states that all pure samples of a chemical compound contain same elements combined in
the same fixed proportion by mass.
Law of Multiple Proportions
The law states that when two elements combine together to form more than one compound, the
different masses of one of the elements combining with a fixed mass of the other one bear a simple
multiple ratio.
Law of Conservation of Mass
The law states that matter can neither be created nor destroyed in a physical or chemical change.
The Structure of Atom
The Fundamental Constituents of atom are Electrons, Protons and Neutrons.
Electrons
J.J. Thomson discovered electrons by the cathode ray tube experiment.
R.A. Millikan calculated the magnitude of charge on electrons by his oil drop experiment.
Discovery of Nucleus of the atom
Rutherford discovered the nucleus of the atom through his gold leaf experiment. Nucleus is the
positively center of the atom.
Protons
Goldstein discovered protons by cathode ray tube experiment.
Neutrons
Chadwick discovered the neutrons while performing some experiments related to the artificial
transmutation of elements.
Modern studies have suggested that atoms are divisible. It consists of subatomic particles such as
electrons, protons and neutrons. An atom has a positively charged center called nucleus. Protons and
neutrons are present in the nucleus and the electrons in the shells outside the nucleus. These shells are
called ‘energy levels’ or ‘energy shells’. The electrons revolve around the nucleus.
Particle
charge Magnitud e
Mass (g)
Location
Electron
1-
1.602 x 10-19 coul.
9.109 x10 -28
outside nucleus
Proton
1+
1.602 x 10-19 coul .
1.672 x 10 -24
nucleus
Neutron
0
1.675 x 10 -24
nucleus
--
An atom is electrically neutral because it contains equal number of protons and electrons.
(i.e. same number of positive and negative charges, so that the net charge is zero)
Atomic number (Z)
Atomic number of an element is the total number of protons in its atom.
Atomic mass, Atomic weight or Mass number (A)
Atomic mass of an element is the total number of protons and neutrons present in its atom.
The unit of atomic mass is amu (atomic mass unit). 1 amu = 1.661 x 10-24 g
Isotopes
Isotopes are the atoms of the same element having same atomic number but different atomic
mass.
Example
1. Hydrogen has 3isotopes.
1
2
3
H (hydrogen)
H (deuterium)
H (tritium)
1
1
1
*Other examples of Carbon, Chlorine and Nitrogen.
Periodic Table of Elements
The periodic Table is a systematic arrangement of elements in the order of increasing atomic
numbers.
Atomic number is taken as the basis for the arrangement of the elements, because when the
elements are arranged in the increasing order of their atomic numbers, elements with similar
properties repeat after a regular interval. This is called Periodic law
The horizontal rows are called periods and the vertical columns are called groups. There are 7
periods and 18 groups. The groups are numbered from 1 through 18. All the elements of the
same group have similar chemical properties because they have same number of valence
electrons.
In the periodic tables, there is a clear distinction between metals, non-metals and metalloids.
Elements on the left and middle are metals and those on the right side are mostly non-metals.
Metals are good conductors of heat and electricity, highly electropositive, lustrous (shining),
malleable and ductile. (Cu, Au, Mg, Al)
Non metals are bad conductors of heat and electricity, electronegative and brittle.(C, P, S )
Metalloids possess both the property metals and non-metals.
Example:
(Boron, Silicon, Germanium, Arsenic, antimony, Tellurium and Polonium)
Elements of group 1 (IA) are called Alkali Metals. They are called so because their hydroxides
are strong bases.
Elements of group 2 (IIA) are called Alkaline Earth Metals.
Elements of the group 17 (VII A) are called Halogens (They are salt producers)
Elements of the group 18(VIII A) are the Noble gases (They are the least reactive elements)
The elements belonging to the first two and the last five groups, except group 18 are called Main
Group Elements.
Elements of groups 3 through 12 (IB – VIIB and VIII) are the Transition Elements. All of them
are metals and form Cations with variable electric charge. They form colored compounds.
Elements with atomic numbers 58-71 are called the Lanthanides and the elements with atomic
numbers 90-103 are called Actinides. Both are collectively known as Inner- Transition
Elements. Actinides are radioactive elements.
Molecules
A molecule is an aggregate of two or more atoms of same or different elements combined in a
fixed proportion by mass.
Ions
An ion is an atom or a group of atoms with positive or negative charges.
