The Periodic Table and Atoms | 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 Periodic Table

The periodic table:

A chart of 118 elements first published by Dmitri Mendeleev in 1869 in which elements having similar chemical and physical properties are grouped together. The elements are organized by atomic number in rows (periods) and columns (groups). Each vertical column corresponds to a group of elements with the same properties

Chemical periodicity

Elements exhibit a periodic pattern when listed in ascending order of atomic number. A period forms a row of the periodic table. You generally need to know the first 3 periods:

  • 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:
Here He Lies Beneath Bed Clothes, Nothing On, Feeling Nervous, Naughty Margret Always Sighs, " Please Stop Clowning Around "

Metal vs. nonmetal

Most elements can be categorized as metals or nonmetals according to their ability to conduct heat and electricity:

  • Metal: good conductor of heat and electricity
  • Nonmetal: poor conductor of heat and electricity
  • Metalloid: element with properties that are intermediate between those of metals and nonmetals

The majority of the elements are metals. From left to right in the periodic table, the properties of the elements gradually change from metallic to nonmetallic

Group Properties

General groups of the periodic table:

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

Special groups

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

Average Atomic Mass

Atomic mass unit (amu):

The mass exactly equal to one-twelfth the mass of an atom of carbon-12
1 amu = 1.6605378 x 10-24 g

Atomic mass (in amu):

The mass of an atom in atomic mass units. Atomic mass is the mass of a single isotope of an element

Average atomic mass (in amu):

Average of the atomic masses of all natural isotopes of an element weighted by their abundance.  The periodic table contains the average atomic mass of each element. For simplicity, the word 'average' is usually omitted when the atomic masses of the elements are discussed

Average atomic mass of the carbon atom:

Carbon has 2 isotopes: 12C (12.000 amu - 98.89%) and 13C (13.003 amu - 1.11%)

  • The atomic mass of 12C is 12.000 amu
  • The average atomic mass of carbon is 12.000 x 0.9889 + 13.003 x 0.0111 = 12.011 amu

The Mole


The amount of a substance that contains as many elementary entities as there are atoms in exactly 12.0 g of 12C. This quantity is used to express the number of atoms in a macroscopic sample of matter. 1 mole corresponds to 6.022 x 1023 atoms:

12.0 g of 12C x 1 amu1.6605 x 10-24 g x 1 atom of C1212 amu = 6.022 x 1023 12C atoms

Avogadro's number NA:

The number used to convert moles in numbers of atoms. NA = 6.022 x 1023 items.mol-1

Masses and Mole Number

Molecular mass (in amu): 

The sum of the atomic masses of the atoms that make up a molecule

Molar mass M (in g.mol-1): 

The mass of one mole of the substance (molecules, atoms, ions). The molar mass of an ion or a molecule is equal to the sum of the molar masses of all the atoms constituting the ion or the molecule

Molecular mass of CH4 = atomic mass of carbon + 4 x atomic mass of hydrogen 
= 12.01 + 4 x 1.008 = 16.04 amu

Molar mass of CH4 = molar mass of carbon + 4 x molar mass of hydrogen
MCH4 = MC + 4 MH = 12.01 + 4 x 1.008 = 16.04 g.mol-1


Mole number n (in mol): 

The 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)