The Atom and Periodic Table General Chemistry: Unit 3 Fall 2010.
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Transcript of The Atom and Periodic Table General Chemistry: Unit 3 Fall 2010.
The Atom and Periodic Table
General Chemistry: Unit 3Fall 2010
Atomic Model Scientists
John Dalton 1803
John Dalton (Wikipedia.org)
Dalton’s Atomic Theory
All matter is made up of tiny, unbreakable particles called atoms (Democritus proposed this in 460-370 BC)
Atoms of the same element are identical, but they are different from atoms of other elements.
Chemical reactions occur when atoms are separated, joined, or rearranged
Successes and Problems
Verified theories and experiments by Democritus and Lavoisier (law of conservation
of matter) and Proust (law of definite proportions).
We know today that atoms are made of smaller particles and can be split, and there can be isotopes of the same element.
Atomic Model Scientists (cont.)
Dmitri Mendeleev 1869 Developed a Periodic
Table based on atomic mass
Dmitri Mendeleev (Wikipedia.org)
Atomic Model Scientists (cont.)
Joseph John Thomson 1897 Found rays bent toward a
positively charged plate and away from a negatively charged plate.
Determined cathode rays are made up of negatively charged particles referred to as electrons.
JJ’s Successes
Scientists were able to use JJ Thompson’s cathode ray tube to discover protons.
The amount of charge on an electron and proton is equal but opposite, but the mass of a proton is much greater that that of an electron.
Thompson Again!
He also discovered that Neon consisted of atoms of two different masses.
Later, these were called isotopes: atoms of an element that are chemically alike but differ in mass (# of neutrons).
Scientists were then able to discover the neutron (1930s): a neutrally charged particle of equal mass to a proton.
Atomic Model Scientists (cont.)
Ernest Rutherford 1911 Discovered the positively
charged nucleus through gold foil experiment.
Ernest Rutherford (Nobelprize.org)
Rutherford’s Gold Foil Experiment
http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter02/Text_Images/FG02_05.JPG
Atomic Model Scientists (cont.)
Niels Bohr 1913
Niels Bohr (The University of New
York)
Bohr Continued
Rutherford’s atomic model did not explain the chemical properties of elements
A description of the behavior of electrons was needed
Bohr described fixed energy levels an electron can possess. In order to move up an
energy level, energy must be absorbed and vice versa but energy is released
The more energy an e- has, the further from the nucleus it is
Bohr concluded
A quantum of energy is the amount of energy needed to move an e- from one level to the next
The higher the energy level, the less energy it takes to move from that level to the next
This idea failed to explain more complex elements… this is where Shrodinger comes in
Erwin Schroedinger(quantum mechanical model)
Electrons are not stuck in a “planetary orbit,” or exact path, around the nucleus.
Instead, they’re spherical regions, of space around the nucleus in which electrons are most likely to be found. You can’t determine the exact location of an e-
Called “electron clouds” or “atomic orbitals” A region of space in which there is a high
probability of finding an electron
Atomic Structure
Atoms are made up of three subatomic particles:
Subatomic = smaller than
an atom
Live where?
Have mass?
What charge?
Protons
Neutrons
Electrons
In nucleus
In nucleus
Outside nucleus
Yes, substantial
Yes, substantial
No, negligible
+
-
none
Elements
Substances made of only one type of atom
Identified by atomic number (protons!!!)
Can not be broken down into simpler substances
Element Symbols
Shorthand name of the element Most are based on the Latin name
Ex: Gold = Au The symbol is either:
1. One capital letter-ex: Carbon = C
2. Two letters…one capitol, one lower case-ex: Krypton = Kr
Electron Clouds
The different layers of clouds hold different numbers of electrons 1st cloud = 2 2nd cloud = 8 3rd cloud = 8
And then it gets complicated...we’ll save that discussion for future chemistry courses
These cloud layers conveniently match the rows
on the periodic table:1st row = 2 elements2nd row = 8 elements
3rd row =8 elements…And then it gets complicated
Electron Clouds (cont.)
To draw the electron clouds: Figure out how many total electrons Fill in the electrons starting with the
first cloud Sulfur has:
16 electrons
Electron Clouds (cont.)
Practice Problem #3 Draw the electron clouds for an Al
atomThis is WAY
too much work…there must be a
simpler way!
Lewis Dot Structures
A smart man named Gilbert Newton Lewis figured out an easier way! For Lewis Dot Structures draw only the
important electrons… The outer, or valence, electrons
Just draw the outer electronsaround the atomic symbol
S
Lewis Dot Structures (cont.)
Practice Problem #4 Draw Lewis Dot Structures for:
A Calcium atom
A Chlorine atom
An Oxygen atom
Ca
O
Cl
Lewis Dot Structures (cont.)
