Atomic Structure and Wave Phenomena

Posted on Nov 21, 2024 in Physics

Item 8: Atomic Structure

Dalton’s Atomic Theory

  1. Matter consists of individual atoms.
  2. Atoms are unchanged in chemical reactions.
  3. Atoms of the same element are identical.
  4. Atoms of different elements have different masses and chemical properties.
  5. Chemical compounds are formed by combining two or more atoms of different elements.
  6. When two or more atoms of different elements combine to form the same compound, they do so in simple whole-number ratios.
  7. In chemical reactions, atoms are not created nor destroyed, only rearranged.

Atomic Particles

  • Electron: Negative charge, smaller mass (9.11 x 10-31 kg, 1.6 x 10-19 C).
  • Proton: Positive charge, larger mass (1.67 x 10-27 kg, 1.6 x 10-19 C).
  • Neutron: No electric charge, similar mass to proton (1.67 x 10-27 kg).

Key Discoveries about Atomic Structure:

  1. The atom is divisible and contains subatomic particles.
  2. Some of these particles (electrons) have a negative electric charge.
  3. The rest of the atom carries most of its mass and a positive charge.
  4. The number of negative charges (electrons) is equal to the number of positive charges (protons) in a neutral atom.

Rutherford Model

  1. Electrification of Matter: Atoms are electrically neutral. Matter becomes electrified by gaining or losing electrons.
  2. Ion Formation: A cation is an atom that has lost one or more electrons. An anion is an atom that has gained one or more electrons.
  3. Rutherford’s Experiment: An atom is mainly empty space. Positive particles aimed at a thin gold foil mostly passed straight through. Some were deflected, and a few bounced back, indicating a dense, positively charged nucleus.

Atomic Spectra and Bohr Model

Atomic emission spectra are the unique sets of electromagnetic radiation emitted by excited atoms of an element. Bohr’s model explained these spectra:

  1. Electrons orbit the nucleus in stable circular orbits without radiating energy.
  2. Each orbit has a specific energy level, increasing with distance from the nucleus.
  3. Electrons radiate energy only when they transition to a lower energy level.

Atomic Identification

  • Atomic Number (Z): The number of protons in an atom.
  • Mass Number (A): The sum of protons and neutrons in the nucleus.
  • Isotopes: Atoms of the same element with the same atomic number but different mass numbers.

Average Atomic Mass Formula: Avg. At. Mass = (m1·%)/100 + …

Radioactivity

Radioactivity is the spontaneous emission of radiation by certain atoms. Types of radiation:

  • Alpha (α) particles: Two protons and two neutrons.
  • Beta (β) particles: Fast electrons.
  • Gamma (γ) radiation: High-energy electromagnetic radiation.

Element Characteristics

Metals

  • Have a gloss.
  • Are opaque.
  • Are good conductors of heat and electricity.
  • Are malleable and ductile.
  • Are solid at ambient temperature (except mercury).

Non-metals

  • Lack luster.
  • Are not good conductors of heat and electricity.
  • Can be solid, liquid, or gaseous at room temperature.
  • Tend to have low melting and boiling points.

Item 7: Wave Movement

Wave movement is a transfer of energy without matter transport. A wave is a periodic disturbance that propagates through a medium or space.

Types of Waves

  • Mechanical waves: Require an elastic medium for propagation (e.g., sound waves).
  • Electromagnetic waves: Can propagate in a vacuum (e.g., light waves).
  • Longitudinal waves: Particle vibrations are parallel to wave propagation.
  • Transverse waves: Particle vibrations are perpendicular to wave propagation.

Wave Terminology:

  • Pulse: A single disturbance.
  • Wave train: A series of repeated disturbances.
  • Focus: The point where the disturbance originates.
  • Wavefront: The line or surface formed by points reached by the disturbance at the same instant.
  • Ray: An imaginary line perpendicular to the wavefront.

Wave Characteristics

  • Propagation velocity (v): Distance traveled per unit time.
  • Wavelength (λ): Distance between two consecutive points vibrating in the same way.
  • Period (T): Time for one wavelength to pass a point.
  • Frequency (f): Number of vibrations per unit time (Hz).
  • Amplitude: Maximum displacement from equilibrium.
  • Intensity (I): Energy flow per unit area per unit time (W/m2).

Formulas:

v = λ/T

f = 1/T

I = P/A

P = E/t

Sound

Sound is a mechanical, longitudinal wave. It requires a medium for propagation and travels faster in solids than in liquids, and faster in liquids than in gases. Key phenomena:

  • Reflection: Sound waves bounce off obstacles. An echo occurs when the reflected sound is heard distinctly from the original sound (reflection surface at least 17 meters away). Reverberation occurs when the reflection is closer than 17 meters, causing a prolonged sound.
  • Refraction: Sound waves bend when passing through media with different temperatures and densities.

Sound Intensity and Pitch:

  • Intensity: Measured in decibels (dB). Human hearing range: 10 dB to 120 dB.
  • Pitch: Determined by frequency (Hz). Low frequency = bass, high frequency = treble. Human hearing range: 20 Hz (infrasound) to 20,000 Hz (ultrasound).

Light

Light is an electromagnetic wave that travels in a straight line at 300,000 km/s in a vacuum. Its speed decreases in denser media.

  • Reflection: Light bounces off surfaces. Specular reflection occurs on smooth surfaces, diffuse reflection on rough surfaces.
  • Refraction: Light bends when passing between media with different refractive indices.

Formulas for Light Refraction:

n = c/v

n = sin(i)/sin(r)

nwater = 1.3