Chapter 4 Atomic Structure
Chapter 4 Atomic Structure
Chapter 4 Atomic Structure
Daltons atomic
theory.
The Greek philosopher Democritus (460 B.C. 370 B.C.) was among the first to suggest the existence of atoms (from the Greek word atomos)
He
believed that atoms were indivisible and indestructible His ideas did agree with later scientific theory, but did not explain chemical behavior, and was not based on the scientific method but just philosophy
3) Atoms of different elements combine in simple whole-number ratios to form chemical compounds 4) In chemical reactions, atoms are combined, separated, or rearranged but never changed into atoms of another element.
change to Daltons atomic theory is that atoms are divisible into subatomic particles:
Electrons,
protons, and neutrons are examples of these fundamental particles There are many other types of particles, but we will study these three
Television
Computer Monitor
Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.
1916 Robert Millikan determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge
Subatomic Particles
Particle Electron (e-) Proton (p+) Neutron (no) Charge Mass (g) Location Electron cloud Nucleus Nucleus
-1
9.11 x 10-28
+1 0
J. J. Thomson
Thomson believed that the electrons were like plums embedded in a positively charged pudding, thus it was called the plum pudding model.
Alpha particles are helium nuclei The alpha particles were fired at a thin sheet of gold foil Particles that hit on the detecting screen (film) are recorded
Rutherfords problem:
In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target? Target #1 Target #2
The Answers:
Target #1
Target #2
Rutherfords Findings
Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected
Conclusions:
a) The nucleus is small b) The nucleus is dense c) The nucleus is positively
charged
Based on his experimental evidence: The atom is mostly empty space All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a nucleus The nucleus is composed of protons and neutrons (they make the nucleus!) The electrons distributed around the nucleus, and occupy most of the volume His model was called a nuclear model
Atomic Number
Elements are different because they contain different numbers of PROTONS The atomic number of an element is the number of protons in the nucleus
Atomic Number
Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.
Element Carbon Phosphorus Gold # of protons 6 15 79 Atomic # (Z) 6 15 79
Mass Number
Mass number is the number of protons and neutrons in the nucleus of an isotope: Mass # = p+ + n0 Nuclide
Oxygen - 18 Arsenic 75 - 31
p+
8 33 15
n0
10 42 16
e- Mass #
8 33
15 18 75 31
Phosphorus
Complete Symbols
Contain the symbol of the element, the mass number and the atomic number. Mass Superscript number
Subscript
Atomic number
Symbols
Find
each of these: a) number of protons b) number of neutrons c) number of electrons d) Atomic number e) Mass Number
80 35
Br
Symbols
If
an element has an atomic number of 34 and a mass number of 78, what is the: a) number of protons b) number of neutrons c) number of electrons d) complete symbol
Symbols If an element has 91 protons and 140 neutrons what is the a) Atomic number b) Mass number c) number of electrons d) complete symbol
Symbols If an element has 78 electrons and 117 neutrons what is the a) Atomic number b) Mass number c) number of protons d) complete symbol
Isotopes
Dalton
was wrong about all elements of the same type being identical Atoms of the same element can have different numbers of neutrons. Thus, different mass numbers. These are called isotopes.
Isotopes
Frederick
Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials.
Naming Isotopes
We
can also put the mass number after the name of the element:
carbon-12 carbon-14 uranium-235
Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.
Isotope Protons Electrons Neutrons Nucleus
1 1 1
1 1 1
0 1 2
Isotopes
Elements occur in nature as mixtures of isotopes. Isotopes are atoms of the same element that differ in the number of neutrons.
Atomic Mass
How heavy is an atom of oxygen? It depends, because there are different kinds of oxygen atoms. We are more concerned with the average atomic mass. This is based on the abundance (percentage) of each variety of that element in nature.
We dont use grams for this mass because the numbers would be too small.
of grams, the unit we use is the Atomic Mass Unit (amu) It is defined as one-twelfth the mass of a carbon-12 atom.
Each
isotope has its own atomic mass, thus we determine the average from percent abundance.
the atomic mass of each isotope by its abundance (expressed as a decimal), then add the results.
not told otherwise, the mass of the isotope is expressed in atomic mass units (amu)
If
Atomic mass is the average of all the naturally occurring isotopes of that element. Symbol Composition of % in nature Isotope
Carbon-12 Carbon-13 Carbon-14
12
Atomic Masses
C C C
13
14
Carbon = 12.011
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Question
Solution Answer
arrangement of elements in which the elements are separated into groups based on a set of repeating properties The periodic table allows you to easily compare the properties of one element to another
them) is called a period Each vertical column is called a group, or family Elements in a group have similar chemical and physical properties Identified with a number and either an A or B More presented in Chapter 6