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Unit 6: Properties of Matter
Chapter 18: Atoms and Elements
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Section 18.1: Atomic Structure
Inside an atom
o Atoms contain subatomic particles; protons, neutrons and electrons
o Protons and neutrons cluster together in the nucleus, while electrons are found in the area
outside the nucleus called the electron cloud
o Protons are positive, neutrons are neutral, electrons are negative
o Protons and neutrons are about equal in mass and are about 2000X more massive than the
electron
proton—positively charged particle that exists in the nucleus of atoms
neutron—particle with no charge that exists in the nucleus of most atoms
electron—negatively charged particle that exists in the space surrounding an atom’s nucleus
subatomic particle—tiny bits of matter that are the building blocks of atoms
nucleus—center of the atom, contains most of the atom’s mass
How big are atoms?
o An electron is 10 million times smaller than an atom
o Most of the atom is empty space
o If the atom was the size of our classroom, the nucleus would be the size of a grain of sand
Particle
atom
nucleus
proton
neutron
electron
─10
─14
─15
─15
Diameter in meters
1 × 10
1 × 10
1 × 10
1 × 10
1 × 10─18
John Dalton and the atomic theory
Unit 6: Properties of Matter
Chapter 18: Atoms and Elements
Name__________________________________
atomic theory—states that all matter is composed of tiny particles called atoms
o John Dalton was one of the first scientists to gather evidence for the atomic theory
o Dalton’s Atomic Theory:
• Each element is composed of atoms
• All atoms of a given element are identical
• Atoms of different elements have different properties
• Atoms are not changed by chemical reactions
• Compounds are formed when atoms of more than one
element combine
• A compound is defined by the number and kind of atoms
it has
o Dalton pictured the atom as a small hard sphere as depicted at
the right
The changing model of the atom
This picture at the left shows a depiction of J.J. Thomson’s model
of the atom. The Thomson model suggested that negative
electrons were imbedded in a positive sphere (Plum Pudding
Model, aka, compliments of Baxter, the chocolate chip cookie
model).
The Bohr Model of the atom showed electrons moving around the
nucleus in fixed orbits. Niels Bohr proposed this model.
Schrodinger’s model was similar to Bohr’s, but demonstrated that
electron position could be predicted, but not pinpointed exactly.
Describe the contribution each of the scientists listed made to further advance modern atomic
theory (see text, pgs. 313-314).
 Democritus

John Dalton

Plucker, Crooks, and J.J. Thomson

J.J. Thomson

Rutherford, Geiger, and Marsden
Unit 6: Properties of Matter
Chapter 18: Atoms and Elements

Rutherford

Niels Bohr

De Broglie, Schrodinger

James Chadwick
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Section 18.2: Comparing Atoms
Atomic number
o The number of protons distinguishes atoms of one element
from another
o The number of protons can only change in nuclear reactions,
not in ordinary chemical reactions
o Elements can be identified by the number of protons in
atoms of that element
atomic number—the number of protons that an atom contains
Atomic mass, mass number, and isotopes
o The total number of protons and neutrons in an atom is its mass number
o Many elements have more than one stable kind of atom (atoms with the same number of
protons but with different number of neutrons)
o These are called isotopes; hydrogen has three, for example
o The atomic mass found on the periodic table is a weighted average of all isotopes in
existence
o When the mass number is shown with the element symbol to indicate a particular isotope,
this is referred to as isotope notation
periodic table of elements—a table that visually organizes the similarities between all known
elements
mass number—the total number of protons and neutrons in the nucleus of an atom
Unit 6: Properties of Matter
Chapter 18: Atoms and Elements
Name__________________________________
isotopes—forms of the same element that have different numbers of neutrons and different
mass numbers
How many different elements are possible?
o The nucleus contains protons, protons (which are all positively charged) repel, so the nucleus
should fly apart??
o The strong nuclear force holds the nucleus together (strong nuclear force is one of the four
fundamental forces of nature)
strong nuclear force—the force that holds protons together when they are very close together
(only 1 × 10─15 meters apart)
o The strong nuclear force is stronger than the electromagnetic (electrostatic) force (the force
of repulsion between like charges) and only acts over very small distances
How are electrons arranged in atoms?
o The area outside the nucleus is referred to as the electron cloud
o Within the electron cloud, energy levels exist; a specific number of electrons fit in each
energy level
o Energy levels close to the nucleus are lower in energy and hold fewer electrons, while energy
levels further away from the nucleus are higher in energy and hold more electrons
Unit 6: Properties of Matter
Chapter 18: Atoms and Elements
Name__________________________________
Section 18.3 The Periodic Table of Elements
Groups of elements
o Elements that have similar properties, both chemical and physical, are arranged
in groups on the periodic table (see illustration to the left)
o The discovery of the atomic mass provided an overall order for the periodic table
The periodic table of elements
o The order of the modern periodic table is by increasing atomic number
o Each row indicates how many electrons are in each region of the electron cloud
o Valence electrons, electrons in the outermost energy level are indicated by group
number (1 has 1, 2 has 2, 13 has 3, etc.) (go to next page)
Unit 6: Properties of Matter
Chapter 18: Atoms and Elements
Name__________________________________
Reading the Periodic Table
o Different tables show various amounts of information for each element
o The chemical symbols is the 1, 2, or 3 letter abbreviation for the element name
o The atomic number is the number of protons in the nucleus of an atom of that element and is
often shown on the P.T.
o Mass numbers may be given for stable isotopes of the element
o Atomic mass is also often given on the
periodic table
atomic mass— the weighted average of all the
isotopes in existence for a particular element
 It is measured in amu’s, or atomic mass
units, which are made-up units of mass to
measure very small things, such as
masses of atoms
 The amu is defined as 1/12th the mass of
a carbon-12 atom