All elements want to be full of electrons: So they gain or lose electrons until they
are full This gives the atom a charge
Negative charge if they gain electrons Positive charge if they lose electrons
Charged atoms are called ions: cations if they are positive and anions if
they are negative
Called the “Octet Rule”
-gaining/losing enough e- to have a full
valence
Atomic Number
Top Number indicates Atomic Number
Atomic Number = Number of Protons Hydrogen:
Atomic Number 1 = 1 proton Magnesium:
Atomic Number 12 = 12 protons Lead:
Atomic Number 82 = 82 Protons IF YOU CHANGE THE
NUMBER OF PROTONS OF AN ATOM, YOU CHANGE ITS IDENTITY!!!!!!!!
1
H12
Mg82
Pb
Atomic Mass Bottom Number indicates Atomic
Mass Atomic Mass = Total Mass (Number of
Protons + Number of Neutrons) Hydrogen:
1 (Protons + Neutrons) Magnesium:
24 (Protons + Neutrons) Lead:
207 (Protons + Neutrons) If we take the Atomic Mass and
subtract the Atomic Number, we can figure out the number of neutrons.
Pb – (Atomic Mass) 207 – Atomic Number) 82 = 125
H1
Mg24
Pb207
Atomic Structure (cont.)
Atomic mass Average mass of an element,
based on amount of each isotope found in nature
Not a whole number because it is an average
Atomic number Number of protons in an
element Also, number of electrons
when it is neutral (has no charge)
Mass number Mass of a particular isotope
7
N14.011
Determining Composition of atoms
Number of neutrons = mass number – atomic number
# of protons = atomic number If neutral atom… # protons = # electrons
Charge is positive? That # fewer electrons than protons
Charge is negative? That # more electrons than protons
Atomic Structure (cont.)
For a lithium atom: What is the
atomic number? How many
protons? How many electrons since it is neutral?
What is the atomic weight?
How many neutrons?
3
3
3
6.939
Usually 4
Who was the scientist that came up with the planetary model of the
atom?
Atomic Structure (cont.)
Practice Problem #1 For a sodium atom:
What is the atomic number?
How many protons? How many electrons
since it is neutral? What is the atomic
weight? How many neutrons?
11
11
11
22.99Usually 12
Atomic Structure (cont.)
Practice Problem #2 For a boron atom:
What is the atomic number?
How many protons? How many electrons
since it is neutral? What is the atomic
weight? How many neutrons?
55
5
10.811
Often 6
Is a tool to organize the elements
By 1860, scientists had discovered 60 elements
They noticed some elements had similar properties.
They also noticed differences between the elements.
1829 Classified elements
into groups of 3 Called them triads.
The elements in a triad had similar chemical properties
Physical properties varied in an orderly way according to their atomic mass
1869 Developed a
Periodic Table based on atomic mass
He left blank spaces
Dmitri Mendeleev (Wikipedia.org)
Realized chemical + physical properties of elements repeated in an orderly way.
Periodicity- the tendency to recur at regular intervals
Luster Conductive Malleable (can be
bent and formed into shapes)
Ductile (can be pulled into wires)
Dull Nonconductive Brittle
(Shaded regions)
Share properties of metals and non-metals
Some are semiconductors
I study metalloids
Columns and Rows
Called “groups” or “families” Called “periods”
An atom can have up to 7 energy levels of electrons.
An element’s period (row) tells us the number of …? For example, a sodium (Na) atom has ____ electron
orbitals? Fluorine (F) has ____ electron orbitals?
An element’s family (aka group) tells us ...? The outer 2 shells of the Group B elements are
considered valence electron orbits. We will be able to ignore Group B for now.
For example Sodium (Na) has ____ valence electrons Fluorine (F) has ____ valence electrons
Group 1A Only one
valence electron
VERY reactive!!!!
Hydrogen is NOT included
Group 2A Two valence electrons Not as reactive as the alkali metals Named because of where they are
found on Earth
Found in the middle of the table
In the “B Groups” Can change their
number of valence electrons
Bottom Rows are also known as the Rare Earth Metals!!!
Group 7A Seven valence
electrons VERY reactive!!!!
Group 8A 8 valence
electrons—outer energy level is full
Very UNREACTIVE—what do they need to be
For carbon (C):(a) How many electron shells does it have?(b) How many valence electrons does it have?(c) Is it a metal, nonmetal, or metalloid?
Answer for carbon (C):(a) 2 electron shells(b) 4 valence electrons(c) nonmetal
For potassium (K):(a) How many electron shells does it have?(b) How many valence electrons does it have?(c) Is it a metal, nonmetal, or metalloid?
Answer for potassium (K):(a) 4 electron shells(b) 1 valence electron(c) metal
For copper (Cu):(a) How many electron shells does it have?(b) Is it a metal, nonmetal, or metalloid?(c) How many protons does it have?
