Atoms and Elements
a) Atomic Models
An atom is the smallest particle of matter.
The discovery of the atoms was made using various experiments:
Law of conservation of mass:
§ This law states that the total mass of the reactants in a chemical change must be equal to the total mass of the products. Nothing is created, nothing disappears.
§ Used to create Daltons model of the atom
Cathode ray tube:
§ Used to create Thomson’s model of the atom: negative particles called electrons placed in a positive dough)
§ The particles emitted by a cathode ray tube are independent of the gas inside the tube, can move a propeller and are attracted to the positive plate: therefore they are common to all elements, particles that have a mass and are negatively charged.
Gold Foil:
§ Experiment that allowed for the discovery that electrons flowed around a positive nucleus
§ Rutherford bombarded a sheet of gold with gamma radiation and measured the impact of the radiation. Some of the particles were deviated (met with electrons), some went through (space between the atoms) and some bounced right back (discovery of a positively charged nucleus)
Photographic plate:
§ Used to discover that electrons had specific orbits which correspond to different energy level.
§ Electrons bombarded with energy, become excited and jump orbits. The electrons rapidly return to their original orbits by releasing the trapped energy in the form of light.
*** Due to questions raised by the Rutherford-Bohr Model, Chadwick discovered the existence of the neutron. The neutron is a neutral particle found in the nucleus and would allow the protons to stay within close proximity without blowing apart the atom.
b) Atomic Representations
I. Lewis Notation
Definition / ExampleSimplified representation of the atom, in which only the valence electrons are illustrated
Steps / Notes
§ Write the symbol of the atom / There should be no more than 8 electrons around the symbol
§ Place the number of valence electrons around the symbol in a clockwise manner / The amount of valence electrons is equal to the roman numeral above the family
II. Rutherford-Bohr
Definition / ExampleSimplified representation of the atom, in which only the protons and electrons are illustrated
Steps / Notes
§ Draw a nucleus / The number of protons should be equal to the number of electrons (unless it is an ion)
§ Write down the amount of protons in the nucleus (equal to the atomic number) / The octet rule must be followed (no more than 8 electrons per orbit
§ Draw the number of orbits (number of the period/line) around the nucleus / Keep in mind that the first orbit only has place for two electrons (2,8,8…)
§ Place the number of electrons around the nucleus in a clockwise manner
III. Simplified
Definition / ExampleSimplified representation of the atom, in which the protons, neutrons and electrons are illustrated
Steps / Notes
§ Draw a nucleus / The number of protons should be equal to the number of electrons (unless it is an ion)
§ Write down the amount of protons in the nucleus (equal to the atomic number) / The octet rule must be followed (no more than 8 electrons per orbit
§ Write down the amount of neutrons in the nucleus (atomic mass rounded –atomic number) / Keep in mind that the first orbit only has place for two electrons (2,8,8…)
§ Draw the number of orbits (number of the period/line) around the nucleus
§ Write down the amount of electrons on each shell
IV. Ball and Stick
Definition / ExampleH2O
Simplified representation of the atom, in which the tom is represented by a ball and its bonds with other atoms are the sticks
Steps / Notes
§ Draw a ball and write the symbol of the atom / Usually used to represent molecules
§ Find its ionic value (how many electrons doe it have a tendency to give/take) and ensure that the number of bonds matches the ionic value
c) Periodic Table
The periodic table of the elements is a visual representation of the elements in groups according to their physical and chemical properties
In other words, each elements position in the periodic table tells it something about itself.
There exist many types of division in the periodic table:
I) Groups (columns)
The elements in a particular group share the same chemical properties because they all have the same number of valence electrons (number of electrons found on the last shell)
Certain groups have been given names
ALKALI METALS
Column Location / 1st column /Column Name / Alkali Metals
Examples / Li, Na, K, Rb, Cs, Fr (except Hydrogen which belongs to no groups)
Characteristics / Number of valence electrons: 1
Soft
Highly reactive. There high reactivity makes that they are never found naturally but are always in compounds (molecules).
