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GENERAL CHEMISTRY
Principles and Modern Applications
PETRUCCI
HERRING
MADURA
Atoms and the
Atomic Theory
PHILIP DUTTON
UNIVERSITY OF WINDSOR
DEPARTMENT OF CHEMISTRY AND
BIOCHEMISTRY
TENTH EDITION
BISSONNETTE
2
Atoms and the
Atomic Theory
CONTENTS
2-1
Early Chemical Discoveries and
the Atomic Theory
2-2
Electrons and Other Discoveries
in Atomic Physics
2-3
The Nuclear Atom
2-4
Chemical Elements
2-5
Atomic Mass
2-6
Introduction to the Periodic Table
2-7
The Concept of the Mole and the
Avogadro Constant
2-8
Using the Mole Concept in
Calculations
2-1 Early Discoveries and the Atomic Theory
 Lavoisier 1774
Formulate the law of conservation of mass
 He heated a sealed glass vessel containing a sample of tin and
some air
 The mass before heating (vessel + tin + air)=after heating (vessel +
tin + air)
 The total mass of the substance present after a chemical
reaction is the same as the total mass of substance before
the reaction.
 Figure show the reaction between
silver nitrate and potassium
chromate to give a red solid (silver
chromate)
(a) Before the reaction, the beaker
with a silver nitrate solution and a
graduated potassium chromate
solution are placed on a single pan
balance displace the combine mass
= 104.5 g
(b) After mixing, a chemical reaction
occurs that forms silver chromate
(red precipitate) in potassium
nitrate solution. The total mass =
104.5 g, remains unchanged.
FIGURE 2-2
Mass is conserved during a chemical reaction
 The low of conversation of mass says that matter is neither
created nor destroyed in a chemical reaction
 Proust 1799
Law of constant composition
All sample of the compound have the same composition- the same
proportions by mass of the constituent elements.
 Consider the compound water made up of two atoms of hydrogen
(H) for every atoms of oxygen (O)
Can be presented chemical formula H20
 Two samples describes below have the same proportions of the two
elements
Exp: determine the percent by mass of hydrogen
 Simple divide the mass of hydrogen by the sample mass and multiply
by 100%. For each sample, you will obtain the same results:11.19% H
Sample A and Its Composition
10.000 g
Sample A and Its Composition
27.000 g
1.119 g H
% H=11.19
3.021 g H
% H=11.19
8.881 g O
% O= 88.81
23.979 g O
% O= 88.81
Dalton’s Atomic Theory: John Dalton
Describes the basis of atomic theory with three assumptions
1.
Each element is composed of small particles called atoms. Atoms
are neither created nor destroyed in chemical reactions.
2.
All atoms of a given element are identical but atoms of one
element are different from those off all other elements
3.
Compounds are formed when atoms of more than one element
combine in simple numerical ratios.
exp: one atom of A to two B (AB2)
 In forming carbon monoxide
(CO), 1.0 g of carbon
combines with 1.33 g of
oxygen.
Figure 2-3
Molecules of CO and CO2
 In forming carbon dioxide
(CO2), 1.0 g of carbon
combines with 2.66 g of
oxygen.
 The second oxide is richer in O
 It contains twice as much O as the first
2-2
Electrons and Other Discoveries in Atomic Physics
 Electricity and magnetism were used in the experiment so that led to the current
theory of atomic structure
Certain objects displays a properties called electric charge, which can be either
positive (+) or negative (-)
An object having equal number of (+) or (-) charged particles carries no net
charge and is electrically neutral
If the number of (+) charge exceed the number of (-) charge , the object has a net
positive charge
If the number of (-) charge exceed the number of (+) charge , the object has a net
negative charge
 (+) and (-) charges attract each other , while two (+) and two (-) charges repel
each other
FIGURE 2-4
Forces between electrically charged objects
 (a) Electrostatically charged comb. If you comb your hair the static charge
develop on the comb and causes bits of paper to be attracted to the comb
(b) Both objects on the left carry negative charge repel each other
The objects in the center lack any electrical charge and exert no force on each other
The object on the right carry opposite charges and attract each other
The Discovery of Electrons
 Faraday discovered cathode rays, a type of radiation emitted by a (-) terminal, or
cathode (it is iron, platinum so on)
The radiation crossed the evacuated tube to the (+) terminal, or anode
FIGURE 2-6
Cathode ray tube
 The high voltage source of electricity creates a (-) charge on the electrode at the left
(cathode) and a (+) charge on the electrode at the right (anode)
Cathode rays pass from the cathode (C) to the anode (A) which is perforated to allow
the passage of a narrow beam of the cathode rays
They are visible only through the green florescence that they produce on the zinc
sulfide-coated screen at the end of the tube. They are in the other part s of the tube
 C rays are deflected by electric and magnetic fields in the manner expected for
negatively charged particles (Figure 2-7 (a) and (b))
 Figure 2-7 (c) determine the mass to charge ratio m/e for the C rays
 Cathode rays subsequently become known as electrons
Electron m/e = -5.6857 × 10-9 g coulomb-1
FIGURE 2-7
Cathode rays and their properties
 Robert Millikan determined the electronic charge , e through a
serious oil drop experiment
 He showed ionized oil drops can be balanced against the pull of
gravity by an electric field
e is -1.6022x10-19 coulomb
 mass of electron= -1.6022x10-19 coulomb x = -5.6857 × 10-9 g
coulomb-1 = 9.1094 x 10-28 g
Figure 2-8
Millikan’s oil-drop experiment
Plum-pudding Model Proposed by Thomson
 Explains how the electron particles were incorporated
into atoms.
