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Chemistry
Wilbraham
Staley
Matta
Waterman
Chapter 4: Atomic Structure
Copyright © 2005 Pearson Education & Prentice-Hall, Inc.
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Element Rap
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ated
Discoveries of the Atom
In 1774, Antoine Lavoiser (Aun-twain la-vwa-zyey) preformed
experiments and measurements that led to the law of
conservation of matter.
Matter can be neither created nor destroyed.
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compounds and developed the law of
constant composition.
a pure chemical compound is always has the
same percentage composition of each
element by mass, regardless of how the
substance was made or where the substance
is found.
So, whenever we talk about water, we know
that there are 2 atoms of hydrogen and 1
atom of oxygen in a molecule of water.
Substance
Formula
Composition
Properties
Water
H2O
2 atoms of hydrogen, 1 atom of oxygen
Clear, colorless, homogeneous liquid that is good for human consumption
hydrogen peroxide
H2O2
2 atoms of hydrogen, 2 atoms of oxygen
Clear, colorless, homogeneous liquid that is harmful if consumed by human (poisonous)
John Dalton’s atomic theory
Which states:
1. All matter is composed of tiny, identical particles,
called atoms, that cannot be destroyed or created.
2. Each element has atoms that are identical to each
other in all of their properties, and these properties are
different from the properties of all other atoms.
3. Chemical reactions are simple rearrangements of
atoms from one combination to another in small wholenumber ratios.
Scientific theory
provides new predictions that are tested by experiments to
support or disprove the theory.
scientists can never prove a theory to be true.
experiments are used to support a theory.
..?.atomic theory led to...
John Dalton proposed the law of multiple proportions
Which states:
When two elements can be combined to make two
different compounds, and if samples of these two
compounds are taken so that the masses of one of the
elements in the two compounds are the same in both
sample, then the ratio of the masses of the other element
in these compounds will be a ratio of small whole
numbers.
In 1834, Michael Faraday showed that electrical
current could cause chemical reaction to occur,
demonstrating the electric nature of elements.
In the 1870s, Sir William Crookes developed
what is know today as cathode ray tube.
He mistakenly thought that the cathode rays
were negatively charged molecules instead of
electrons.
How can you describe the
structure of the nuclear
atom?
J.J. Thomson determined that
cathode rays were a
fundamental part of matter he
called electrons.
He determined their charge to
mass ratio (e/m= -1.76 x 108
coulomb’s gram-1)
Led to the “plum pudding”
model of the atom, which had
electrons bathed in a sea of
position charges similar to
raisins in English pudding.
Cathode-ray tube
Cathode-ray tube
A cathode ray is deflected by
electrically charged metal
plates, as shown beside. A
positively charged plate attracts
the cathode ray, while a
negatively charged plate repels
it. Thomson knew that opposite
charges attract and like repel,
so he determined the cathode
ray to be stream of negatively
charged particles moving at
high speed. Thomson called
these particles corpuscles;
later named electrons.
Checkpoint
How do negatively charged plates affect the path of cathode
rays?
Checkpoint
How do negatively charged plates affect the path of cathode
rays?
A negatively charged plate repels a cathode ray.
Subatomic Particles
What are the three kinds of subatomic particles?
Subatomic Particles
The three kinds of subatomic particles are electrons, protons,
and neutrons.
Electrons are negatively charged particles shown by
Thomson’s experiments
Ernest Rutherford was interested in
radioactive materials and identified
alpha and beta particles in his
research.
A alpha particle contains two protons
and two neutrons, the nucleus of a
helium atom.
A beta particle is a naked negativelycharged electron
Through the gold foil experiment
Rutherford deduced the nuclear
model of the atom.
Rutherford’s gold foil experiment
Deduces the nuclear model of the atom.
In 1911, Rutherford and co-worker at University of Manchester,
England, wanted to test the existing plum-pudding model of
atomic structure.
Their test used alpha particles, which are helium atoms that have
lost their two electrons and have a double positive charge- two
remaining protons.
A narrow beam of alpha particles were directed at a very thin
sheet of gold foil.
Rutherford’s gold foil experiment
Rutherford’s results:
most alpha particles went
straight through the gold
foil, or were slightly
deflected.
However, what was
surprising is that a small
fraction of the alpha
particles bounced off the
gold foil at very large
angles.
Rutherford’s gold foil experiment
Rutherford concluded that most of
the alpha particles pass through
the gold foil because the atom is
mostly empty space. The mass
and positive charge are
concentrated in a small region of
the atom. Rutherford called this
region the nucleus. Particles that
approach the nucleus closely are
greatly deflected.
In 1919, Rutherford discovered
that the basic unit of positive
charge in the atom and named
it proton.
The proton has a positive
charge which is exactly equal in
magnitude to the electron
charge.
mass of 1.67 x 10-24 which is
1836 times heavier than the
electron.
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In 1932, James Chadwick
discovered a very penetrating
form of radiation.
The third major particle that
makes up the atom, and it was
named the neutron since it is
neutral.
The neutron has a mass almost
equal to that of the proton.
What are their relative
charges? and symbol?
Electrons?
Protons?
Neutrons?
Name
Symbol
Relative Charge
Electrons
-1
e or e-
Protons
+1
p
Neutrons
0
n
Structure of the Atom
Principal energy levels (shells)
Sub-levels (sub-shells)
Orbitals
Electronic Structure of the Atom
Electronic Configuration
abbreviated electronic configurations
Valance electrons
How to count valance electrons
Distinguishing Among Atoms
What makes one element different from another?