Reacts highly to water.
ALKALINE EARTH METALS
Column Location / 2nd column /Column Name / Alkaline Earth Metals
Examples / Be, Mg, Cs, Sr, Ba, Ra
Characteristics / Number of valence electrons: 2
Highly malleable
Easily burned in the presence of heat.
Not usually found naturally, but are often found in rocks or earth
HALLOGENS
Column Location / Before Last /Column Name / Hallogens
Examples / F, Cl, Br, I, At
Characteristics / Number of valence electrons: 7
Non-metals that react easily to form compounds with alkali metals to form salt
NOBLE GASES
Column Location / Last Column /Column Name / Noble Gases
Examples / He, Ne, Ar, Kr, Xe, Rn
Characteristics / Number of valence electrons: 8
Very stable, they do not react with other atoms
.
II) Periods (rows)
All the elements in a period have the same number of electron shells.
III) Metals vs. Non-metals and metalloids
METALS / NON-METALS / METALLOIDSLeft of the periodic Table / Right of the periodic table and Hydrogen / 7 elements (B, SI, Ge, As,Sb, Te, At)
Generally good conductors of electricity and heat / Poor conductors or electricity and heat / Semi-conductors – there conductivity depends on the conditions.
Often ductile and malleable / Easily reduced to powder
Usually shiny
Molecules and ions
a) Molecule, Bonds and Ions
A molecule is a group of two or more chemically bond atoms.
Atoms bond together in order to complete their last shell of electrons. The periodic table helps determine the tendency an atom has to gain to loose electrons. (Keep in mind that they want to get to a full shell the fastest way possible).
Tendency to gain or lose electrons based on the periodic table
Group # / IA / IIA / IIIA / IVA / VA / VIA / VIIA / VIIIANumber of Valence e- / 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8
Tendency / Loose 1 e- / Loose 2 e- / Loose 3 e- / Gain or Loose 4 e- / Gain 3 e- / Gain 2 e- / Gain 1 e- / None (stable)
Rule of Octet:
General rule that states that all atoms try to obtain 8 electrons (exceptions: Li, Be and B who want only 2 electrons on their last shell.
Hydrogen is another exception, in some cases he will gain one electron, and in others he will loose his only electron.
An ion
An ion is an atom that has become electrically charged by loosing (+) or gaining (-) one or more electrons.
Solutions
A solution is a homogeneous mixture whose component substances cannot be distinguished, even with the aid of a magnifying instrument.
Solvent vs. Solute
The solute is dissolve in the solvent
An aqueous solution is a solution in which the solvent is water
I. Solution Characteristics
A solution has many different characteristics such as solubility, concentration, electrical conductivity and pH
i) Solubility
Solubility can be defined as the amount of solute that one can dissolve in a given solvent.
ii) Electrical Conductivity
The electrical conductivity of a solution is the measure of the solution’s ability to allow an electrical current to flow.
An electrolyte is a substance that, when dissolved in water, allows an electric current to flow through the solution.
Electrolytic dissociation is the physical separation of a dissolved compound into two ions of opposite charges
e.g.
