Electron Configuration and Chemical Periodicity
Electron Configuration
Pauli Exclusion Principle
Two electrons in the same atom cannot have identical quantum numbers. Since electrons in the same orbital are identical, an orbital cannot contain more than two electrons.
The maximum number of electrons per energy level is 2n2
Electronic Configuration of the Elements
Spectral Notation
Spectral notation schematically represents the distribution of electrons in an atom based on Bohr’s atomic model. Electrons tend to occupy the lowest energy orbitals.
The figure below shows the order of filling orbitals.
Electron orbital MAXIMUM LEVEL BY LEVEL
Example: Calcium (Z = 20) has the spectral notation: 1s22s22p63s23p64s2
Knowing this order ensures the third electron goes to the 2s orbital. Therefore, lithium’s electron configuration is 1s22s1. Beryllium, with four electrons, places its fourth electron in the 2s orbital, resulting in the configuration 1s22s2.
Orbital Diagram Notation
Orbital diagrams use arrows to represent electrons (↑ for spin +1/2 and ↓ for spin -1/2).
Example: Orbital diagrams for the first five elements:
- 1H: 1s1 (↑)
- 2He: 1s2 (↑↓)
- 3Li: 1s22s1 (↑↓)(↑)
- 4Be: 1s22s2 (↑↓)(↑↓)
- 5B: 1s22s22p1 (↑↓)(↑↓)(↑)
Conventional Notation
Conventional notation has two forms:
- Total Configuration: Lists all orbitals (e.g., 11Na: 1s22s22p63s1)
- Partial Configuration: Abbreviates filled levels with letters (e.g., 11Na: (K, L)3s1 or [Ne]3s1)
Example: 20Ca: [Ar]4s2
Hund’s Rule
Hund’s rule applies when a p, d, or f orbital is occupied by more than one electron. It states that unpaired electrons with parallel spins remain in orbitals of equal energy, with each orbital having at least one electron before any orbital holds two.
Example: 15P: [Ne]3s23p3 (↑↓)(↑)(↑)(↑) and not [Ne]3s23p3 (↑↓)(↑↓)(↑)
Chemical Periodicity
Introduction
Modern atomic theory explains why certain elements have similar properties based on electronic structures. Elements with similar outer electron configurations (valence electrons) share many properties. This relationship between structure and properties forms the basis of the periodic law.
Initial Periodic Rankings
- Johann W. Dobereiner: Classified elements into triads (groups of three with similar properties).
- John Newlands: Arranged elements in octaves (groups of eight) by atomic weight, noting recurring properties every eighth element.
- Dmitri Mendeleev and Lothar Meyer: Arranged elements by atomic weight, grouping those with similar properties.
Current Periodic Table
In 1913, Henry Moseley used X-ray experiments to determine atomic numbers, leading to the modern periodic table.
Periodic Law
The periodic law states: “The chemical properties of elements are a periodic function of their atomic numbers.” This means that when elements are arranged by increasing atomic number, groups with similar chemical and physical properties appear periodically.
Organization of the Periodic Table
Elements are arranged in rows (periods) and columns (groups or families). Groups are labeled with Roman numerals and letters (A and B). Group A elements are representative elements, while Group B elements are transition elements. Inner transition elements (lanthanides and actinides) are placed separately.
The periodic table classifies elements into metals, nonmetals, and metalloids. Metals are good conductors, malleable, ductile, and lustrous. Nonmetals are poor conductors, not malleable or ductile, and brittle. Metalloids have intermediate properties.
Location of Elements
An element’s position is determined by its electron configuration: the last energy level indicates the period, and the valence electrons indicate the group.
Representative Elements: Group number equals the sum of s and p electrons in the outermost level.
Example: Element with Z = 35 (1s22s22p63s23p64s23d104p5) is in period 4 and Group VIIA.
Transition Elements: Group number is determined by the sum of d and s electrons.
Example: Element with Z = 47 (1s22s22p63s23p64s23d104p65s14d10) is in period 5 and Group IB.
Rare Earth Elements: Lanthanides are in period 6, and actinides are in period 7.
Behavior of Properties in the Table
- Atomic Radius: Increases down a group and decreases across a period.
- Ionization Energy: Decreases down a group and increases across a period.
- Electronegativity: Increases across a period and decreases down a group.
- Electron Affinity: Increases across a period and decreases down a group.
Properties of Matter
Matter is anything that occupies space and has mass.
General Properties
- Mass: Amount of material in a body.
- Weight: Force of gravity on a body.
- Extension: Property of occupying space.
- Impenetrability: Two bodies cannot occupy the same space simultaneously.
- Inertia: A body remains at rest or in motion unless acted upon by an external force.
- Porosity: Space between molecules.
- Elasticity: Ability to regain original shape after deformation.
- Divisibility: Matter can be divided.
Specific Properties
Specific properties distinguish substances, such as color, smell, taste, state of aggregation, density, boiling point, and solubility.
Classification of Matter
- Heterogeneous Mixture: More than one phase (e.g., water and oil).
- Homogeneous Mixture: Single phase (e.g., salt and water).
- Solution: Homogeneous mixture of more than one substance (e.g., soda).
- Pure Substance: Unchanging chemical composition (e.g., ethanol).
- Element: One type of atom (e.g., N2, Ag).
- Compound: Different types of atoms (e.g., CO2).
Changes of Matter
- Physical Change: Change in state, size, or form without change in composition (e.g., melting ice).
- Chemical Change: Change in composition (e.g., burning paper).
- State Changes: Transitions between solid, liquid, gas, and plasma.
Representation of Compounds
- Symbol: Letter(s) representing an element (e.g., Al).
- Molecule: Two or more atoms bonded in definite proportions.
- Formula: Represents a compound, indicating the type and number of atoms.
Chemical Units
- Mole: Avogadro’s number (6.023 x 1023) of particles.
- Atomic Weight: Weight of one mole of atoms.
- Molecular Weight: Weight of one mole of molecules.
Solutions
A solution is a homogeneous mixture of two or more substances.
Solubility
Solubility is the maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature.
Factors Affecting Solubility
- Contact Surface
- Agitation
- Temperature
- Pressure
Colloids
Colloids are mixtures intermediate between solutions and heterogeneous mixtures.
Properties of Colloids
- Brownian Motion
- Tyndall Effect
- Adsorption
- Electric Charge