Answer for copper (Cu):(a) 4 electron shells(b) metal(c) 29 protons
For uranium (U):(a) How many electron shells does it have?(b) Is it a metal, nonmetal, or metalloid?(c) How many protons does it have?
Answer for uranium (U):(a) 7 electron shells(b) metal(c) 92 protons
Radioactivity is the process of nuclear decay, in which an unstable nucleus gives off matter and energy.
Nuclei with too many or too few neutrons compared to the number of protons are radioactive.
The three types of nuclear radiation are alpha, beta, and gamma radiation
When alpha radiation occurs, an alpha particlemade of two protons and two neutrons is emitted from the decaying nucleus
• A second type of radioactive decay is called beta decay.
Sometimes in an unstable nucleus a neutron decays into a proton and emits an electron.
Because the atom now has one more proton, it becomes the element with an atomic number one greater than that of the original element.
However, because the total number of protons and neutrons does not change during beta decay, the mass number of the new element is the same as that of the original element.
They have no mass and no charge and travel at the speed of light.
Isotopes Same element, different number of
neutrons Ex. Uranium-235 & Uranium-237 Also written as
UU 23792
23592 &
Some isotopes are radioactive, while
others are not.
_________ and _____________ are always conserved due to the Law of Conservation of Mass.
Mass # atomic #
RaTh 228232
RnHe 22086
42
147
24195
He4290 88
Ra22488
Am N Ra22788
Alpha decay of U-238
Beta decay of Th-235
Gamma decay of Th-235
ThHeU 23490
42
23892
PaeTh 23591
01
23590
00
23590
23590 ThThm
Alpha decay emits a Helium atom
Beta decay decomposes a
neutron to a proton and emits an electron
Gamma decay causes rearrangement of the
nucleus and emits gamma radiation
Alpha, beta, and gamma rays: particles or waves emitted during
radioactive decay
Form of radiation
Symbol Charge Penetration
Alpha
Beta
Gamma
+
-No charge
Least
Middle
Most
α
β
γ
Most damaging radiation is… Alpha, if it gets in the body
If alpha radiation gets in the
body it does NOT
leave!
People are exposed to radiation Naturally
Cosmic rays, radioisotopes from rocks, soil, groundwater
People are exposed to radiation Human sources
Nuclear weapons testing, air travel, nuclear power, X-rays, medical treatments with radiation
Some smoke detectors give off alpha particles that ionize the surrounding air.
Radon is…
A noble gas that decays into radioactive materials
Radon in homes is a concern because… It seeps in through cracks in foundations It stays trapped in air-tight homes It decays into heavy metals that emit alpha
particles
10-14% of U.S. deaths from lung cancer attributed
to radon
Radon levels in the U.S. Zone 1
> 4 pCi/L Zone 2
2-4 pCi/L Zone 3
2 pCi/L
12% of lung cancer deaths are caused by
Radon
2 ways of using radioisotopes in medicine To locate
problems To kill cancer
cells
Tracers are… Radioisotopes injected in
people for diagnostic purposes.
They are injected and then traced as they travel through the body.
Irradiation Kills cancer cells It is applied directly to a cancerous
location on the body
Irradiation is… Exposure to
radiation of any kind
Usually refers to food irradiation
The FDA requires that all irradiated foods be
labeled with the above symbol, the radura
Irradiate strawberries, poultry, potatoes, etc. Reduces spoiling Kills bacteria and
parasites that lead to foodborne illnesses
Half-life is… The time
it takes for half the atoms to decay
Hydrogen-3 has a half-life of 12 years…start with 1,000 atoms After 12 years
There will be 500 atoms After 24 years
There will be 250 atoms After 48 years
There will be 62.5 atoms
7,000 atoms of Radon-222, half life of 3.28 days, in 6.56 days there are how many atoms? 1,750 atoms
100,000 atoms of Uranium-238, half life of 4.5 billion years, in 13.5 billion years there are how many atoms? 12,500 atoms U238
92
Rn22286
60,000 atoms of Potassium-40, half life of 1,280,000,000 years, how long until 7,500 atoms left? 3,840,000,000 years
Radon-222, half life of 3.28 days, how long until 12.5% of initial sample? 9.84 days
K4019
Rn22286
500 atoms of Radon-222, half life of 3.28 days, it has been 19.68 days since the experiment began, how much did you begin with? 32,000 atoms
Carbon-14, half life of 5,730 years, what fraction remains after 11,460 years? ¼
Rn22286
C146
Uranium powers our nuclear power plants. After the fuel is spent, we are left with radioactive waste.
High-level radioactive waste with U-238 is a problem because…
Uranium’s half life is 4.5 billion years, it will be
around for a long time.
Nuclear fission: The process of splitting a nucleus into several smaller nuclei.
Nuclear fusion Combining of two nuclei to form a new,
heavier nucleus
Fusion is much more
powerful than Fission