Understanding Atomic Orbitals and Element Properties
Atomic Structure and Electron Configuration
Quantum Numbers:
- n: Principal quantum number, describes the energy level (1 to 7).
- l: Angular momentum or azimuthal quantum number, describes the subshells (s, p, d, f) within an energy level.
- m: Magnetic quantum number, describes the orientation of orbitals in space (-1, 0, +1, etc.).
Principles of Electron Configuration:
Aufbau Principle (Building-Up Principle): Electrons first fill the orbitals with the lowest energy. Higher energy orbitals are occupied only after lower energy orbitals are filled.
The energetic level of the orbitals can be determined by:
- The diagonal rule.
- The sum of n + l.
Pauli Exclusion Principle: No two electrons in an atom can have the same set of four quantum numbers. This means that a maximum of two electrons can exist in a single atomic orbital, and they must have opposite spins.
Hund’s Rule of Maximum Multiplicity: When filling degenerate orbitals (orbitals with the same energy, e.g., 3p, 5d, 7f), electrons will individually occupy each orbital before pairing up in any one orbital. Electrons in singly occupied orbitals have parallel spins.
The electron configuration indicates the number of electrons (z) and their placement in the system.
Noble Gases: Noble gases fulfill the octet rule (except for helium, which follows the duet rule).
Special Cases: Some chemical elements undergo rearrangements in their electron configuration. This is especially true for elements with incomplete d orbitals, where electrons may shift to achieve a more stable configuration (e.g., from 4 to 5 electrons or from 9 to 10 electrons).
Differentiating Electron: The most energetic electron; the last electron added in the electron configuration.
Valence Electrons: Electrons located in the outermost energy level. Electrons in incomplete subshells at a previous level can also be considered valence electrons.
Properties of Elements
Metals: Generally found on the left and central parts of the periodic table. Good conductors of heat and electricity, lustrous, ductile, and solid at room temperature.
Nonmetals: Generally found on the upper right side of the periodic table. Poor conductors of heat and electricity (insulators). Can be solids, liquids, or gases. Non-lustrous, brittle, and not ductile.
Metalloids: Exhibit intermediate properties between metals and nonmetals.
The Periodic Table
The periodic table is divided into groups/families (18 columns) and periods (7 rows).
- Groups A (Representative Elements): Elements where the outermost electrons are in s or p orbitals.
- Groups B (Transition Elements): Elements where the outermost electrons are in d or f orbitals. Elements within the same group have similar chemical properties because they have the same number of valence electrons.
- Periods: Elements in the same period have the same number of electron shells (energy levels).
History of the Periodic Table
The modern periodic system began in the 19th century. Between 1860 and 1870, Julius Lothar Meyer and Dmitri Mendeleev organized the then-known 60 chemical elements according to their atomic mass. In 1869, Mendeleev presented his work to the Russian Chemical Society with six postulates:
- If elements are arranged according to their atomic masses, they exhibit a clear periodicity of properties.
- Elements with similar chemical properties have similar atomic masses.
- The arrangement of elements in order of their atomic weights corresponds to their valence.
- The most abundant elements in nature have small atomic masses, and these elements have well-defined properties.
- The atomic mass value allows for predicting the properties of an element.
- The atomic mass of an element can be corrected if the properties of adjacent elements are known.
Mendeleev concluded that the sequence should follow a logical order and proposed that spaces should be left for undiscovered elements.
Errors in Mendeleev’s Table:
- No fixed place for hydrogen.
- Considered only one valence for each element, but some elements have more than one.
- Lanthanides were placed in a single row as if they were a single element.
- The most important principle is not always in the correct order, as with atomic mass.
Groups Names
1A-1 = Alkali Metals
2A-2 = Alkaline Earth Metals
3A-13 = Boron Group
4A-14 = Carbon Group (Carbonoids)
5A-15 = Nitrogen Group (Nitrogenoids)
6A-16 = Chalcogens
7A-17 = Halogens
18 = Noble Gases