Foundations of College Chemistry, 14th Ed.
Morris Hein and Susan Arena
Lightning occurs when electrons move to neutralize charge difference between the storm clouds and Earth.
5 Early Atomic Theory and Structure
What are the building blocks of matter?
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Early Greeks
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Air, Earth, Water, Fire
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Democritus (470-370 BC)
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Atoms
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John Dalton (1766-1844)
Dalton’s Atomic Theory
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Elements (matter) is composed of small,
indivisible particles called atoms.
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Atoms of a given element are identical in mass
and behavior.
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Atoms of different elements differ in mass and
behavior.
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Chemical combination of elements to make
different substances occurs when atoms join
together in small whole number ratios.
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Chemical reactions only rearrange the way the
Law of Constant Composition
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The composition of a substance is always the
same, regardless of how the substance was
made or where the substance is found.
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Atoms of two or more elements may combine
in different ratios to form different
compounds.
Law of Multiple Proportions
1. Elements can be decomposed under certain conditions.
2. Not all atoms of the same element have identical mass. These are called isotopes.
Revisions to Dalton’s Theory
3. Atoms are not indivisible.
Atoms are composed of subatomic particles.
Properties of Electric Charge
1. Charge may be either positive or negative.
2. Opposite charges (positive and negative) attract
while like charges (i.e. negative and negative) repel.
3. Charge may be transferred from one object to another, by contact or induction.
4. The force of attraction between charges is related to the distance between charges
Electric Charge
Ions are atoms (or groups of atoms) that carry a positive or negative charge.
Ex. NaCl in water dissociates into two ions, Na+ and Cl–.
The Na+ (cation) produced is attracted to the negatively
charged electrode (cathode).
The Cl– (anion) produced is attracted to the positively
charged electrode (anode).
Electrons are gained from atoms to give anions. Electrons are lost from atoms to give cations.
Atoms can become ions by gaining or losing electrons from this sphere.
The Effect of Subatomic Particles
In 1911, Ernest Rutherford established the nuclear model of the atom by bombarding gold atoms with α particles.
This suggested the gold atoms must have a densely, positively charged nucleus to affect the path of an
α particle (a positively charged He atom).
Most of the particles passed through the gold foil, but some were deflected and some even bounced back!
Because most of the particles were not deflected, this suggested most of the atom is empty space.
Protons and neutrons are located in the nucleus. Electrons are dispersed throughout the remainder
of the atom (mainly open space).
Neutral atoms contain the same number of protons and neutrons to maintain charge balance.
Nuclear Model of the Atom
Atoms are composed of
Protons
+ charge 1.67x10
-24g nucleus
Neutrons no charge 1.67x10
-24g nucleus
Atomic Number: Number of protons in the nucleus of an atom.
Atomic numbers for every element are above the element’s symbol in the periodic table.
The atomic number determines the identity of the atom.
27
Co
Atomic Number
Atomic Number
Isotopes
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Atoms which differ only in the number of
neutrons present in the nucleus.
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Neutrons help keep the protons together by
adding to the strong nuclear force without
adding to the mutually repulsive electrical force
of the protons.
–
Generally 1-1.5 neutrons per proton in an atom’s
1 proton 1 neutron
Standard Isotopic Notation
Mass number: Total number of protons and neutrons for an element.
Example: Isotopes of Hydrogen
1 proton 0 neutrons
1 proton 2 neutrons
Protium Deuterium Tritium
A
E Element Symbol Z
Atomic Number Mass Number
Isotopes of the Elements
Practice:
How many protons, neutrons, and electrons are found in each of the following isotopes?
64 Cu 29
Example
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How many protons, neutrons, and
electrons are found in each of the
following isotopes?
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Write the isotope symbol that
corresponds to an element with 24
protons and 28 neutrons?
Atomic Mass
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The average relative mass of the isotopes of
an element compared to the atomic mass of
carbon-12 (exactly 12 amu)
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Atomic mass unit (amu)
Average atomic mass of Cu:
Measuring Cu isotope abundances by using mass spectrometry.
(62.9298) x (0.6909) + (64.9278) x (0.3091) = 63.55 amu
Atomic Mass % Abundance Atomic Mass % Abundance
Atomic Mass and Isotope Distribution
Silver exists as two isotopes with atomic masses of 106.9041 and 108.9047 amu.
Determine the average atomic mass for silver if the % abundance for each isotope is 51.82 and
48.18%, respectively.
Average atomic mass of Ag:
(106.9041) x (.5182) + (108.9047) x (0.4818) = 107.8680 amu
Atomic Mass % Abundance Atomic Mass % Abundance