The losing of electrons forms positive ions. They are also called cations. The gaining of electrons
forms negative ions. They are also called anions.
Molecular Formula and Empirical formula.
The formula that gives the actual number of atoms of elements present in one molecule of the
compound is called the molecular formula.
Empirical Formula
The formula that gives the simplest whole number ratio of the constituting atoms in a molecule
of the compound is called the empirical formula.
Molecular Formula = n x Empirical formula.
n = Molecular Formula mass / Empirical formula mass.
Formula of ionic compounds
Ionic compounds do not contain molecules. They exist as ionic pairs.
Group number
# of valence electrons
Ion charge
1(IA)
1
1+
2(IIA)
2
2+
13(IIIA)
3
3+
14(IVA)
4
4+/-
15(VA)
5
3-
16(VIA)
6
2-
17(VIIA)
7
1-
Predicting formulas of ionic compounds:
(2) By Cross over approach
i.
Ca2+
N3-
ii.
K1+
O2-
K2O
iii.
Al3+
N3-
AlN
Ca3N2
Naming of ionic compounds.
Ionic compounds contain cations and anions. The metal ion (cation, +) is named first, followed
by the non-metal ion (anion, -). For naming the cation, the element’s name is written
unchanged.
For naming mono atomic and simple anions, the first part of the element’s name is combined
with the suffix (-ide).
KCl
Potassium Chloride
AlN
Aluminum Nitride
CaO
Calcium Oxide
Covalent compounds (molecular compounds.)
These are formed by the mutual sharing of electrons between atoms. Such compounds are
usually formed between the atoms of non-metals.
Naming of Covalent Compounds:
Greek Prefixes are used to indicate the number of atoms.
1- mono
2-di
3 - tri
4 -tetra
6 -hexa
7 - hepta
8 - octa
9 - nona
5 - penta
10 - deca
The first element is named as it is. The second element’s name is altered to accommodate the
suffix (-ide).
N2O
dinitrogen monoxide
CO
carbon monoxide
CO2
carbon dioxide
The prefix “mono” is never used for naming the first element. No prefix is used to indicate the
number of H atoms.
Naming of Acids
An acid is a substance capable of giving H+ ions when dissolved in water (aqueous solution).
They contain one or more ‘H’ atoms and an anionic group. Their name starts with a prefix
‘Hydro’ and ends in a suffix ‘ic’ in the case of anions whose name ends in ‘ide’.
HCl - Hydrochloric Acid
HCN – Hydro cynic Acid
Naming Oxoacids.
Oxoacids contain Hydrogen, Oxygen and a central element.
H2SO4 - Sulfuric acid, HNO3 - Nitric acid, HClO3 - Chloric acid.
There may be two or more oxoacids with same central atom but a different number of ‘O’
atoms. The following rules are observed for naming such acids, starting with the reference acid
whose name ends with ‘ic’.
For example let us take the reference acid, HClO3 - Chloric acid.
 Addition of one ‘O’ atom to the ‘ic’ acid changes the name of acid to include a prefix
‘per’
HClO4 - Perchloric acid.
 Removal of one ‘O’ atom from the ‘ic’ acid changes the name of acid to include a suffix
‘ous’
HClO2 - Chlorous acid.
 Removal of two ‘O’ atom from the ‘ic’ acid changes the name of acid to include a prefix
‘Hypo’ and a suffix ‘ous’
HClO - Hypochlorous acid.
Naming Oxoanions.
 When all the ‘H’ ions are removed from the ‘ic’ acid, the anions name ends with ‘ate’
 When all the ‘H’ ions are removed from the ‘ous’ acid, the anions name ends with ‘ite’
 When all the ‘H’ ions are not removed, the number of H ions present must be indicated in
the name.
Naming of Bases
A base is a substance that yields OH- ions (Hydroxide ions) when dissolved in water.
NaOH - Sodium Hydroxide
Hydrates
They are compounds containing a specific number of water molecules attached to them.
CuSO4.5H2O – Copper Sulfate pentahydrate
MgCl2.2H2O – Magnesium Chloride dihydrate
Organic Compounds
They are compounds containing Carbon and Hydrogen (Hydrocarbons) and their derivatives. They
form a special class of compounds and a special branch of chemistry known as Organic chemistry.
Ex. CH4 - methane, C2H6 -ethane, CH3OH - methanol, C2H5OH - ethanol, CCl4 - carbon tetrachloride.