 He thought that the positive charge necessary to
counter balance the negative charges of electrons in a
neutral atom was the form of a nebulous cloud.
 He suggest, electrons floated in a diffuse cloud of
positive charge
 A helium atom would have a +2 cloud of (+) charge and
two electrons (-2)
 If helium atom loses one electron, it becomes charged
and is called an ion referred to He+ has a net charge of 1+
If the helium atom loses both electron the He2+ ion forms
X-Rays and Radioactivity
 X-ray is form of high energy electromagnetic radiation
 Radioactivity is the spontaneous emission of radiation
from a substance
Two types of radiation form from radioactive material
were identified by Ernest Rutherford
 Alpha (a): a-particles carry two fundamental units of
positive charge and the same mass as helium atoms.
This particle are identical to He2+ions
 Beta (b): b-particles are negatively charged and have
the same properties as electrons
 Gamma (g) rays: is not effected by electric or magnetic
field. It is not made of particles. It is electromagnetic
radiation of extremely high penetrating power.
2-3 The Nuclear Atom
 Rutherford used the a-particles to study inner structure of the atoms
 The telescope travels in a circular track around at evacuated chamber containing
the metal foil.
 Most a-particles pass thought the metal foil undeflected , but some are deflected
through large angles
Figure 2-11
The scattering of a- particles by metal foil
 The nuclear atom have these features below
 Most of mass and all of positive charge of an atom are centered in a very
small region called nucleus. The remainder of the atom is mostly empty space
 The magnitude of the positive charge is different for the different atoms and
is approximately one-half the atomic weight of the element
There are as many electrons outside the nucleus as there are unit of positive
charge on the nucleus. The atom as a whole is electrically neutral.
Properties of Protons, neutrons and Electrons
 Protons: positively charged fundamental
particles of the matter in the nuclei of
atoms
 Neutrons: penetrating radiation
consisted of beam of neutral particles
 The number of protons in a given atom
is called the atomic number, or the
proton number, Z
The number of electrons in the atom is
equal to Z because the atom is electrically
neutral
The total number of proton and neutrons
in an atom is called the mass number, A
The number of neutron is A-Z and
electrically neutral.
Figure 2-13
The nuclear atom – illustrated by the helium atom
2-4 Chemical Elements
 Each element has a name and distinctive symbol
 Exp: carbon:C, oxygen:O, neon:Ne, iron:Fe
To represent a particular atom we use symbolism
Symbol of element
A= mass number
Z = atomic number
 Has 13 protons and 24 neutrons in its nucleus
27
13
Al
and 13 electron outside the nucleus (recall that
an atom has the same number of electrons as
protons)
Isotopes
 atoms that have the same atomic number (Z) but different masss number (A) are
called isotopes.
 Exp: all neon atoms have 10 protons in their nuclei, and most have 10
neutron as well. A very few neon atoms have 11 neutrons and some have 12
20
Ne
10
Ions
21
Ne
10
22
Ne
10
 When atoms lose or gain electrons the species formed are called ions and carry
net charges.
 Removing electrons result in positively charged ion
 The number of proton does not change when an atom becomes an ion.
 Exp: 20 + 10 protons 10 neutrons and 9 electrons
Ne
10
22
2+
Ne
10
10 protons 12 neutrons and 8 electrons
16 2O
8
8 protons 8 neutrons and 10 electrons
Isotopic masses
 Used when original mass of an atoms can not be determined
 That must be done by experiment
 One type of atom has been chosen and assigned a specific mass. This standard
is an atom of the isotope carbon-12
 Next the masses of the other atoms relative to carbon -12 are determined with
a mass spectrometer
Figure 2-14 A mass spectrometer and a mass spectrum
2-5 Atomic Mass
 The average of the isotopic masses, weighted according to the
naturally occurring abundance of the isotopes of the elements
Weighted Average
Atomic Mass of an Element
Equation (2.3)
fractional
atomic
fractional
atomic
abundance x mass of + abundance x mass of + ……
of isotope 1 isotope 1 of isotope 2 isotope 2
=
Aave
=
where
x1 x
A1
+
x2 x A2
x1 + x2+ + xn = 1.0
+ …… xn x An
2-6 Introduction to The Periodic Table
 The classification system we need known as the
periodic table of the elements
 Read atomic masses
 Read the ions formed by main group elements
 Read the electron configuration
 Learn trends in physical and chemical properties
We will discuss these in detail in Chapter 9.
Noble Gases
The Periodic table
Alkali Metals
Alkaline Earths
Halogens
Main Group
Transition Metals
Main Group
Lanthanides and Actinides
2-7
The Concept of the Mole and the Avogadro Constant
 A mole: is the amount of the substance that contains the same number of
elementary entities (atoms, molecules and so on)
 Avogadro constant or Avogadro number, NA: The amount of elementary
entities in a mole
NA = 6.02214179 x 1023 mol-1
 Exp:
 1 mol 12C = 6.02214179 x 1023
12C
atoms = 12 g
 1 mol 16O = 6.02214179 x 1023
16O
atoms = 15.9949 g (and so on)
 Molar mass, M: the mass of one mole of substance , from a table of atomic masses
 Exp: the molar mass of lithium is 6.941 g/mol Li