NaCl Na+ + Cl-
Electrolyte Characteristics
Strong / All molecules split into ionsWeak / Only some molecules split into ions
Non-Electrolytes / No molecules split into ions
Types of electrolytes
Type:Acids / Substance that releases H+ ions in an aqueous solution
Identified by: / chemical formula which often starts with an H and followed by a non-metals
When Blue Litmus paper turns red
Examples: / HCl H + + Cl-
H2SO4 2H + + SO4 2-
CH3COOH H+ + CH3COO-
Type:
Bases / Substance that releases OH- ions in an aqueous solution
Identified by: / chemical formula which often starts with a metal and ends with an OH
When Red Litmus paper turns blue
Examples: / NaOH Na + + OH-
Mg(OH)2 Mg 2+ + 2 OH-
NH4OH NH4+ + OH-
Type:
Salts / Substance made up usually of metal and non-metal ions (other than H+ and OH-)
Examples: / NaCl Na + + Cl-
AgNO3 Ag+ + NO3-
CaCl2 Ca2+ + 2 Cl-
iii) pH
The pH of a solution in the concentration of H+ ions
– pH of 7 = neutral = 0.000 000 1 mol/L of H+
• Or 1 x 10-7 mol/L
• A pH of 2 has 0.01 mol/L of H+ or 1 x 10-2 mol/L
– pH > 7 = basic = less concentrated
– pH < 7 = acidic = more concentrated
*** To neutralize an acid you must add a base. When mixing an acid and a base together you will always get a salt and water as your products.
iv) Concentrations
The concentration of a solution is the amount of solute in a given amount of solution
**** Although concentration is always the amount of solute divided by the amount of solvent, we can express it many different ways such as in ppm, % or mol/L.
a) Concentration in % (over 100 ml)
This concentration is the number of parts of solute found in a hundred parts of solvent.
This concentration can be measured by:
- Mass/mass (m/m) which refers to the amount in g of solute in 100 g of solution
- Mass/volume (m/V) which refers to the amount in g of solute in 100 ml of solution
- Volume/Volume (V/V) which refers to the amount in ml of solute in 100 ml of solutions
b) Concentration in ppm (parts per million)
This concentration is the number of parts of solute in a million parts of solution. This unit of measurement is extremely important for the very small concentrations
1 ppm = 1 g/1000 000 g = 1 mg/1000 kg = 1mg/kg or 1g/1000L
2 ppm = 2g/1000 000 g = 2 mg/1000 kg = 2 mg/kg or 2g/1000L
Example Question:
You are required to make a solution of Kool-Aid with a concentration of 50 ppm. What is the concentration in g/L?
Answer:
Desired Concentration is 50 ppm which corresponds to 50 g/ 1000 000 ml or to 50 g/1000 L (see conversions above)
Therefore, 50 g can be found in 1000 L and I want to know how many grams will be in 1 L. I will then cross multiply:
____50 g___ = ___x___ x = 0.05 g
1000 L 1L
Equivalent concentration is: 0.05 g/L
c) Concentration in mol/L
This concentration is the number of moles of a solute in a liter of solution
Example: You must dissolve 64 g of sugar (C6H12O6) in 2 liter of water, what is the molar concentration?
Answer:
Step 1: Find the molar Mass of Sugar
C = 12 g = 6 x12 72g
H = 1g = 12x1 12g
O = 16g = 16 x 6 96g
One mole of sugar = 180 g
Step 2: Find out how many moles do you have (cross multiply)
If …
One mole is worth 180 g
X moles is worth 64 g
0.356 moles
Step 3: Complete with the equations C = n/V
C = n/V
C = 0.356/ 2 L
C = 0.178 mol/L
1. Change a concentration
In order to change a concentration there are many different things you can do:
Changes / EffectDilution (add solvent) / Reduces concentration
Dissolution (add solute) / Increases concentration
Evaporation (decrease solvent) / Increases concentration
2. Prepare a solution with a specific concentration
1. Find the amount of solute you need for the given volume of solvent using the C= m/V formula
2. Carefully weigh the necessary amount of solute in a weighing boat
3. Poor the solute in the container and add half of the amount of solvent
4. Stir
5. Add the remaining amount of solvent
Chemical Changes
Chemical Reactions do at least one of the following:
– Emit energy (light)
– Release or absorb heat
• A reaction that releases heat = exothermic
• A reaction that absorbs heat = endothermic
– Release a gas
– Change color
– Form a precipitate
• (2 liquids together gives a solid)
a) Conservation of mass
This law states that the total mass of reactants (molecules and elements before the arrow) is always equal to the total mass of products (after the arrow).
For example, look at the following balanced equation: