Atomic Structure, Electron Configuration, and Chemical Properties
Bohr’s Atomic Model
Bohr’s model for the hydrogen atom aimed to explain its electronic structure and justify its atomic spectrum. It was based on three main postulates:
- The electron revolves around the nucleus in specific circular orbits of fixed energy.
- Only orbits where electrons possess specific, quantized energy values are allowed. These orbits are called energy levels.
- When an electron transitions from a higher energy level to a lower one, the energy difference is emitted as light (a photon).
Electron Energy Levels and Sublevels
As higher-resolution spectral equipment became available, some spectral lines that initially appeared simple were found to consist of multiple finer lines. This observation, combined with the Bohr model’s limitations in predicting the spectra of multi-electron atoms, led Arnold Sommerfeld to propose an extension to his teacher’s model. Sommerfeld suggested that each principal energy level (designated by the quantum number n) is subdivided into n sublevels (s, p, d, f, etc.).
Electron Configuration
Electron configuration describes the arrangement of electrons within the various energy levels and sublevels of an atom’s electron shell, following the order of increasing energy.
Ions: Anions and Cations
Atoms can gain or lose electrons to form ions:
- When an atom gains one or more electrons, it becomes a negatively charged ion, known as an anion. The magnitude of the negative charge equals the number of electrons gained.
- When an atom loses one or more electrons, it becomes a positively charged ion, known as a cation. The magnitude of the positive charge equals the number of electrons lost.
Development of the Periodic Table
The classification of elements evolved over time:
- 1813: Jöns Jacob Berzelius introduced an early classification, dividing elements into metals and nonmetals.
- J. A. R. Newlands: Proposed the Law of Octaves, noting that when elements are arranged by increasing atomic mass, properties repeat every eighth element.
- Lothar Meyer: Independently identified periodicity, particularly in atomic volume, when elements were arranged by atomic mass.
- Dmitri Mendeleev: Created a periodic table arranging elements by increasing atomic mass, grouping elements with similar chemical properties vertically (families or groups). He famously left gaps for undiscovered elements and predicted their properties.
- Henry Moseley: Established that ordering elements by increasing atomic number (Z) provided a more accurate basis for periodicity than atomic mass.
- Alfred Werner and Friedrich Paneth: Contributed to the development of the modern long-period form of the periodic table based on atomic number.
Properties of Ionic Compounds
- Typically solid at room temperature, forming crystalline lattices (not discrete molecules).
- Often soluble in water.
- Conduct electricity when molten or dissolved in an aqueous solution.
- Hard but brittle.
- Possess high melting and boiling points.
Properties of Molecular Substances
- Composed of discrete molecules.
- Intermolecular forces are relatively weak, leading many to be gases or liquids at room temperature; solid forms are generally soft.
- Exhibit low melting and boiling points.
- Typically do not conduct electricity.
- Solubility in water varies greatly (often low for nonpolar molecules).
Properties of Covalent Network Solids
- Solids composed of atoms joined by covalent bonds extending in three dimensions throughout the material.
- Generally insoluble in common solvents.
- Typically electrical insulators (graphite is a notable exception).
- Very hard and often brittle.
- Possess very high melting and boiling points.
Properties of Metals
- Typically solid at room temperature (except Mercury, Hg), forming metallic lattices.
- Exhibit metallic luster (shine).
- Good conductors of heat and electricity due to the mobility of their electrons (often described as an ‘electron sea’).
- Generally insoluble in water (though some may react chemically).
- Ductile (can be drawn into wires) and malleable (can be hammered into thin sheets).
- Vary in hardness but are generally tenacious (resist being pulled apart).
- Melting and boiling points range from moderate to very high (e.g., Hg at -39°C to W at 3422°C).
- Generally possess high densities.