Distinguishing Among Atoms
Elements are different because they contain different
numbers of protons.
Distinguishing Among Atoms
Atoms are composed of protons, neutrons, and electrons.
Protons and neutrons make up the nucleus.
Electrons surround the nucleus.
Atomic number
The atomic number of an element is the number of protons
in the nucleus of an atom of that element.
ex: all hydrogen atoms have one proton, the atomic number is
1.
All oxygen atoms have eight protons, the atomic number is 8.
The atomic number identifies an element.
In table 4.2:
The number is protons equals the number of electrons.
Atoms are electrically neutral. Thus, the number of electrons
(negative charged particles) must equal the number of
protons (positively charged particles).
Understanding Atomic Number
The element nitrogen (N), shown here is liquid form, has an
atomic number of 7. How many protons and electrons are in a
neutral nitrogen atom?
The atomic number gives the number of protons, which in a
neutral atom equals the number of electrons.
The atomic number of nitrogen is 7, which means that a
neutral nitrogen atom has 7 protons and 7 electrons.
Practice Problems:
Complete this table:
Element
Atomic
number
Protons
Electrons
K
19
(a)
19
(b)
(c)
(d)
5
S
16
(e)
(f)
V
(g)
23
(h)
Mass Number
The total number of protons & neutrons in an atom is called
the mass number.
The number of neutrons in an atom is the difference between
the mass number and the atomic number.
Number of neutrons = mass number - atomic number.
The chemical symbol Li
appears with two numbers
written to the left.
What does it mean?
What do the numbers stand
for?
The chemical symbol for
Lithium, Li appears with two
numbers written to the left.
The mass number is the
superscript.
The atomic number is the
subscript.
You can also refer to atoms by using the mass number and
the name of the element.
Ex: 197 79Au may be written as gold-197.
How many electrons does gold have?
Gold has 79 electrons.
How do you calculate mass number?
How do you calculate mass number?
Mass number = number of neutrons + atomic number.
Isotopes
Isotopes: are atoms that have the same number of protons
but different numbers of neutrons.
Because isotopes of an element have different numbers of
neutrons, they also have different mass numbers.
Isotopes are chemically alike because they have identical
numbers of protons and electrons, which are the subatomic
particles responsible for chemical behavior.
Chemical symbols of
Isotopes
Diamonds are a naturally occurring form of elemental carbon.
Two stable isotopes of carbon are carbon-12 and carbon-13.
Write the symbols for each isotope using superscripts and
subscripts to represent the mass number and atomic number.
Diamonds are a naturally occurring form of elemental carbon.
Two stable isotopes of carbon are carbon-12 and carbon-13.
Write the symbols for each isotope using superscripts and
subscripts to represent the mass number and atomic number.
Isotopes are atoms that have the same number of protons but
different numbers of neutrons. The composition of a atom can
be expressed by writing the chemical symbol, with the atomic
number as a subscript and the mass number as a superscript.
Based on Table 4.2, the symbol for carbon is C and the
atomic number is 6. The mass number for each isotope is
given by its name. For carbon-12, the symbol is 126C. For
carbon-13, the symbol is 136C.
Atomic Mass
Mass of a proton or neutron is very small (1.67 x 10-24 g).
Mass of an electron is 9.11 x 10-28 g).
The mass of even the largest atom is incredibly small.
An atomic mass (amu) is defined as one twelfth of the mass
of a carbon-12 atom.
helium-4 atom, with a mass of 4.0026 amu, has about 1/3 the
mass of a carbon-12 atom.
Carbon-12 has six protons and six neutrons in its nucleus,
and its mass is set as 12 amu.
What is the atomic mass of an element?
Atomic mass is a weighted average mass of the atoms in a
naturally occurring sample of element.
The atomic mass of copper is 63.546 amu. Which of copper’s
two isotopes is more abundant: copper-63 or copper-65?
Analyze
The atomic mass of an element is the weighted average mass
of the atoms in a naturally occurring sample of the element. A
weighted average mass reflects both the mass and the
relative abundance of the isotope as they occur in nature.
Solve
The atomic mass of 63.546 amu is closer to 63 than to 65.
Because the atomic mass is a weighted average of the
isotopes, copper-63 must be more abundant than copper-65.
Practice Problems:
Boron has two isotopes: boron-10 and boron-11. Which is
more abundant, given that the atomic mass of boron is 10.81?
Practice Problems:
There are three isotopes of silicone; they have mass numbers
of 28, 29, and 30. The atomic mass of silicon is 28.086 amu.
Comment on the relative abundance of these three isotopes.
To calculate the atomic mass of an element, multiply the mass
of each isotope by the natural abundance, expressed as a
decimal, and then add the products.
Atomic mass of carbon = (12.00 amu x 0.9889) + (13.003
amu x 0.0111) = 12.011 amu
Facts and Figures
Carbon-14 Dating
All living organisms contain carbon-12 and carbon-14 in a
fixed ratio. After an organism dies, this ratio changes as the
carbon-14 decays. Paleontologists and archaeologists use
this fact to establish the age of fossils and ancient artifacts.
Review
The periodic table
How many elements are in period 2?
In group 2A?
Review
The periodic table
How many elements are in period 2?
There are eight elements in period 2.
In group 2A?
There are 6 elements in group 2A.
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