Download Atom

Properties of Atoms and
the Introduction to the
Periodic Table
Structure of the Atom
• Elements are abbreviated in
scientific shorthand – first
letter or two of element’s
name
• Atom – smallest piece of
matter that still has the
properties of the element
Hydrogen
1
1
H
Protons: 1
Neutrons: 0
Electrons: 1
Atom
•
•
•
•
Protons have electrical charge of
1+
Neutrons do not have an
electrical charge
Electrons have electrical charge
of 1Protons and neutrons are in the
nucleus of an atom; electrons
surround the nucleus
Example: Lithium atom
Lithium has:
• 3 protons (red)
•4 neutrons (blue)
•3 electrons
(black)
Protons and neutrons are
made up of smaller
particles called quarks
•
•
Quarks are studied by
colliding accelerated
charged particles with
protons, which leave
tracks in a bubble
chamber
Six quarks are known to
exist; the sixth is called
the top quark
Aristotle
• Said all matter
consisted of
four elements:
•
•
•
•
Earth
Water
Air
Fire
Democritus (Greek) 400BC was first to
propose an Atomic Theory
• Believed all matter was
composed of small indivisible &
indestructible particles he
called atoms, from Greek word
atomos.
− Not based on experimentation
John Dalton (early 1800’s) – atomic
theory based on experimentation
• All elements are composed of atoms
• Atoms of the same element are
identical, atoms of different
elements are different
• Atoms of different elements can join
together in single whole number
ratios to form compounds
• Chemical reactions occur when
atoms are separated, joined or
rearranged
• Here are some of the symbols Dalton used
for atoms of elements and molecules of
compounds. He probably used a circle for
each because, like the ancient Greeks, he
thought of atoms as tiny, round hard
spheres.
J.J. Thomson (English) 1897
• Discovered electrons using a
cathode ray tube.
• Sealed tube with vacuum inside
and electrodes on each end. When
connected to electric current, a
beam traveled from cathode to
anode. He called the beam
“cathode rays.” They were
electrons.
Thomson’s Cathode Ray
Tube
Thomson cont.
• Proposed “Plum Pudding” model of
the atom.
• Was a field of evenly distributed
positive and negative charge.
The 'Plum Pudding' Model of an Atom
Rutherford’s
Experiments:
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf
In 1911, Ernest Rutherford interpreted these
results and suggested a new model for the atom.
He said that Thomson's model could not be right.
The positive charge must be concentrated in a
tiny volume at the center of the atom, otherwise
the heavy alpha particles fired at foil could never
be repelled back towards their source. On this
model, the electrons orbited around the dense
nucleus (center of the atom).
Compare Thomson’s and
Rutherford’s models:
Niels Bohr (early 1900’s)
The next important development came in 1914 when
Danish physicist Niels Bohr revised the model again.
It had been known for some time that the light given
out when atoms were heated always had specific
amounts of energy, but no one had been able to
explain this. Bohr suggested that the electrons must
be orbiting the nucleus in certain fixed energy levels
(or shells).
The nucleus is the centre of an atom, containing
protons and neutrons. The energy must be given out
when 'excited' electrons fall from a high energy level
to a low one.
Bohr Atom Model
Scientists use scaled-up models
to represent atoms.
• Early models of atoms used
a solid sphere.
• Current quantum
mechanical model shows
electrons traveling in
specific energy levels
around a nucleus of protons
and neutrons
Where are protons,
neutrons and electrons?
• Protons and neutrons are in the
center nucleus surrounded by
electrons traveling in specific
energy levels.
Electron Orbitals
Masses of Atoms
Atomic Mass – composed mostly
of the protons and neutrons in
the nucleus
•
Unit of measurement for atomic
particles is atomic mass unit
(amu) which is one-twelfth the
mass of a carbon atom
containing six protons and six
neutrons.
• Atomic number – the number
of protons in an atom; number
of protons also identifies the
element
• The sum of the number of
protons and neutrons in the
nucleus of an atom is the mass
number.
How many protons, neutrons and
electrons are in a neutral atom?
