Atomic Structure, Bonding, and Properties of Matter
1. The Nuclear Model of the Atom
Democritus proposed the concept of atoms. J. Dalton’s atomic theory stated that atoms were indivisible and unchanged by chemical processes.
1.1 The Gold Foil Experiment
E. Rutherford bombarded a thin gold foil with alpha rays. He expected all particles to pass through uniformly. However, some particles were deflected, demonstrating that:
- The atom has a central core (nucleus) containing most of its mass, including protons.
- Electrons revolve in concentric circular orbits around the nucleus.
- There is mostly empty space between the nucleus and the electrons.
Atomic Number (Z): Number of protons in the nucleus.
Mass Number (A): Number of protons and neutrons.
2. The Atomic Shell
2.1 The Bohr Model and Hydrogen Spectrum
N. Bohr proposed an atomic model for hydrogen stating that:
- Electrons revolve around the nucleus in fixed energy orbits.
- Electrons can only occupy orbits with specific energy levels.
- When an electron transitions from a higher to a lower energy level, light is emitted.
3. Periodic Table and Electronic Structure
Each block in the periodic table corresponds to a sublevel in the electron shell.
- s-block: Groups 1 and 2 (alkali and alkaline earth metals).
- p-block: Groups 13 to 18.
- d-block: Groups 3 to 12 (transition metals).
- f-block: Inner transition metals.
All elements in the same group have the same number of valence electrons (electrons in the outermost shell), which determine their chemical behavior and properties.
4. Groupings of Atoms: Chemical Bonding
A chemical bond is the union of two or more atoms to form a stable structure. The energy released during this process is called binding energy, which is equal to the energy required to separate the bonded molecules and crystals.
4.1 Atoms and Molecules
- Molecules: Stable clusters of a fixed number of atoms (usually small). If the atoms are the same, it’s an element; otherwise, it’s a compound.
- Crystals: Stable clusters of a variable and large number of atoms or ions forming solids with a regular arrangement in all directions.
4.2 Octet Rule
When atoms bond, they tend to gain, lose, or share electrons to achieve a noble gas electron configuration (eight valence electrons) in their outermost shell.
5. Metallic Bonding and Metallic Compounds
Metals are always elements. Metallic bonding involves:
- A three-dimensional structure of positive ions at lattice points.
- A “sea” or cloud of delocalized valence electrons that can move throughout the entire structure.
Properties of Metals:
- Solid at room temperature.
- Good conductors of heat and electricity.
- Ductile and malleable.
- Generally soft and tough.
6. Covalent Bonding and Covalent Compounds
A covalent bond is formed when two atoms share one or more electrons. Each shared pair constitutes a bond.
Lewis Diagrams:
G.N. Lewis proposed a way to represent valence electrons:
- The Lewis symbol for an element is the element symbol with dots representing valence electrons.
- Dots are placed around the symbol (up to four sides).
- Each shared pair of electrons (bond pair) is represented by a line.
- Covalent valence is the number of covalent bonds an atom can form.
Molecular Substances:
Characterized by strong intramolecular forces compared to weaker intermolecular forces. Most are gases at room temperature.
Covalent Crystals:
Formed when covalent bonds extend in three dimensions, creating large crystalline structures. They result in very stable, hard, insoluble, and non-conductive compounds.