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Chapter 2 - Cont.
23 January 2014
The Structure of the Atom
and the Periodic Table
Determining the Composition of an
Atom
Determine the number of protons, neutrons
and electrons in each of the following:
11
5
B
55
26
Fe
1
Isotopes
• Isotopes - atoms of the same element
having different masses
4
– contain same number of protons
– contain different numbers of neutrons
Isotopes of hydrogen
Hydrogen
(Hydrogen - 1)
Deuterium
(Hydrogen - 2)
Tritium
(Hydrogen - 3)
Isotopic Calculations
• Isotopes of the same element have identical
chemical properties
• Some isotopes are radioactive
• Find chlorine on the periodic table
• What is the atomic number of chlorine?
17
• What is the mass given?
35.45
• This is not the mass number of an isotope
2
Atomic Mass
• What is this number, 35.45?
• The atomic mass - the weighted average of
the masses of all the isotopes that make up
chlorine
• Chlorine consists of chlorine-35 and
chlorine-37 in about 3:1 ratio
• Weighted average is an average corrected
by the relative amounts of each isotope
present in nature
Ions
• Ions - electrically charged particles that result
from a gain or loss of one or more electrons by
the parent atom
• Cation - positively charged
– result from the loss of electrons
– 23Na  23Na+ + 1e-
• Anion - negatively charged
– results from the gain of electrons
– 19F + 1 e-  19F-
3
2.2 Development of Atomic
Theory
• Dalton’s Atomic Theory - the first
experimentally based theory of atomic
structure of the atom.
Postulates of Dalton’s Atomic Theory
1. All matter consists of tiny particles
called atoms
2. An atom cannot be created, divided,
destroyed, or converted to any other
type of atom
3. Atoms of a particular element have
identical properties
4
4. Atoms of different elements have
different properties
5. Atoms of different elements
combine in simple whole-number
ratios to produce compounds (stable
aggregates of atoms)
6. Chemical change involves joining,
separating, or rearranging atoms
Postulates 1, 4, 5 and 6 are still regarded
as true.
Evidence for Subatomic Particles:
Electrons, Protons and Neutrons
• Electrons were the first subatomic
particles to be discovered using the
cathode ray tube
5
Evidence for Protons and
Neutrons
• Protons were the next particle to be discovered, by
Goldstein
– Protons have the same size charge but opposite in sign
– Proton is 1837 times as heavy as electron
• Neutrons
– Postulated to exist in 1920’s but not demonstrated to
exist until 1932
– Almost the same mass as the proton
Evidence for the Nucleus
• Initial assumed protons and neutrons were
uniformly distributed throughout the atom
• Ernest Rutherford’s “Gold Foil
Experiment” lead to the understanding of
the nucleus
– Most alpha particles pass through the foil
without being deflected
– Some particles were deflected, a few even
directly back to the source
6
Rutherford’s Gold Foil Experiment
• Most of the atom is empty space
• The majority of the mass is located in a
small, dense region
Models of the Atom
(a) Thomson
(b) Rutherford
7
2.3 Light, Atomic Structure, and
the Bohr Atom
• Rutherford’s atom – tiny, dense, positively
charged nucleus of protons surrounded by
electrons
• How do we describe the relationship of the
electrons to each other and the nucleus?
• Use the measurement of particle energy
rather than position.
Light and Atomic Structure
• Spectroscopy - absorption or emission of light
by atoms.
– Used to understand the electronic structure.
• To understand the electronic structure, we must
first understand light, electromagnetic
radiation
– travels in waves from a source
– speed of 3.0 x 108 m/s
8
Wavelengths
• Light is propagated (moves) as a collection
of sine waves
• Wavelength is the distance between identical
points on successive waves
• All wavelengths travel at the same velocity,
but have their own characteristic energy
Electromagnetic Spectrum
high energy
short wavelength
low energy
long wavelength
9
Bohr Theory
• Atoms can absorb and emit energy via
promotion of electrons to higher energy levels
and relaxation to lower levels
• Energy that is emitted upon relaxation is
observed as a single wavelength of light
• Spectral lines are a result of electron
transitions between allowed levels in the atoms
• The
emission
emission
spectrum
spectrum
- light
of hydrogen
emitted when
lead to
a the
substance
modernisunderstanding
excited by an of
energy
the
source.
electronic structure of the atom
10
The Bohr Atom
Electrons exist in fixed
energy levels
surrounding the nucleus
Promotion of
electron occurs as
it absorbs energy
Energy is released as
the electron travels
back to lower levels
Quantization of energy
Excited State
Relaxation
The Bohr Atom
11
Electronic Transitions
• Amount of energy absorbed in jumping from one
energy level to a higher energy level is a precise
quantity
• Energy of that jump is the energy difference
between the orbits involved
• Orbit - what Bohr called the fixed energy levels
• Ground state - the lowest possible energy state
Bohr Theory
•
•
•
•
Allowed levels are quantized energy levels, orbits
Electrons are found only in these energy levels
Highest-energy orbits are farthest from the nucleus
Atoms
– absorb energy by excitation of electrons to higher energy
levels
– release energy by relaxation of electrons to lower energy
levels
• Energy differences may be calculated from the
wavelength of light emitted
12
Modern Atomic Theory
• Bohr’s model of the atom when applied to
atoms with more than one electron failed to
explain their line spectra
• One major change from Bohr’s model is that
electrons do not move in orbits
• Atomic orbitals - regions in space with a
high probability of finding an electron
• Electrons move rapidly within the orbital
giving a high electron density
The periodic law and the
periodic table
• Dmitri Mendeleev and Lothar Meyer - two
scientists working independently developed
the precursor to our modern periodic table.
• They noticed that elements have distinct
regular variation of their properties when
listed in order of atomic mass.
• Periodic law - the physical and chemical
properties of the elements are periodic
functions of their atomic numbers.
13
Classification of the Elements
Periods
Parts of the periodic table
• Period – a horizontal row of elements in the
periodic table. They contain 2, 8, 8, 18, 18, and
32 elements,
• Group – also called families, are columns of
elements in the periodic table.
• Elements in a particular group or family share
many similarities.
14
Category Classification of
Elements
• Metals - elements that tend to lose electrons
during chemical change, forming positive ions.
• Nonmetals - a substance whose atoms tend to
gain electrons during chemical change, forming
negative ions.
• Metalloids - have properties intermediate
between metals and nonmetals.
Classification of the Elements
Periods
15
Classification of Elements
Metals
• Metals:
– Their atoms tend to lose electrons during chemical
change
– Elements found primarily in the left 2/3 of the
periodic table
• Properties:
–
–
–
–
High thermal and electrical conductivities
High malleability and ductility
Metallic luster
Solid at room temperature
Classification of Elements
Nonmetals
• Nonmetals:
– A substance whose atoms may gain electrons,
forming negative ions
– Elements found in the right 1/3 of the periodic table
• Properties:
– Brittle
– Powdery solids or gases
– Opposite of metal properties
16
Atomic Number and Atomic Mass
• Atomic Number:
– The number of protons in the nucleus of an
atom of an element
– Nuclear charge or positive charge from the
nucleus
• Most periodic tables give the element
symbol, atomic number and atomic mass
17