The Atomic Theory
Dalton
• Relied on the premise that their existed a different kind of
atom for each element
• Key points for Dalton’s Model of the Atom:
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Elements are composed of indivisible particles called atoms
Atoms of same element are alike in size and mass
Atoms of different elements have different sizes and masses
Compounds form when two or more atoms of different
elements combine
– Atoms combine in simple whole number ratios; 1:1, 1:2, 2:3,
ect…
– Atoms of two elements may combine in different ratios to form
different compounds: CO, CO2
Exceptions to Dalton
• Atoms consist of subatomic particles
• Atoms of the same element may have
different masses
• Under specific circumstances, atoms are able
to be broken down
Composition of Compounds
• Compounds contain the same elements in the
same proportion, by mass
• Example:
– H2O
– Water is a 2:1 ratio of hydrogen atoms to oxygen
atoms, always
– It is 11.2 % hydrogen to 88.8% oxygen by mass
Hydrogen and oxygen may combine in another ratio
however it would not form water: H2O2 is 5.9 % H to
94.1% O by mass
Law of Definite Composition
• A compound always contains two or more
elements combined in a definite proportions
by mass
Law of Multiple Proportions
• Atoms of two or more elements may combine
in different ratios to produce more than one
compound
Electric Charge
• Positive and negative charge
• Charge may transfer
– Contact
– Induction
• Unlike charges attract
• Force of charges increases as distance
between the charges decreases
The Ion
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Farday and Arrhenius
Conduct electricity when dissolved in water
Ions are atoms with a positive or negative charge
Cation: positively charged atom
Anion: negatively charged atom
How does this happen?
Stoney and Thompson show existence of the
electron: negatively charged subatomic particle
in the atom
Subatomic particles
• Electron
– e– Negatively charged subatomic
particle
– Mass of 9.110 x 10-28 g
– 1/1837 the mass of the
hydrogen atom
– Electrical charge is -1
Subatomic particles
• Proton
– Goldstein and Thompson
– Mass is ~1837 x that of an
electron
– 1.673 x 10-24 g
– p+
– Equal in magnitude to the
electron but opposite in charge
Modifications to Dalton’s Model
• Thompson offers new theory to Atomic model
• Although it seemed as if the atom was
indivisible, it is now clear that it is composed
of subatomic particles
• This clearly negates parts of Dalton’s Model of
the Atom
• Modifications were suggested by Thompson
Thompson’s Model of the Atom
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Electrons are embedded in the atomic sphere
Atoms are neutral therefore they must contain an equal number of positive
protons
Ions are formed by gaining or losing electrons
•
Cations form by losing electrons
– Group I forms 1+ ions by losing 1 electron
– Group 2 forms 2+ ions by losing 2 electrons
– Group 3 forms 3+ ions by losing 3 electrons
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Anions form by gaining electrons
– Nitrogen group forms 3- ions by gaining 3 electrons
– Oxygen group forms 2- ions by gaining 2 electrons
– Halogens form 1- ions by gaining 1 electron
Subatomic Particles
• Neutron
– Chadwick
– Neutral
– n◦
– 1.675 x 10-24 g
Dimensional Analysis and the mass of
an atom
• Be familiar with the example problems on
page 89, at the bottom…
Rutherford and the Nucleus
of the Atom
• Rutherford used the radioactive alpha particles discovered
by Becquerel to establish the nature of the nucleus of the
atom
• He directed the positively charged He atoms (alpha
particles) at a piece of gold foil
• Most passed through
• Some were deflected
• A few bounced back
• Rutherford was able to reason that their must be a positive
portion in the center of the atom because like repels like
• This portion was referred to as the nucleus
• Most of the atom is empty space
Mass of an atom?
• Mass of the atom is usually referred to as the
nucleus of the atom
• The nucleus contains protons and neutrons
• 99.9% of the total mass of the atom
• Mass of electrons is mostly negligible
Bell Ringer
Pick up one of each of the three
papers off the front demo table.
Schedule
1. Discuss Midterm
2. Atom Notes
You also need your composition notebook
for notes today.
How many protons, neutrons, and electrons
are in a neutral atom of Carbon?
What is Carbon’s mass number? Atomic
number? Atomic mass?
HOMEWORK: Practice
Problems
The train of failure
usually runs on the
track of laziness.
The Neutral Atom
• Atoms consist of a dense
portion called the nucleus
• The nucleus contains the
positively charges protons
and the neutral neutrons
• Electrons are found outside
the nucleus
• The number of protons in a
neutral atom equal the
number of electrons
Elements and their Numbers
• Atomic number is the whole number on the
peridic table that increases from left to right
• The atomic number = the number of protons
in the nucleus of an atom
• Atomic number is unique for each atom as it
determines the identity for each element
• Atomic # 1 = Hydrogen , 2 = Helium, 3 =
Lithium
• In a neutral atom, the number of protons will
= the number of electrons
• The mass number = the number of protons +
the number of neutrons
• The number of neutrons = mass number – the
number of protons
Isotopes
• Atoms of the same element
have the same atomic number
• If the same atoms have the
same atomic number but
different mass numbers, they
are isotopes
• Atoms of elements with the
same number of protons and
electrons but different
neutrons are called isotopes
Proper Notation
A number is mass number ( protons + neutrons
or nucleons)
Z number is number of protons (atomic number)
• A=Z+N
• N= A – Z
Remember:
A = number or nucleons or protons plus
neutrons…A is the mass number
Z is the number and N is the number of
neutrons
Example
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Protons = 79
Electrons = 79
Neutrons = 197 – 79 = 118
Nucleons= 197
Atomic Mass
• Read section in Chapter and walk through
example problems on pages 96 and 97.