The Periodic Table and Atomic Mass | General Chemistry 1

The periodic table:

The periodic table is a chart of 118 elements first published by Dmitri Mendeleev in 1869. Elements with similar chemical and physical properties are grouped together. The elements are arranged by atomic number, with vertical columns representing groups of elements with similar properties, and horizontal rows representing periods.

Elements in the table:

Each element is represented by a tile containing:

• Name of the element (chemical symbol, one or two letters)
• Atomic number
• Average mass (usually with 4 significant figures)

Chemical periodicity

The chemical elements have a periodic pattern when arranged in ascending order of atomic number. A period forms a row of the periodic table. You should be familiar with 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 in the periodic table:

A vertical column of elements in the periodic table is known as a group, with a total of 18 groups. Elements within the same group have the same number of electrons in their outermost shells, leading to similar chemical properties and bond types.

Classification of elements:

• Main-group elements: Elements in the 2 groups on the left and the 6 groups on the right of the periodic table (Groups 1, 2, 13-18).
• Transition metals: Elements in the groups between the main-group elements (groups 3-12).
• Inner transition metals: Elements in the bottom two rows, known as lanthanides and actinides.

Common groups and their characteristics:

• Alkali metals (Group 1): lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs)
  ⇒ soft, silvery metals that react rapidly and often violently with water.
• Alkaline earth metals (Group 2): beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra)
  ⇒ lustrous, silvery metals that react with O₂; less reactive with water compared to alkali metals.
• Chalcogens (Group 16): oxygen (O), sulfur (S), selenium (Se), tellurium (Te), polonium (Po)
• Halogens (Group 17): fluorine (F), chlorine (Cl), bromine (Br), iodine (I)
  ⇒ colorful, corrosive nonmetals found in nature combined with other elements.
• Noble gases (Group 18): helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn)
  ⇒ colorless gases with very low chemical reactivity, considered inert.

Classification:

Most elements can be categorized as metals or nonmetals based on their conductivity of heat and electricity.

• Metal: Good conductor of heat and electricity, typically hard, metallic-looking solids with high melting and boiling points. Found on the left side of the periodic table.
• Nonmetal: Poor conductor of heat and electricity, softer elements, often colored, with lower melting and boiling points than metals. Found on the right side of the periodic table (excluding hydrogen).
• Metalloid: Elements with properties intermediate between metals and nonmetals. Divide the periodic table in a zigzag line between metals and nonmetals.

Metallic trend:

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

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

• Metallic character decreases from left to right in each period: outer electrons experience an increasing effective nuclear charge, requiring more energy for removal, decreasing 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