The Periodic Table and Atoms | General Chemistry 1

The periodic table:

A chart of 118 elements first published by Dmitri Mendeleev in 1869, in which elements with similar chemical and physical properties are grouped. The elements are arranged by atomic number. Each vertical column corresponds to a group of elements with the same properties, each horizontal line corresponds to a period
 

Elements in the table

Each element of the periodic table is represented by a tile containing the name of the element (chemical symbol composed of one or two letters), the atomic number and the average mass (usually indicated with 4 significant figures)

Chemical periodicity

The chemical elements have a periodic pattern when they are arranged in ascending order of atomic number. A period forms a row of the periodic table. You should generally know the first 3 periods:

• 1^st period: H, He
• 2^nd period: Li, Be, B, C, N, O, F, Ne
• 3^rd 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 "

Groups of the periodic table:

A vertical column of elements is called a group. There are 18 groups in total. Members of the same group have the same number of electrons in the outermost shells of their atoms and form bonds of the same type. The elements can be classified into:

• The main-group elements: elements in the 2 groups on the far left and the 6 groups on the far right of the periodic table (groups 1, 2, 13-18)
• The transition metals: elements in the groups between the elements of the main group (groups 3-12)
• The inner transition metals: elements in the 2 rows at the bottom of the table. The top-row elements are called lanthanides and the bottom-row elements are actinides
   

Special groups

Several groups of representative elements are known by common names:

• Alkali metals: 1^st column = lithium, sodium, potassium, (rubidium, caesium and francium)
  ⇒ very reactive metals and can explode if they are exposed to water
• Alkaline-earth metals: 2^nd 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

Metals, nonmetals and metalloids

Most elements can be classified as metals or nonmetals based on their ability to conduct heat and electricity:

• Metal: good conductor of heat and electricity. They tend to be hard, metallic-looking solids with high melting and boiling points. The majority of elements are metals
• Nonmetal: poor conductor of heat and electricity. They are generally softer elements, often colored, with lower melting and boiling points than most metals
• Metalloid: an element with properties intermediate between metals and nonmetals. Metalloids divide the periodic table in a zigzag line between metals on the left and nonmetals on the right

Metallic trend

The metallic character is essentially the ease with which an atom gives up an electron

• Metallic character increases down a group. Going down a group, the atomic radius increases: the outer electrons are farther from the nucleus and are easier to remove, thus increasing the metallic character

• Metallic character decreases from left to right in each period of the periodic table. The outer electrons have an increasing effective nuclear charge and thus require more energy for their removal decreasing the metallic character

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 atomic mass is the mass of a single atom of a chemical element in atomic mass units. It includes the masses of the 3 subatomic particles that make up an atom: protons, neutrons and electrons
 

Average atomic mass (in amu):

The 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

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. . It represents the link between the microscopic and the macroscopic. 1 mole corresponds to 6.022 x 10²³ atoms
 

12.0 g of ¹²C x $\frac{1 \mathrm{amu}}{1.6605 \mathrm{x} {10}^{-24} \mathrm{g}}$ x $\frac{1 \mathrm{atom} \mathrm{of} {}^{12}\mathrm{C}}{12 \mathrm{amu}}$ = 6.022 x 10^23 12C atoms

Avogadro's number N_A:

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

 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 making up 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
M_CH4 = M_C + 4 M_H = 12.01 + 4 x 1.008 = 16.04 g.mol^-1

Mole number n (in mol): 

The number of moles in a sample