ATOM
In chemistry an atom is the smallest possible particle of a chemical element that retains its chemical properties.

• Every element is made of atoms - piles of paper clips.
• All atoms of any element are the same - all the paper clips in the pile are the same size and color.
• Atoms of different elements are different (size, properties) - like different sizes and colors of paper clips.
• Atoms of different elements can combine to form compounds - you can link different sizes and colors of paper clips together to make new structures.
• In chemical reactions, atoms are not made, destroyed, or changed - no new paper clips appear, no paper clips get lost and no paper clips change from one size/color to another.
• In any compound, the numbers and kinds of atoms remain the same - the total number and types of paper clips that you start with are the same as when you finish.

Atoms are the smallest representative particles of the elements. They are composed of 3 different subatomic particles:

• protons
• neutrons
• electrons

Positively charged protons and uncharged neutrons are placed in the nucleus (in the center) and negatively charged electrons rotate around the nucleus.
Protons and neutrons make up almost all the mass of atom. They possess the same mass (1 amu). But the mass of electron is negligible when compared with the masses of neutron and proton. 

ELEMENTS

Elements: are pure substances those contain only one kind of atom and they are the basic units of matter that cannot be decomposed into simpler substances.

• Today 114 elements are known, 92 of them are found in nature and the rest are produced artificially in lab.
• Elements are shown by symbols.
• Everything in universe is made up of 92 naturally occurring elements.
• The most abundant element in the earths crust is oxygen, but in the universe hydrogen.
• Names, symbols and properties of elements can be found from the periodic table of the elements.

Symbols of Elements
Hydrogen: H         Helium: He                    
Lithium: Li            Carbon: C
Nitrogen: N           Oxygen: O
Fluorine: F            Neon: Ne
Sodium: Na          Magnesium: Mg
Aluminum: Al        Silicon: Si
Phosphorus: P      Sulfur: S
Chlorine: Cl          Argon: Ar
Potassium: P         Calcium: Ca 
Iron: Fe                Copper: Cu
Zinc: Zn               Bromine: Br
Silver: Ag             Iodine: I
Barium: Ba           Mercury: Hg
Lead: Pb               Boron: B

Types of Elements
According to their physical and chemical properties elements are classified into three groups: metals, nonmetals and metalloids.

Metalloids: some elements show both metallic and nonmetallic properties, these elements are called metalloids.

Metals

1. They are good conductors of heat and electricity
2. They are solid at room temperature. (except Hg)
3. They have metallic shiny color.
4. They can be drawn into wires.
5. They can be hammered to sheets.

 

Nonmetals

1. They are very poor conductors. (except: graphite)
2. They can be solid, liquid or gas at room temperature.
3. Their surfaces do not have metallic shiny color.
4. They cannot be drawn into wires.
5. They cannot be hammered.

 

Main Components of Air and Earth’s Crust
Air:                                Earth’s Crust:
78% Nitrogen                          49% Oxygen
21% Oxygen                           26% Silicon
0.9% Argon                             7% Aluminum
0.003% Carbon dioxide           5% Iron
                                              3.4% Calcium
                                              2.6% Sodium

Relative Atomic Mass
The relative atomic mass of an element is the mass of one of the element's atoms relative to the mass of an atom of Carbon 12, the most common isotope of carbon.

Scientists have assigned an atomic mass of 12 to Carbon 12. Therefore, if an element has an atomic mass equal to half that of Carbon 12, it has a relative atomic mass of six. If an element has an atomic mass twice that of Carbon 12, it has a relative atomic mass of 24. And so on.

Atoms are so small that ordinary mass units cannot be used to for the atoms. Instead, atomic mass unit (amu) is used. 1 amu is equal to the 1/12 of the mass of a carbon-12 atom. Relative atomic mass of an element is shown as Ar_(E) where E is the symbol of the element.

1amu = m(C)/12
          =1,99.10^-23g/12
          =1,66.10^-24 g

To find the relative atomic mass of an element divide the mass of the atom to 1 amu (=1,66.10^-24 g)

Example: Find the relative atomic mass of fluorine if its mass is 3.15 * 10-23g.

Solution: Ar(F) = m(F)/1amu
                          = 3,15.10^-23/1,66.10^-24
                          = 19g

Example:mass of H₂SO₄
Solution:
Mr(H₂SO₄) = 2×Ar(H) + Ar(S) + 4×Ar(O)
= 2×1.0079 + 32.066 + 4×15.999
= 2.0158 + 32.066 + 63.996
= 98.078

VALENCY
The atoms in compounds can form a fixed number of bonds. The measure of this ability is called valence.
Hydrogen is monovalent (makes 1 bond), oxygen is divalent (makes 2 bonds), aluminum is trivalent (makes 3 bonds) and carbon is tetravalent (makes 4 bonds). 
                                                          
Although there are some exceptions, we can find the valencies of elements with a simple generalization:

• Valency of an element is equal to its group number. So if the group number is 3 then the valency of the element is 3 too.
• Most non-metals tend to have more than 1 valency. The second valency can be found by subtracting group number from 8. So for an element in 5th group the valency might be 3 (8-5 = 3) as well as 5.

So there is such a trend:

1st group	2nd group	3rd group	4th group	5th group	6th group	7th group
1	2	3	4	5and3	6and2	7and1

 

Just to cross the valencies is enough to write the formulas of most compounds.
I         II
H       O       then the formula is H₂O

III         II
Al         O   then the formula is Al₂O₃

Non metals take lowest possible valency when they combine with metals. For example when chlorine (a nonmetal) combines with a metal such as calcium, it takes the valency I (not VII)
            VII
II           I
Ca        Cl      then the formula is CaCl₂ (not Ca₇Cl₂)

We do some simplifications when necessary
IV         II
C          O      then the formula is CO₂ (not C₂O₄)

It is very useful to memorize valencies of some elements.