Introduction to chemistry

Chemistry Unit Notes
8th Grade Science


Basic Vocabulary
 Matter: Anything that has mass and volume
 Mass: Amount of matter in an object
 Weight: Measure of the force of attraction between
objects due to mass and gravity
 Volume: Amount of space an object takes up
 Density: Measurement of how much mass is
contained in a given volume


More Vocabulary
 Atoms: Smallest particle of an element that has all the
properties of matter:
– Protons- particles in the nucleus with positive charge
– Electrons- particles orbiting around nucleus with negative charge
– Neutrons- particles in the nucleus with no charge

 Elements: Simplest form of a pure substance
 Compounds: Two or more elements chemically combined
to form a new substance


Sub-Atomic Particles
Part of
Atom

Charge

Location

Mass/Size

Electron

- negative

outside
nucleus

.0006 amu
(too little to count)

Proton

+ positive

inside nucleus 1 amu

Neutron

no charge

inside nucleus 1 amu


Periodic Table

Using the Periodic Table
17



Atomic Number
– Equal to # protons = # electrons
– Periodic Table is arranged by this number

Cl
35.5



Symbol
– “Shorthand” for the element – Note 2nd letter is
always lowercase



Atomic Mass Number
– Total AVERAGE mass of Protons + Neutrons +
Electrons


Electron Energy Levels




Electrons are arranged in “Shells” around nucleus in
predictable locations
Fill “seats” closest to nucleus first (concert – best seats)
“Seats” available
–
–
–
–
–
–



Shell #1
Shell #2
Shell #3
Shell #4
Shell #5
Shell #6

2 electrons
8 electrons
8 electrons
18 electrons
32 electrons
50 electrons

Ex. Carbon has 6 total electrons so…
Two electrons on first energy level

Four electrons on second energy level
Question: Could we fit more electrons on the second energy level if there were more electrons in carbon??


Atomic Structure
Total # of protons and electrons (in a neutral atom)
17 protons in nucleus
17 electrons orbiting nucleus

17

Cl

Element Name
Chlorine

35.5

Total Mass of Nucleus
36 - 17 = 18 neutrons
(Round Atomic Mass)

Notice: electrons follow energy level rules
from previous slide.


Atomic Mass – Fractions?
Look at Chlorine (atomic number 17)
 Atomic mass of 35.5? I dont’ get it!
 Where does the 35.5 come from?



– 0.5 protons? 0.5 neutrons?  No


Atomic Mass = average number of protons
and neutrons in nature


More Practice


Determine the name, number of protons,
neutrons and electrons for each element
shown and draw…
15

8

26

P

O

Fe

31

16

56


Isotopes


An isotope is a variation of an element
(same protons) but can have diff. # of
neutrons



Ex: carbon (atomic mass = 12.011)
– Carbon (14) and carbon (12) exist in
nature


Ions
Change in electrons which gives an atom a
charge (+ or -)
 You can only add or subtract electrons!


(protons don’t change)

– Ex. Count the number of electrons below…

Carbon ion (-1 charge)
7 electrons (-)

6 protons (+)

Neutral Carbon
6 electrons (-)
6 protons (+)

Carbon ion (+1 charge)
5 electrons (-)
6 protons (+)


Valence Electrons





An electron on the outermost energy shell of an atom
Important to understand because this is a key factor
in how atoms will BOND with each other
Octet rule – stable atom will have 8 electrons in that
outer shell
Practice – Valence # of
– Chlorine?
– Neon?
– Nitrogen?
– Oxygen?


Electron Dot Diagrams







a diagram that represents the # of valence
electrons in an atom of an element.
The amount of electrons is displayed by dots
around the symbol of the element.
Ex.

/>ssons/lesson38.htm


Types of Chemical Bonds


Ionic- Two elements bond by transferring electrons to create ions

that attract together (+ is attracted to - after an electron is transferred)



Covalent- Two elements bond by sharing electrons (strongest
bond type)



Metallic- Two metals bond and form a “common electron cloud”.

This is a cluster of shared electrons (weakest bond type)


Examples of Bonding
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Predicting Bonds
Ionic Bond = metal to non-metal
 Covalent = non-metal to non-metal
 Metallic = metal to metal


Do you understand why? HINT: the numbers at the top of the table indicate the # of valence electrons for each column


Oxidation Numbers


Oxidation numbers are assigned to each element



They represent a predicted “charge” of an atom/ion
when it bonds with another element.
 (tells us if the atom would prefer give or take electrons, and how many).



They help us to predict what compounds will form
when two elements get together.



Oxidation numbers are labeled like this:
 Na 1+
 O 2-


How to Use Oxidation Numbers
Oxidation Number indicates the number of electrons lost, gained or shared when
bonding with other atoms.

Ex. Na wants to lose an electron. If an electron is lost, it
becomes a +1 charge
SO: oxidation number for Na = 1+

Ex. Cl wants to gain an electron. If an electron is gained, it
becomes a -1 charge
SO: oxidation number for Cl = 1-


Oxidation Numbers


Each column going down the periodic table
has elements with the same oxidation
number.




Label the oxidation numbers on your periodic table at the top of each
column as shown here:

1+

2+

3+ 4(+/-) 3- 2- 1- 0


Rules for using oxidation
numbers to create compounds
1. Positive ions can only bond with negative ions and vice
versa
2. The sum of the oxidation numbers of the atoms in a
compound must be zero (the key is to stay balanced)
3. If the oxidation numbers are not equal to zero, then you
must add additional elements until they balance at zero.
4. When writing a formula the symbol of the Positive (+)
element is followed by the symbol of the negative (-)
element.


Examples of Forming Compounds
Ex. Na (+1) + Cl (-1) = NaCl
Are these oxidation numbers already equal to zero?
If so, you don’t need to add any extra elements to combine them into a compound, so the answer is
simply NaCl

Ex. H (+1) + O (-2) = H2O
How many +1 would you need to balance the -2 to zero?
Since you need 2 atoms of the 1+ to balance the 2- to zero the resulting compound would be H 2O
In other words: to combine H with O, you MUST have 2 H to balance the oxidation numbers to zero
2+ and 2- = ZERO

Ex. Al (+3) + S (-2) = Al2S3
This one is tricky…we are not even close to balancing + and - to zero.
Because of this we must have more than one Al and more than one S in our final equation.
By using 2 Aluminums instead of just1 we would have 6+
By using 3 sulfers instead of just 1 we would have 6Since these are now equal to zero, we combine 2 Aluminums and 3 Sulfers to make Al2S3


Chemical vs. Physical Change
– Physical Change: A change that can occur
without changing the identity of the
substance.
– Ex. Solid, Liquid, Gas (Phase change)

– Chemical Change: Process by which a
substance becomes a new and different
substance
– Ex. Fire


Chemical Reactions


Chemical Reaction: a process in which the
physical and chemical properties of the

original substance change as new
substances with different physical and
chemical properties are formed