Electrons in Atoms and the Periodic Table

Introduction: Reactivity and Inertness of Elements

The chemical behavior of elements, such as the reactivity of hydrogen and the inertness of helium, can be explained by the arrangement of electrons in atoms. Understanding these arrangements is essential for predicting chemical properties and trends in the periodic table.

Models of the Atom

Development of Atomic Models

Atomic models have evolved to explain the observed chemical properties of elements. Early models, such as the Bohr model, were later refined into the quantum-mechanical model, which better accounts for the behavior of electrons in atoms.

Light: Electromagnetic Radiation

Nature of Light

Light is a form of electromagnetic radiation, a type of energy that travels through space at the speed of light, m/s. Light exhibits both wave-like and particle-like properties, described as photons.

Wave Properties of Light

• Wavelength (\(\lambda\)): The distance between adjacent wave crests.

• Frequency (\(\nu\)): The number of wave cycles passing a point per second.

• Wavelength and frequency are inversely related:

Visible Light and Color

White light contains all visible wavelengths, which can be separated into colors using a prism. Red light has the longest wavelength (~750 nm), and violet has the shortest (~400 nm).

The Electromagnetic Spectrum

The electromagnetic spectrum includes all types of electromagnetic radiation, from gamma rays (shortest wavelength, highest energy) to radio waves (longest wavelength, lowest energy). Visible light is only a small portion of this spectrum.

Photons and Energy

• Photon: A particle of light, or a packet of electromagnetic energy.

• The energy of a photon is inversely proportional to its wavelength: , where is Planck's constant.

Atomic Spectra and Models

Emission Spectra of Elements

When elements are energized, they emit light at specific wavelengths, producing line spectra unique to each element. This phenomenon is explained by the arrangement of electrons in atoms.

The Bohr Model of the Atom

The Bohr model proposes that electrons travel in fixed orbits around the nucleus, each with a specific energy. Electrons can move between orbits by absorbing or emitting energy as photons.

The Quantum-Mechanical Model

Orbitals and Probability Maps

The quantum-mechanical model replaces fixed orbits with orbitals—regions of space where electrons are likely to be found. These are described by probability maps, not exact paths.

Quantum Numbers and Orbital Types

• Principal quantum number (n): Specifies the energy level (shell).

• Subshells: Indicated by letters (s, p, d, f), each with a characteristic shape.

Electron Configurations

Writing Electron Configurations

Electron configurations show how electrons occupy orbitals in an atom. The order of filling is determined by increasing energy, following the Aufbau principle, Pauli exclusion principle, and Hund's rule.

• Aufbau principle: Lower-energy orbitals fill before higher-energy orbitals.

• Pauli exclusion principle: Each orbital holds a maximum of two electrons with opposite spins.

• Hund's rule: Orbitals of equal energy are singly occupied before any is doubly occupied.

Noble Gas Core Notation

For elements beyond neon, electron configurations can be abbreviated using the symbol of the previous noble gas in brackets, followed by the remaining configuration.

Valence and Core Electrons

Valence electrons are those in the outermost shell and are responsible for chemical bonding. Core electrons are all other electrons in lower energy shells.

The Periodic Table and Electron Configurations

Blocks and Patterns

The periodic table is divided into blocks (s, p, d, f) based on the type of orbital being filled. Elements in the same group have similar valence electron configurations and chemical properties.

Periodic Trends

Atomic Size

• Atomic size decreases across a period (left to right) due to increasing nuclear charge.

• Atomic size increases down a group due to increasing principal quantum number (n).

Ionization Energy

• Ionization energy increases across a period (harder to remove electrons as atoms approach noble gas configuration).

• Ionization energy decreases down a group (outer electrons are farther from the nucleus and easier to remove).

Metallic Character

• Metallic character decreases across a period and increases down a group.

• Metals tend to lose electrons, forming cations; nonmetals tend to gain electrons.

Applications and Health Connections

Ionizing Radiation and Cancer Treatment

X-rays and gamma rays are forms of ionizing radiation that can damage biological molecules. In medicine, these are used to target and destroy cancer cells, though care is taken to minimize damage to healthy tissue.

Ions in Biological Systems

Sodium (Na+) and potassium (K+) ions are essential for nerve signal transmission in the human body, maintained by ion pumps in cell membranes.

Summary of Key Concepts

• Light is electromagnetic radiation with both wave and particle properties.

• The Bohr model explains hydrogen's emission spectrum but is limited for multi-electron atoms.

• The quantum-mechanical model describes electrons in orbitals, predicting chemical properties and periodic trends.

• Electron configurations determine the arrangement of electrons and explain periodic trends in atomic size, ionization energy, and metallic character.