Element
H
Atomic #
I
Pb
Protons
1
O
K
Mass #
Neutrons
Electrons
0
16
8
19
20
127
53
82
125
• Isotopes – atoms of the same
element with different
numbers of neutrons
•
•
•
Different isotopes have different
properties
Numbers of neutrons is equal to
mass number minus atomic
number
Name of element followed by
mass number identifies the
isotope
Isotopes Examples:
• Average atomic mass is the
weighted-average mass of an
element’s isotopes
• Average atomic mass is
closest to its most abundant
isotope
How do atomic number
and mass number differ?
• Atomic number is the number of
protons in an atom; mass
number is the number of
protons and neutrons in an
atom.
The Periodic Table
• Elements are organized in the
periodic table by increasing
atomic number.
•
•
In the late 1800’s, Dmitri
Mendeleev devised the first
periodic table based on atomic
mass.
In 1913, Henry Moseley arranged
the elements by atomic number
rather than atomic mass.
The Periodic Table
• Vertical columns in the
periodic table are groups of
elements with similar
properties.
•
Elements in the same group have
the same number of electrons in
their outer energy level
The Periodic Table
• Each of the seven energy
levels can have a maximum
number of electrons.
•
•
Energy level one can contain at
most two electrons
Energy level two can contain at
most eight electrons
•
More on this later! (MUCH MORE!)
The Periodic Table
• Rows are called periods
• Each row in the periodic table
ends when an outer energy
level is filled
• Electron dot diagrams use the
element symbol and dots to
represent outer energy level
electrons.
Molecular
Formula
CH4
Electron Dot
Diagram
H
··
H : C : H
··
H
Structural
Formula
H
|
H - C - H
|
H
The Periodic Table
• Periods – horizontal rows of
elements that contain
increasing numbers of protons
and electrons.
•
•
Elements are classified as
metals, nonmetals, or metalloids
Elements are synthesized in
laboratories all over the world
The Periodic Table
• The same elements exist all
over the universe
•
•
Hydrogen and helium are the
building blocks of other naturally
occurring elements
Supernovas spread heavier
elements throughout the universe
What do elements in the
same group have in
common?
• The same number of electrons
in the outer energy level
Electromagnetic Radiation (Light) as a
Key to Understanding Electron Paths
• Early scientists discovered that
Electromagnetic radiation (light)
is given off by atoms of an
element when they have been
excited by some form of energy
• Furthermore, atoms of different
elements give off different colors
of light when they are excited.
When salts
containing Li,
Cu, and Na
dissolved in
methyl
alcohol are
set on fire,
brilliant
colors result.
Cu
Li
Na
Spectral Analysis of Emitted
Light from Excited Atoms
• When the emitted light from excited
atoms was passed through a prism, a
curious spectrum of discrete lines of
separate colors, separate energies,
was observed rather than a continuous
spectrum of ROY G BIV.
• Furthermore, different elements show
totally different line spectra.
• In fact, line spectra are used to
identify the presence of different
elements
The Continuous Spectrum
Continuous Spectrum
Atomic Line Spectrum
Na
Interpretation of Atomic
Spectra
• The line spectrum must be related to
energy transitions in the atom.
• Absorption = atom gaining energy
• Emission = atom releasing energy
• Since all samples of an element give the
exact same pattern of lines, every atom of
that element must have only certain,
identical energy states
• The energy of an atom is quantized –
limited to discrete values
• If the atom could have all possible
energies, then the result would be a
continuous spectrum instead of lines
Interpretation of Atomic Line
Spectra in terms of Electron
Paths
• Electrons may be thought of as traveling in
concentric shells or energy levels about the
nucleus.
• The energy of the shells increase as one
proceeds away from the nucleus.
• When an atom absorbs energy, electrons are
promoted from an inner, low energy, shell to
an outer, higher energy shell.
• Conversely, when an excited atom emits
energy, electrons drop down from an excited
outer, higher energy, shells to an inner, lower
energy, shells
An excited lithium atom emitting
a photon of red light to drop to a
lower energy state.
A sample of H atoms receives energy from an
external source
The excited atoms (H) can release the
excess energy by emitting photons.
When an excited H atom returns to a
lower energy level, it emits a photon
that contains the energy released by
the atom.
When excited hydrogen atoms
return to their lowest energy
state, the ground state, they emit
photons of certain energies, and
thus certain colors.
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