Chapter 2.3
Periodic Table and Atomic Theory
Do Now: Based on your knowledge
draw a detailed model of an atom
Valance Electrons
• Electrons are grouped into patterns within energy
levels
• The electrons in the outermost energy level are called
valance electrons
• All Alkali Metals have one valance electron
• Alkaline Earth Metals have two valance electrons
• All Halogen Gases have seven valance electrons
• All Noble Gases have eight valance electrons
– Except Helium that has two valance electrons
– All Noble gases have their valance energy shell filled so
they are stable, even Helium is stable with 2 valance
electons
Bohr-Rutherford Diagram
• Periodic Table
– Non-metals appear on the right
– Elements with similar properties line up in
columns
• These elements have similar electron arrangements
• Bohr-Rutherford Diagram shows how
electrons are arranged in an atoms energy
shells
Bohr-Rutherford Diagram
• Energy Shells around the nucleus of an atom
– First shell nearest the nucleus
• 0 to 2 electrons
– Second shell
• 0 to 8 electrons
– Third shell
• 0 to 8 electrons
– Fourth shell
• 0 to 18 electrons
Bohr-Rutherford Diagram
• See diagram 2.21 and 2.23 on pages 60 and 61
respectfully
• The diagram contains
– Element symbol
– Number of protons
– Number of neutrons
Valence Electrons
• Valence Electrons are in the outermost electron
shell…called the valence shell
– have the strongest influence on the properties of an
atom
• Most elements in the same family have the same
number of valence electrons
– Halogens have 7 valence electrons
– Noble Gases have 8 valence electrons, making them
the valence shell full and the element stable
• Helium is the exception, with only 2 valence electrons, but it
is still full and stable
Valence Electrons
• Period Number of the periodic table indicates
the number of electron shells that an element
has
• Elements in the same period have the valence
elections in the same valence shell
Noble Gas Stability
• Noble Gases are very stable and unreactive
– He = Helium
– Ne = Neon
– Ar = Argon
– Kr = Kryton
– Xe = Xenon
– Rn = Radon
– Uuo = Ununoctium
Noble Gas Stability
• Unreactive elements do not form new
substances with other atoms
• Valence shells are “full” of valence electrons
making them stable and unreactive
• With a full valence shell atoms are unable to
lose or gain electrons or share
• Atoms with a “filled” valence shell will not
trade or share electrons easily, making them
stable
Noble Gas Stability
• Other families of atoms try to achieve noble
gas stability
– They do this by gaining or losing electrons
• Metals will lose 1 to 3 valence electrons to
achieve the stability of the noble gas that is in
their periodic period
• Non-Metals will gain 1 to 3 valence electrons
to achieve the stability of the noble gas in
their periodic period
Reactive Elements
• Elements in the same group share physical and chemical
properties
– That’s because their electron configuration is the same,
especially in the valence shell
• Alkali Metals have 1 valence electron
• Halogens have 7 valence electrons
– They are short one valance electron for stability
– Missing one valence electron makes Halogens very reactive
• Alkaline Earth Metals have two valence electrons
– Making them slightly reactive, willing to lose electrons
• Noble Gases have 8 valence electrons
– Valence shell is full = stability
Reactive Elements
• Elements that are most reactive are missing
just one valence electron to achieve stability
– They will readily react with an element that has 1
valence electron in its valence shell
• Least reactive elements have a full valence
shell, they are stable