Transcript example - SCIS Teachers

• The elements of aluminum, Iron, Oxygen, and Silicon make
up about 88 percent of the earth's solid surface. The air is 99
percent nitrogen and oxygen. Hydrogen, oxygen, and
carbon make up 97 percent of a person. Thus almost
everything you see in this picture is made up of just six
elements.
Rev 121914
Ave. Atomic Mass
Symbol
Atomic Number
EXAMPLE
How many protons, neutrons and electrons are found in an
atom of
133
55
Cs
Atomic number = protons
There are 55 protons (and 55 electrons!)
Mass number = sum of protons and neutrons
133 – 55 = 78 neutrons
(note: if looking at only 1 atom, there is no
average, so Ave Atomic Mass = Mass #)
Hydrogen
1
1
H
Protons: 1
Neutrons: 0
Electrons: 1
Sodium
23
11
Protons: 11
Neutrons: 12
Electrons: 11
Na
Rhenium
186
75
Re
Protons: 75
Neutrons: 111
Electrons: 75
Rhenium isotope
187
75
Re
Protons: 75
Neutrons: 112
Electrons: 75
• Periodic Patterns
– The chemical behavior of elements is determined by its
valence electrons
– Energy levels are quantized. so roughly correspond to
layers of electrons around the nucleus.
– Each period begins with a new outer electron shell
(energy level)
– Each period ends with a completely filled outer shell that
has the maximum number of electrons for that shell.
– The outer shell electrons are responsible for chemical
reactions, and are called valence electrons.
– So now…explain WHY atoms get smaller as you move
to the right on the periodic table, and what they do as you
move up the periodic table, and why (so need to explain
2 mechanisms since they are not the same).
There are 2 stages to an atom’s life:
1. Atoms are “born” on the periodic table with the same
number of protons and electrons, so they are neutral.
2. Then they grow up and become ions, so they gain or lose
electrons and have a charge.
Ions
– When an atom gains or loses an electron it becomes an
ion.
• A cation has a positive charge (the “t” is like a “+”)
• An anion has has a negative charge
– (Groups 1, 2, or 3). Elements with 1, 2, or 3 electrons in
their outer shell tend to lose electrons to get a full outer
shell, and become cations.
• These are the metals which always tend to lose electrons.
– Elements with 5 to 7 electrons in their outer shell tend to
gain electrons to fill their outer shell and become anions.
• These are the nonmetals which always tend to gain electrons.
EXAMPLE
Q: What would the charge be on a sodium ion?
A: Since sodium is in the first Group, it has 1 valence e-,
and would want to get rid of it to get a complete valence shell
Group 1A elements lose 1 electron
So the charge would be +1
*Remember an electron is negatively charged. When you
lose them the atom becomes positively charged…
when you gain them it becomes negatively charged
Q1: How many TOTAL electrons is
beryllium, Be “born” with (when neutral)?
Q2: How many valence electrons is
beryllium, Be “born” with (when neutral)?
Be
A1: It is #4 on periodic table, so it has 4 protons and
4 electrons.
A2: It is in the 2nd Group, and the pattern of the periodic
table is based on valence e-’s, so it has 2 valence electrons
EXAMPLE
Q: Will Be want to gain or lose e-’s to become happy?
Q: How many valence electrons will Be want to gain or lose?
Q: What charge will it have when it becomes happy?
Be
A: Lose
A: It is in the 2nd Group so it has 2 valence e-’s. It needs a
full valence shell, so it will give away the 2 e-’s which will
leave it with the (full) shell underneath.
A: losing 2 e-’s makes it +2
Q1: How many TOTAL electrons is oxygen, O, “born” with?
Q2: How many valence electrons is oxygen, O, “born” with?
O
A1: 8 protons, so 8 electrons (it’s neutral).
A2: It is in the 6th Group (we don’t count the transition
metals), and the pattern of the periodic table is based on
valence e-’s, so it has 6 valence electrons.
EXAMPLE
Q: Will O want to gain or lose e-’s to become happy?
Q: How many valence electrons will O want to gain or lose?
Q: What charge will it have when it becomes happy?
O
A: Gain
A: It is in the 6th Group so it has 6 valence e-’s. It needs a
full valence shell (8), so it will gain 2 e-’s which will
give it 8 valence electrons (8 is great).
A: Adding 2 e-’s makes it -2 charge (an anion).
EXAMPLE
Q1: How many valence electrons does sodium have on the
periodic table? A: 1
Q2: How would you write the charge symbol for the
sodium CATION?
+1
Na