Download finished 2.3 notes - Mr Schmitt

2.3 – The Periodic
Table and Atomic
Theory
Key Ideas - Outcomes
• You will need to know how to draw a Bohr diagram for the
first 20 elements on the periodic table
• Electrons are arranged in a specific pattern
• The electrons in the outermost shells are called valence
electrons
• Determine the difference between an atom and an ion
• Use atomic structure to explain why elements behave
differently
Periodic Table
• The periodic table is full of patterns
• Elements that share properties line up in columns because
they share the same arrangement of electrons
• One way to show the arrangement is with a Bohr diagram
Bohr Diagrams
• A Bohr model shows how many electrons are in each shell
surrounding the nucleus
• The regions surrounding the nucleus of an atom are often
called electron shells
Bohr Diagrams
• Niels Bohr described how the electrons are arranged in the
shells around a nucleus in an atom
• His theory has 3 ideas:
1.
2.
3.
Electrons move around the nucleus in shells
Each shell is a certain distance away from the nucleus and can
hold a definite number of electrons
After the shell closest to the nucleus is full, electrons start
filling the next shell
How to Draw an Atom:
• Draw a circle and put the symbol and number of protons (and
neutrons if possible) inside of it
• Add the appropriate number of shells (this is determined by the
period number)
• EX: Magnesium is in the 3rd period so it will have 3 shells
Mg
12p+
12no
There are now 3
shells in total
Filling up Electron Shells
1.
2.
3.
4.
5.
Maximum of 2 electrons in the first shell (1st shell)
Maximum of 8 electrons in the next two shells (2nd and 3rd
shells)
Maximum of 18 electrons in the 4th shell and all shells after
that (we won’t be making these!!)
Electrons occur in pairs
There must be 4 single electrons before the electrons will
occur in pairs
• Remember the pattern: 2, 8, 8, 18
• This pattern applies to all atoms although not all atoms have that
many electrons
Magnesium example
• EX: Magnesium has 12 electrons (from atomic number)
• So, 2 electrons in the first shell, 8 in the second and 2 in the
last
• Watch how the electrons are added (up, down, side to side)
Electron Shells and Reactivity:
• The way an element chemically
reacts depends on the number
of electrons in its outer shell.
• Atoms are stable when their
outer shells are full of electrons.
• If a shell is not full, the atom is
reactive, and it either wants to
fill its outer orbital or get rid of
it altogether.
Shell
(orbital)
Maximum
# of e-
1
2
2
8
3
8
4
18
Just use this table for our
purposes in grade 9
Examples when outer shell is not full
Look at the outer shell of each:
• Outer shells not full.
• Reactive!
Lithium
Lithium will donate 1 e- and
Fluorine borrow 1 eFluorine
Bohr Models of the First 20 Elements
Valence Electrons
• This is a fancy way of saying “outer shell electrons”
• If you are ever asked for the number of valance electrons,
they are just asking how many electrons are found in the last,
outermost, shell only
• EX: Magnesium atom has
2
valence electrons
Valence Electrons
• Most elements in the same family have the same
number of valence electrons
Examples:
Alkali Metals = 1 valence electron
Alkaline Earth Metals = 2 valence electrons
Halogens = 7 valence electrons
Noble Gases - are stable and non-reactive
because they are full in the outer most valence
shell.
Atoms VS Ions
• Atoms are elements as you see them on the Periodic Table:
with equal amounts of p+ and e• EX: sodium atom has 11 p+ and 11 e- so it is neutral
• Ions are elements that have gained/lost electrons and now
have a charge
• EX: sodium ion has 1+ charge because it has lost 1 e-
Practice!
State if the following are Atoms or Ions
Na
K
H+
Cl-
Ca
O2-
Ne
Be2+
Al
N3-
Na+
K+
Be
Cl
Ca2+
Hint: Anything with a charge is an ion, all the others are atoms
Ions and Electrons
 To determine the number of electrons in an ion you need to
subtract the ion charge from the atomic number
◦ EX: Calcium ion has an atomic number of 20 and a +2 ion
charge.......so, its ion has 20 - 2 = 18 electrons
◦ EX: Fluorine ion has an atomic number of 9 and a -1 ion
charge.......so, its ion has 9 – (-1) = 10 electrons (remember
subtracting a negative is like adding!)
Element Behaviour
• GOLDEN RULE: all atoms want a full outer shell (it makes them
more stable) – they will gain/lose electrons to achieve this!
• Thus, elements that have the same number of valence
electrons will behave the same way as they need to gain/lose
the same number of electrons!
THINK ABOUT IT!
• Why do sodium and cesium behave the same way?
• Why do oxygen and sulphur behave the same way?
• Why are the noble gases un-reactive (inert)?
• Why is this statement true?
“All atoms are trying to become like their nearest Noble Gas”
Bohr Models for Ions
• Draw your normal atom Bohr model (neutral – no charge)
• Place square brackets around the entire Bohr model
• Decide the easiest way for the outer shell to become full by either
adding electrons or subtracting electrons
Your ion charge should match the
periodic table if you did it correctly!!
• If you add electrons, write a negative charge outside the brackets
equal to the number of electrons you added
-1
Fluorine Ion
Bohr Models for Ions
Add electrons to non-metals to make
full outer shells!!! (forms negative ions)
-1
Fluorine Ion
Bohr Models for Ions
• Draw your normal atom Bohr model (neutral – no charge)
• Place square brackets around the entire Bohr model
• Decide the easiest way for the outer shell to become full by either
adding electrons or subtracting electrons
Your ion charge should match the
periodic table if you did it correctly!!
• If you subtract electrons, write a positive charge outside the brackets
equal to the number of electrons you lost (erase the outer shell that
just lost electrons too!)
+1
Sodium Ion
Bohr Models for Ions
Subtract electrons from metals to make
full outer shells!!! (forms positive ions)
+1
Sodium Ion
Summary
• All atoms want full outer electron shells
• All metals lose electrons
• All non-metals gain electrons