The periodic table and Chemical Periodicity | General Chemistry 1

The periodic table and chemical periodicity are studied in this chapter: law of constant composition, molecular and molar masses, difference between empirical and molecular formula, chemical reactions and how to balance a chemical equation, groups and periods in the periodic table and their properties.

The Law of Constant Composition

Samples of a pure compound always contain the same elements in the same mass proportions.
The mass percentage of each element, for example A in AB(s), is given by: 

mass% of A in AB = mass of Amass of AB x 100



Mass, Mole and Avogadro's Number

Molecular mass (in amu): sum of the atomic masses of the atoms in a molecule

Molar mass M (in g.mol-1): mass of one mole of molecules, atoms, ions
= sum of the molar masses of the atoms in a molecule

1 mole = 6.022 x 1023 items
Avogadro's Number NA = 6.022 x 1023 mol-1 ⇒ makes working with large number easier


Molecular mass of CH4 = atomic mass of C + 4 x atomic mass of H
Molecular mass of CH4 = 12.01 + 4 x 1.008 = 16.04 amu

Molecular mass of CH4 = molar mass of C + 4 x molar mass of H
MCH4 = MC + 4 MH = 12.01 + 4 x 1.008 = 16.04 g.mol-1


Mole number n (in mol): number of moles in a sample

n = mM

m = mass of the substance (in g)
M = molar mass of the substance (in g.mol-1)


n = NNA

N = number of particles in the substance
NA = Avogadro’s number (in mol-1)


Empirical vs. Molecular Formula

Empirical formula shows the simplest whole-number ratio of atoms in a compound
Molecular formula shows the number of each type of atom in one molecule


Molecule of glucose : 

Empirical formula = CH2O
Molecular formula = C6H12O6

Chemical Reactions

Chemical reaction: process in which one or more substances (the reactants) are converted to one or more different substaces (the products)

A chemical reaction is represented by a chemical equation:

- chemical formulas of the reactants on the left-hand side 
- chemical formulas of the products on the right-hand side
- reactants and products are separated by an arrow
- each individual substance’s chemical formula is separated from others by a '+' sign.

The states of the substance (solid (s), liquid (l), gas (g) or aqueous (aq)) can be added after each individual substance’s chemical formula.


Mg (s) + Cl2 (g) → MgCl2 (s)

Balancing Chemical Equations

Atoms are neither created nor destroyed in a chemical reaction
⇒ chemical equation must be balanced (same number of each type of atom on both sides)

Li + Br2 → LiBr    is not balanced
2 Li + Br2  → 2 LiBr      is balanced


Easy steps for balancing chemical equations:

- write the unbalanced chemical equation
- determine how many atoms of each element are present on each side of the arrow
- balance atoms present in a single molecule of reactant and product first
- balance any oxygen or hydrogen atoms last
- at the end, check your equation to make sure you cannot reduce the balancing coefficients

Group Properties

Certain chemical elements have similar chemical properties ⇒ they form a group.
A group of elements forms a vertical column in the periodic table.

General groups on the periodic table:

- Main groups: the 2 groups on the far left and the 6 on the far right
- Transition metal groups: groups immediately between the main group elements
- Inner transition metal groups: the 14 groups shown at the bottom of the table (lanthanides and actinides)

Several groups of representative elements are known by common names:

- Alkali metals = 1st column = lithium, sodium, potassium, (rubidium, caesium and francium)
⇒ very reactive metals and can explode if they are exposed to water.

- Alkaline-earth metals = 2nd column = beryllium, magnesium, calcium, (strontium, barium, radium)
⇒ react with O2, less reactive to water than alkali metals

- Halogens = penultimate column = fluorine, chlorine, bromine, iodine, (astatine)
⇒ colorful and corrosive nonmetals, very reactive

- Noble gases = last column = helium, neon, argon, (krypton, xenon, radon)
⇒ very stable elements, generally considered to be inert gases


The elements show a periodic pattern when listed in order of increasing atomic number.
A period forms a row in the periodic table.

You generally should know the three first rows/periods of the periodic table:

1st period: H, He
2nd period: Li, Be, B, C, N, O, F, Ne
3rd period: Na, Mg, Al, Si, P, S, Cl, Ar

Mnemonic Device:
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