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 atomic mass unit (amu), also known as the dalton (Da), is defined as exactly 1/12 the mass of an atom of ${}_6{}^{12}\mathrm{C}$. It is equal to 1.6605378 x 10^-24 g.
 

Atomic mass (in amu):

Atomic mass refers to the mass of a single atom of a chemical element, measured in atomic mass units. It accounts for the masses of the 3 subatomic particles: protons, neutrons and electrons.
 

Average atomic mass (in amu):

The average atomic mass is the weighted average of the atomic masses of all naturally occurring isotopes of an element, taking into account their relative abundances. The periodic table typically lists the average atomic masses of elements. For simplicity, the word 'average' is usually omitted when the atomic masses of the elements are discussed
 

Average atomic mass of carbon atoms:

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

• 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 mole represents the amount of a substance that contains the same number of elementary entities as there are atoms in exactly 12.0 g of ¹²C.

It serves as a crucial link between the microscopic and macroscopic realms in chemistry, allowing for the expression of the number of atoms or molecules in a macroscopic sample. 1 mole corresponds to 6.022 x 10²³ atoms or molecules:
 

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:

Avogadro's number N_A is used to convert between the number of moles of a substance and the number of atoms or molecules it contains. N_A = 6.022 x 10²³ items.mol^-1

 Molecular mass (in amu): 

The molecular mass of a compound is the sum of the atomic masses of all the atoms that make up a molecule. It is expressed in atomic mass units (amu) and provides insight into the relative mass of a molecule.
 

Molar mass M (in g.mol^-1): 

Molar mass represents the mass of one mole of a substance, whether it be molecules, atoms, or ions. It is calculated by summing the molar masses of all the atoms in a molecule or ion and is expressed in g.mol^-1.
 

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 mole number denotes the quantity of moles present in a sample of a substance. It can be determined using the formulas: