Electrons in Atoms and the Periodic Table: Study Notes for Chapter 9

Electrons in Atoms and the Periodic Table

Introduction

This section explores the arrangement of electrons in atoms, the structure of the periodic table, and how these concepts explain chemical properties and periodic trends. Understanding electron configurations and periodic trends is essential for predicting the behavior of elements in chemical reactions.

Electromagnetic Radiation and Atomic Structure

• Electromagnetic Radiation: Energy that travels through space as waves. Examples include visible light, gamma rays, and radio waves.

• Speed of Light: All electromagnetic waves travel at the speed of light in a vacuum, given by: $c = 3.00 \times 10^8 \ \text{m/s}$

• Wavelength and Frequency: Wavelength ($\lambda$) is the distance between two consecutive peaks; frequency ($\nu$) is the number of waves passing a point per second. They are related by: $c = \lambda \nu$

• Energy of Photons: The energy of a photon is given by: $E = h\nu$ where $h = 6.626 \times 10^{-34} \ \text{J} \cdot \text{s}$ (Planck's constant).

• Electromagnetic Spectrum: Gamma rays have the highest energy and shortest wavelength; radio waves have the lowest energy and longest wavelength. Visible light is only a small portion of the spectrum.

• Color Perception: Objects appear colored because they reflect certain wavelengths and absorb others. For example, a red shirt appears red because it reflects red light and absorbs other colors.

Atomic Spectra and Electron Energy Levels

• Atomic Emission Spectrum: Each element emits light at specific wavelengths, producing a line spectrum unique to that element.

• Bohr Model: Electrons occupy specific energy levels (shells) around the nucleus. When electrons move between levels, they absorb or emit energy as photons.

• Principal Quantum Number ($n$): Specifies the main energy level of an electron. Higher $n$ means higher energy and larger orbitals.

Quantum Mechanical Model of the Atom

• Orbitals: Regions in space where there is a high probability of finding an electron. Types include s, p, d, and f orbitals.

• Pauli Exclusion Principle: No two electrons in an atom can have the same set of four quantum numbers. Each orbital can hold a maximum of two electrons with opposite spins.

• Hund's Rule: When filling orbitals of equal energy, electrons fill them singly first, with parallel spins, before pairing up.

• Aufbau Principle: Electrons fill the lowest energy orbitals first.

• Maximum Electrons in Orbitals:

  • s orbital: 2 electrons

  • p orbital: 6 electrons

  • d orbital: 10 electrons

  • f orbital: 14 electrons

Electron Configurations

• Notation: Electron configurations show the arrangement of electrons in an atom's orbitals. Example for fluorine: $1s^2 2s^2 2p^5$

• Noble Gas Abbreviation: Use the symbol of the previous noble gas in brackets to simplify configurations. For bromine: [Ar] $4s^2 3d^{10} 4p^5$

• Valence Electrons: Electrons in the outermost shell, important for chemical reactivity. For chlorine (Cl): 7 valence electrons.

• Examples:

  • Potassium (K): $1s^2 2s^2 2p^6 3s^2 3p^6 4s^1$

  • Arsenic (As): $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^3$ (33 electrons)

Periodic Trends

• Atomic Size: Increases down a group and decreases from left to right across a period.

• Ionization Energy: The energy required to remove an electron from an atom. Decreases down a group, increases across a period.

• Metallic Character: Increases down a group and decreases across a period. Metals are typically found on the left and bottom of the periodic table.

Sample Table: Subshells and Maximum Electrons

Subshell	Number of Orbitals	Maximum Electrons
s	1	2
p	3	6
d	5	10
f	7	14

Summary of Key Concepts

• Electrons occupy orbitals in a specific order, following the Aufbau principle, Pauli exclusion principle, and Hund's rule.

• Electron configurations determine the chemical properties and reactivity of elements.

• The periodic table is organized based on atomic number and recurring chemical properties, which are explained by electron configurations.

• Periodic trends such as atomic size, ionization energy, and metallic character can be predicted using the periodic table.

Additional info: These notes synthesize the main topics and sample questions from the provided pre-test, expanding on electron configurations, quantum numbers, and periodic trends for clarity and completeness.