Key Properties of Ionic, Molecular, Covalent, and Metallic Substances

Key Properties of Chemical Substances

Ionic Compounds

• Solid at room temperature
• Form crystal lattices, not individual molecules
• Soluble in water
• Conduct electricity when molten or dissolved
• High hardness and brittle
• High melting and boiling temperatures

Molecular Substances

• Composed of molecules
• Weak intermolecular forces
• Often gaseous (e.g., O₂, N₂, NH₃) or liquid (e.g., H₂O)
• Soft solids
• Low melting and boiling temperatures
• Poor electrical conductors
• Not very soluble in water

Covalent Crystals

• Solid
• Form crystals with atoms joined by covalent bonds in three dimensions
• Insoluble in all solvents
• Generally do not conduct electricity (except graphite)
• Very hard and brittle
• High melting and boiling temperatures (1200 to 3600 °C)

Metals

• Solid at room temperature (except Hg, which is liquid)
• Form metallic networks
• Metallic luster
• Conduct heat and electricity due to electron mobility
• Insoluble in H₂O
• Ductile, soft, malleable, and tenacious
• Moderate to high melting and boiling points (-39 to 3400 °C)
• High densities

Atomic Models and the Periodic Table

The idea that matter is composed of tiny, indivisible particles dates back to the Greek philosopher Democritus. J. Dalton, an English chemist, revived this idea in his atomic theory of matter in 1805, stating that atoms are indivisible and remain unchanged in chemical processes.

Rutherford's Gold Foil Experiment

E. Rutherford bombarded a thin gold foil with alpha particles. Contrary to expectations, some particles were deflected or bounced back, leading to the discovery of the atomic nucleus.

Bohr's Model of the Hydrogen Atom

Bohr proposed a model for the hydrogen atom, explaining its electronic structure and atomic spectrum. Key points include:

• Electrons orbit the nucleus in fixed energy levels.
• Electrons can only exist in specific energy levels.
• When an electron moves to a lower energy level, the energy difference is emitted as light.

Development of the Periodic Table

J. Berzelius introduced the first classification scheme in 1813, dividing elements into metals and nonmetals. J. Newlands proposed the law of octaves, noting that every eighth element had similar properties when arranged by increasing atomic mass. L. Meyer observed regularities in atomic volume. D. Mendeleev presented a periodic table classifying elements by increasing atomic mass, with elements of similar properties arranged vertically. H. Moseley later ordered elements by increasing atomic number (Z). A. Werner and F. Paneth suggested the current long-form periodic table.