Chapter 17: Properties
of Atoms and the
Periodic Table
17.1 – Structure of the Atom
17.2 – Masses of Atoms
17.3 – The Periodic Table
Symbols
 Chemical symbols have one capital letter, or one capital
letter plus one or two small letters
 For some, the symbol is the first letter(s) of the elements
name
 Some are named after scientists or for their properties
 Some come from Latin
 Ex. Argentum – Latin for “silver” (Ag on periodic table)
What are atoms?
 Atom: the smallest particle of an element that still retains
the properties of the element (the element H only has H
atoms)
 Atoms contain subatomic particles that affect its
properties
- nucleus – the center of the atom which contains
protons, neutrons, and is surrounded by an electron cloud
- protons – particles w/ a positive electrical charge
(1+)
- neutrons – particles that do not have an electrical
charge
- electrons – particles with a negative electrical
charge (1-)
Atomic Model
 400 B.C. – idea that atoms make up all substances
 Another Greek philosopher proposed that this was
incorrect and matter was uniform throughout and not
composed of smaller particles
 Finally in the 1800s, John Dalton proved that atoms exist
 Created the atomic model
Atoms
 Atoms are neutrally charged because the number of
electrons is the same as the number of protons
 The atom of a given element may lose or gain electrons
yet it still remains the same element
 The particles in the nucleus of an atom DO NOT
change in a chemical reaction
 Chemical reactions occur in the electron cloud
Electron Cloud Model – the
electron cloud is the area around
the nucleus of an atom where its
electrons are most likely found
There is an attractive force between electrons and protons
There is a repulsive forces between electrons and electrons/
protons and protons
How Much Do These Things
Weigh?
 Most of the mass of an atom is contained within the
nucleus of an atom
 Protons and neutrons each have a mass of 1.67 x 1024
grams
 Electrons have a mass about 2000 times smaller
(1,836 exactly), therefore electrons are usually not
considered when determining the mass of an atom
***You don’t have to know the exact mass of the particles
Quarks
 Scientists think electrons are not made of smaller particles
– they themselves are one of the most basic types of
particles
 Protons and neutrons are made up of quarks




There are 6 different quarks
3 quarks held together – proton
Another arrangement of 3 quarks – neutron
Understanding the composition of protons and neutrons is an
ongoing effort
Protons Identify the Element
 Protons identify the elements – If you are given the
number of protons an element has in its nucleus, you
can determine the name and symbol of the atom
 The number of protons in an atom is equal to the
atomic number
 Example: an element with 7 protons will ALWAYS be
Nitrogen (N) and an element with 79 protons will
ALWAYS be Gold (Au)
 Please give the name and symbol for the following
elements 14, 10, 1, 92
Further Identifying Atoms
 Mass Number = Number of Protons + Number of Neutrons
(P+N)
If you know the mass # and the atomic #, you can calculate the #
of neutrons
 Isotopes – Atoms of the same element that have different
numbers of neutrons (will always have the same number of
protons)
 Ex. Carbon-12 and Carbon-14
Practice
 Complete the following table (MN = #P + #N)
Element
Symbol
#P
Atomic #
#N
Mass
Number
Silicon
Si-29
14
14
15
29
Potassium
K-40
Copper
Cu-64
Krypton
Kr-84
Remember the mass number is for a specific isotope and that
the number of neutrons = mass number – number of protons
Average Atomic Mass
 The Atomic mass of an element is the weighted average of
the masses of the naturally occurring isotopes of an
element.
 The atomic mass given on the periodic table is the average
atomic mass. Why?
 Because there is usually more than one isotope of an given
element.
Elements
Symbol of the element
He-4
Mass number
The Periodic Table
 Periodic – something that has a repeated pattern ex. Days of
the week, 1st period followed by 2nd period…
 Dmitri Mendeleev – placed the known elements of his time
in order of increasing atomic mass and found a pattern that
repeated – followed predictable changes in physical &
chemical properties
 Henry G.J. Mosely Improved the Periodic Table by placing
elements according to atomic number, how it is today
Construction of the Periodic Table
 Groups – vertical columns on the periodic table (1-18)
 Elements in each group have similar properties
Examples:
Group #1 Hydrogen, Lithium, Sodium, etc. – highly reactive
Group #11 (Copper, Silver, Gold) – shiny metal, good
conductor of electricity and heat
Group #18 “The Noble Gases” – non-reactive (except under
certain conditions) non-metals
Electron Cloud Structure
 In a neutral atom, the number of electrons = the
number of protons
 Electrons closer to the nucleus have lower amounts of
energy, electrons further from the nucleus have more
energy
 Elements in the same group have the same number of
outer (valence electrons) which gives them similar
properties (similar chemical reactivity)
Electron Cloud Structure
 There is a specific order in
which the energy levels fill
 Energy Level 1 fills first with
two electrons then it is full
 Energy Level 2 fills with eight
electrons then it is full
 Energy Level 3 fills with eight
electrons then it is full
 An atom is not stable unless it
has a complete outer energy
level (8 electrons)
Rows on the Periodic Table
 Periods on the periodic table of the elements run
horizontally across the table
 Period numbers represent the number of energy levels an
atom has
Regions (Families) on the Periodic
Table of The Elements
 Alkali Metals (Group 1)
 Alkaline Earth Metals (Group 2)
 Transition Metals (Groups 3-12)
 Metalloids (Stair step Line)
 Non-Metals (To the Right of the stairs)
 Noble gases (Group 18)
 Lanthanide Series (Elements 58-71)
 Actinide Series (Elements 90-104 + Radioactive)
Valence Electrons
 The number of electrons in the outer-most energy level are
called valence electrons
Group
Name
# valence electrons
1
Alkali Metals
1
2
Alkaline Earth Metals
2
13
3
14
4
15
5
16
Oxygen Group
6
17
Halogens
7
18
Noble Gases
8 (except He)
Trend in The Periodic Table
 Trends in the number of valence electrons
- From L to R
- From Top to Bottom
 Trends in the number of energy levels
- From left to right across any period
- From top to bottom within any group
Atoms
 Stable atoms contain 8 valence electrons – If an atom does not
have 8, it tends to gain, lose, or share valence electrons to become
stable
Group 1 & 2 : lose 1 and 2 electrons, respectively
Group 18 : stable, do not readily gain/lose
Group 16 & 17 : gain 2 and 1 electrons, respectively
Group 13-15: less likely to gain/lose
Most transition elements (Groups 3-12) : tend to lose
electrons