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 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:

• 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 "

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

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: 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

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

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 $\frac{1 \mathrm{amu}}{1.6605 \mathrm{x} {10}^{-24} \mathrm{g}}$ 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 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
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