Download 22013 Lecture-Ch2 Chem

Chapter 2
Atoms, Molecules, and Ions
semester 2/2013
2.2 The Structure of Atom
2.3 Atomic Number, Mass Number, and Isotopes
2.4 The Periodic Table
2.5 Molecules and Ions
2.6 Chemical Formulas
Ref: Raymond Chang/Chemistry/ Eleventh Edition
Prepared by A. Kyi Kyi Tin
2.2 The Structure of Atom
Atom..the basic unit of an element that can enter into
chemical combination (extremely small and indivisible)
Three subatomic particles…electrons , protons, and
neutrons.
The Electron (Joseph John Thomson (1856-1940)
Negatively charged particles
Source: (from cathode ray tube experiment, forerunner of
the television tube)
The cathode ray is attracted by the plate bearing positive
charges and repelled by the plate bearing negative
charges, it must consist of negatively charged particles.
Cathode Ray Tube
J.J. Thomson, measured mass/charge of e(1906 Nobel Prize in Physics)
Radiation…emission and transmission of energy
through space in the form of waves..
Radioactivity (Antoine Henri Becquerel(1852-1908)
spontaneous emission of particles and / or radiation /
Any element that spontaneously emits radiation is
“Radioactive”.
(Uranium compound)
2.2
Three types of rays produced by decay of radioactive
substances such as “Uranium”..
(i) Alpha ( ) rays..positively charged particles ( )
particles.. deflected by positively charged plate
(ii) Beta () rays..electrons..deflected by negatively
charged plate
(iii) Gamma () rays..high-energy rays..no charge and
are not affected by an external field.
(1908 Nobel Prize in Chemistry)
 particle velocity ~ 1.4 x 107 m/s
(~5% speed of light)
1. atoms positive charge is concentrated in the nucleus
2. proton (p) has opposite (+) charge of electron (-)
3. mass of p is 1840 x mass of e- (1.67 x 10-24 g)
The Proton and the Nucleus
1910,Ernest Rutherford’s experiment
(Refer text bk. Pg.47,48)
Nucleus…a dense central core within the atom
Proton…positively charged particles in the nucleus
Source: whenever “. Came close to a nucleus in the
scattering experiment, it experienced a large repulsive
and therefore a large deflection. (i.e nucleus is composed
of positively charged particles, which called proton)
The Neutron (James Chadwick, 1932)
-electrically neutral particles having a mass slightly greater
than that of protons
Particle
Proton
Mass (g)
9.10 x 10 -28
1.67 x 10 -24
Neutron
1.67 x 10 - 24
Electron
Charge unit
-1
+1
0
Source: When a thin sheet of beryllium is
bombarded with -particles, a very high-energy
radiation similar to - rays was emitted by the metal
i.e Neutron
mass p = mass n = 1840 x mass e2.2
2.3 Atomic Number, Mass Number, and Isotopes
Atomic Number (Z)…the number of protons in the
nucleus of each atom of an element.
Mass Number (A)… the total number of neutrons and
protons present in the nucleus of an atom of an element.
Isotopes…atoms that have the same atomic number but
different mass number.
1
1
H ,................12 H ,...............13 H
hydrogen......deuterium......tritium
Mass number
Atomic number
A
Z
X
2.4 The Periodic Table
-A chart in which elements having similar chemical and
physical properties are grounded together.
-Groups…Vertical Columns
-Periods….Horizontal Rows
-Metals….Good conductor of Heat & Electricity
-Nonmetals…Poor conductor of heat & electricity
-Metalloids….Intermediate b/w Metal & Non-metal
-Alkali metals(1A)/ Alkaline Earth metals(2A)/
Halogens( 7A)/ Noble gases (or) Rare gases (8A)
Noble Gas
Halogen
Group
Alkali Metal
Alkali Earth Metal
Period
2.5 Molecules and Ions
Molecule…an aggregate of at least two atoms in definite
arrangement held together by chemical forces (Chemical
bonds) SAME ELEMENTS (or) TWO (or) MORE
ELEMENTS IN A FIXED RATIO
Diatomic molecules
same elements (H2,O2,Cl2)
different elements (HCl, CO)
Polyatomic molecules
more than two atoms
(O3, H2O,NH3)
Ions..an atom or a group of atoms that has a net positive or
negative charge.
Cations / Anions / monoatomic ions / polyatomic ions
2.6 Chemical Formulas
Molecular Formula…shows the exact number of atoms of each
element in the smallest unit of a substance.
Allotrope..one of two or more distinct forms of an element
(carbon..diamond and graphite)
Structural Formula…shows how atoms are bonded to one
another in a molecule.
Empirical Formula..which elements are present and the simplest
whole number ratio of their atoms..
Molecular
formula
H2O
Empirical
formula
H2O
H2O2
HO
NH3
NH3
H2C2O4
HCO2
O3
O
Ionic compounds consist of a combination of cations and an
anions. (example Na+Cl-)