Chemistry Notes: Atomic Structure, Periodic Table, and Chemical Compounds

Calculation of Atomic Mass

• Naturally occurring chlorine consists of two isotopes:

  • Chlorine-35: $75.77\%$ abundance (mass = $34.97$ amu)

  • Chlorine-37: $24.23\%$ abundance (mass = $36.97$ amu)

• To calculate the atomic mass, multiply the fractional abundance of each isotope by its mass and sum the results:

  • Atomic Mass of Chlorine $= (0.7577 \times 34.97 \text{ amu}) + (0.2423 \times 36.97 \text{ amu})$

  • Atomic Mass of Chlorine $= 26.4950 \text{ amu} + 8.9566 \text{ amu}$

  • Atomic Mass of Chlorine $= 35.4516 \text{ amu}$

Development of the Periodic Table

• Elements with similar chemical characteristics have been systematically grouped over time.

• The modern periodic table is the culmination of many years of refining these groupings.

• Arrangement Principles:

  • Elements are arranged horizontally in order of increasing atomic number.

  • Elements are grouped vertically by similar chemical properties.

• Historical Note: Initially, elements were ordered by atomic mass, but the modern table uses atomic number.

Noble Gases

• These are a group of elements characterized by being colorless, odorless, and mostly inert (unreactive).

• They were discovered after the periodic table was first developed, hence their inclusion came later.

Using the Modern Periodic Table

• Periods:

  • Elements within a horizontal row are in the same period.

  • There are seven periods in the periodic table.

  • The sixth and seventh periods include the inner transition elements, which are typically displayed separately below the main table.

• Groups/Families:

  • Elements within a vertical column are in the same group or family.

  • Elements in the same group exhibit similar chemical properties due to their similar electron configurations (especially valence electrons).

Group/Family Designations

• Older Numbering System: Uses Groups $1-8$ with an A or B designation (e.g., $1A, 2A, 1B$).

• Modern Numbering System: Simply uses numbers $1-18$ from left to right across the table.

• Common Group Names:

  • Groups 1 and 2: Alkali metals and alkaline earth metals, respectively.

  • Group 11: Contains the coinage metals (e.g., Copper, Silver, Gold).

  • Group 17: Halogens.

  • Group 18: Noble gases.

Larger Groups on the Periodic Table

• Main Group Elements:

  • Found in groups $1, 2, 13-18$ (using modern numbering).

  • These are more common elements.

  • They exhibit more predictable chemical behavior.

• Transition Elements:

  • Found in groups $3-12$ (using modern numbering).

  • These are all metals.

• Inner Transition Elements:

  • Typically displayed in two rows beneath the main periodic table.

  • Lanthanoids: Elements $58-71$ (after Lanthanum, from Cerium to Lutetium).

  • Actinoids: Elements $90-103$ (after Actinium, from Thorium to Lawrencium).

Metals, Nonmetals, and Metalloids

• Metals:

  • Constitute the majority of elements.

  • Located on the left side and center of the periodic table.

  • Generally good conductors of heat and electricity, ductile, malleable, and lustrous.

• Nonmetals:

  • Consist of hydrogen plus the elements found on the upper-right side of the periodic table (e.g., C, N, O, F, Cl, S, P, Se, Br, I, He, Ne, Ar, Kr, Xe, Rn, H).

  • As solids, they are typically brittle.

  • Poor conductors of heat and electricity.

• Metalloids:

  • Elements that form a diagonal border between metals and nonmetals (e.g., B, Si, Ge, As, Sb, Te, Po, At).

  • Share properties with both metals and nonmetals (e.g., silicon is a semiconductor).

Nomenclature and Formulas

• Chemical Nomenclature: The systemic approach or rules for naming chemical compounds.

• Chemical Formulas: Combine elemental symbols and subscripts to represent compounds.

  • Show a formula unit of a compound.

  • Represent the ratio of atoms in a compound.

Compounds Exist in Different Forms: Molecular Compounds

• Definition: Contain nonmetal atoms bonded to each other.

• Bonding: Held together by covalent bonding, which involves the sharing of electrons between atoms.

• Form: Exist as individual discrete particles called molecules, each made up of bonded atoms.

• Example (Chlorine molecule): Two chlorine atoms (each with $17$ electrons and $17$ protons) share electrons to form a $Cl_2$ molecule.

Compounds Exist in Different Forms: Ionic Compounds

• Definition: Composed of metal atoms and nonmetal atoms.

• Bonding: Held together by ionic bonds.

  • Ionic bonds form through the transfer of electrons from a metal atom to a nonmetal atom.

  • This transfer results in the formation of oppositely charged ions (cations and anions) that are attracted to each other.

• Example (Sodium Chloride):

  • A sodium (Na) atom (with $11$ electrons and $11$ protons) loses one electron to become a positively charged sodium ion ($Na^+$) (with $10$ electrons and $11$ protons).

  • A chlorine (Cl) atom (with $17$ electrons and $17$ protons) gains one electron to become a negatively charged chloride ion ($Cl^-$) (with $18$ electrons and $17$ protons).

  • The electrostatic attraction between $Na^+$ and $Cl^-$ forms the ionic compound NaCl.

Atoms, Molecules, and Formula Units

• (a) Atoms of Ar: Argon exists as individual unbonded atoms.

• (b) Molecules of $Br_2$: Bromine exists as diatomic molecules, meaning two bromine atoms are covalently bonded together.

• (c) Formula units of NaCl: Sodium chloride exists as an extended lattice of $Na^+$ and $Cl^-$ ions, with the formula unit (NaCl) representing the simplest ratio of ions.

Molecules of Elements

• Some elements are never found in nature as individual atoms but instead occur as small molecules containing two or more atoms.

• Seven Diatomic Elements: These elements always exist as molecules composed of two atoms when uncombined:

  • $H_2$ (Hydrogen)

  • $N_2$ (Nitrogen)

  • $O_2$ (Oxygen)

  • $F_2$ (Fluorine)

  • $Cl_2$ (Chlorine)

  • $Br_2$ (Bromine)

  • $I_2$ (Iodine)