Electromagnetic Radiation

• Ancient Greek philosophers questioned if matter could be infinitely divided or if there was a limit to the process.
• Democritus (460-370 BC) proposed that matter consists of indivisible particles called "atomos" (atoms), a philosophical idea not widely accepted until 1808.

Dalton's Atomic Theory

• John Dalton developed an atomic theory that gained broad acceptance in 1808.
• The theory includes:
• Postulates of Dalton's atomic theory
• Postulates of the modern atomic theory
• Laws of conservation of mass, definite proportions, and multiple proportions
• Using Dalton's atomic theory to explain the laws of definite and multiple proportions.
• Scientific laws develop based on previous scientific findings, such as the law of conservation of mass and the law of definite proportions forming the basis for Dalton's atomic theory.
• Dalton formulated a law based on the conservation of mass and definite proportions but is not explicitly stated.

Laws of Chemical Combination

• Burning wood illustrates that mass is conserved when burning in a closed container, despite the ashes having a different mass than the original wood.
• Sugar (C, H, O) turns into black carbon when burned in a crucible and the hydrogen and oxygen atoms are released as gases.
• Water is always 11.2% hydrogen and 88.8% oxygen by mass.
• Example: 18.0 g of water contains 2.02 g of hydrogen and 15.98 g of oxygen.
• Example: 1.00 g of water contains 0.112 g of hydrogen and 0.888 g of oxygen.

Law of Multiple Proportions

• Compound A contains 1.750 g of nitrogen per 1 g of oxygen.
• Compound B contains 0.8750 g of nitrogen per 1 g of oxygen.
• Compound C contains 0.4375 g of nitrogen per 1 g of oxygen.
• The ratios of nitrogen masses combining with 1 g of oxygen in each pair of compounds are small whole numbers, supporting the law of multiple proportions.
• Calculating ratios: A/C = 4/1, B/C = 2/1.

Significance and Limitations of Dalton's Atomic Theory

• Some of Dalton's postulates are retained in the modern atomic theory.
• Atomic theory explains that the decomposition of 1.00 g of water yields consistent amounts of 0.112 g of hydrogen and 0.888 g of oxygen, regardless of the water source.
• Some of Dalton's postulates are inconsistent with later observations. Dalton's model is still useful despite inconsistencies.

Modern Atomic Theory

• Generalizations from experiments are presented as postulates of the modern atomic theory, starting with John Dalton.
• Dalton's work focused on how atoms combine to form new compounds, while modern theories explore the internal structure of atoms.
• J.J. Thomson's discovery of the electron in 1897 marked the beginning of modern theories about the physical structure of atoms.

Discovery of the Electron

• Cathode rays are deflected by electric and magnetic fields.
• J.J. Thomson (1856-1940) discovered the electron by experimenting with cathode rays.
• Thomson showed that cathode ray characteristics are independent of the cathode material.
• Cathode rays consist of negatively charged particles (electrons) that are constituents of all matter.
• Thomson calculated the ratio of the mass of an electron to its charge as -5.686 x 10^-12 kg/C.
• Robert A. Millikan measured the charge of the electron in 1909 as e = -1.602 x 10^-19 C.
• Electron's mass: 9.109 x 10^-31 kg, calculated using Thomson's ratio and Millikan's charge.

Radioactivity

• Radioactivity is the spontaneous emission of particles and/or radiation from unstable atomic nuclei
• Contradicts Dalton's idea of atoms
• Three types identified:
• Alpha (α) rays: Positively charged particles, mass ~4x hydrogen, charge twice electron's magnitude (identical to helium nuclei)
• Beta (β) rays: Electrons from inside the nucleus, deflected by negatively charged plate
• Gamma (γ) rays: High-energy rays, no charge, not affected by external fields

Structure of the Nucleus

• Thomson proposed the "plum-pudding" model where electrons and protons were randomly distributed.
• Rutherford's experiment (1911): Positively charged particles aimed at gold foil.
• If Thomson's model was correct, the particles would travel straight.
• Result: Some particles deflected, a few went back, disproving Thomson's model.
• Most alpha-particles went through foil undeflected because most of the atom is empty space
• Only a small fraction of alpha-particles was slightly deflected, due to encountering positive charge
• Few alpha-particles bounced back, indicating a concentrated mass and charge in nucleus.

Discovery of the Neutron

• Neutron discovered through alpha-particle scattering experiments: Beryllium irradiated with alpha rays produced penetrating radiation.
• James Chadwick (1891-1974) showed radiation consists of neutral particles (neutrons).
• Neutron mass: (mn = 1.67493 x 10^-27 kg), nearly identical to proton mass, ~1840 times electron mass

Atomic Structure

• Protons (hydrogen nuclei) form when alpha particles strike lighter elements such as nitrogen and have a positive charge equal to electron's magnitude.
• Proton mass: (mp = 1.67262 x 10^-27 kg) about 1840 times the mass of electrons
• Table 1.1 compares relative masses and charges of subatomic particles.
• "amu" (atomic mass unit) equals the mass of 1 atom of carbon-12

Atomic Number

• Atomic number (Z): The number of protons in an atom's nucelus; same for all atoms of a particular element.
• Mass number (A): Total number of protons and neutrons in an atom’s nucleus

Isotopes

• Atoms of an element with identical atomic numbers but different mass numbers.
• Differ in neutron number.
• Carbon: Carbon atoms have six protons, 98.89% have six neutrons(A = 12), small percentage have seven neutrons (A = 13), even fewer have eight (A = 14).
• Carbon has three isotopes: 12C, 13C, and 14C.
• Atomic mass: Average mass for atoms in an element averages relative isotope masses, weighted by observed fractional abundances.
• For n isotopes with relative masses A1, A2 ...An abundances f1, f2 ... fn average relative atomic mass (A) is: A = A1 f1 + A2 f2 + ... + An fn

Calculating Atomic Mass

• Silver has two isotopes: 107Ag (106.90509 amu, 51.84% abundance), 109Ag (108.90476 amu, 48.16% abundance).
• Portion of atomic mass from 107Ag: 106.90509 amu x 0.5184 = 55.42 amu
• Portion of atomic mass from 109Ag: 108.90476 amu x 0.4816 = 52.45 amu
• Atomic mass of silver: 55.42 amu + 52.45 amu = 107.87 amu

Electromagnetic Radiation

• Characterized by: wavelength, frequency, and speed
• Wavelength (λ): Distance wave travels during one cycle.
• Expressed in meters (m).
• Also nanometers (nm), picometers (pm), or angstrom (Å).
• Frequency (v): Number of cycles wave undergoes per second (1/s or hertz, Hz)
• Speed: Depends on wave type and medium travels through
• wave speed (c) equals to the product of the wavelength times frequency
• In vacuum, electromagnetic waves (EMR) travel at 3 x 10^8 m/s (speed of light).
• EMR: Emission and transmission of energy in the form of electromagnetic waves
• Includes range of frequencies (electromagnetic spectrum)
• Different wavelengths in visible light have different colors: red (λ = 750 nm) to violet (λ = 380 nm)
• Radiation transfers energy: Sun reaches Earth with visible and ultraviolet
• Fireplace coals transmit heat energy via infrared radiation

Relating Frequency and Wavelength

• Ethiopian National Radio, Addis Ababa, broadcasts AM signal at 2400 kHz, resulting in radio wave wavelength equals 3.00 x 10^8 m/s / 2.4 x 10^6 s =125m , so, λ = c/v = 3.00 x 10^8 m/s / 2.4 x 10^6
• Earth's most intense radiation emitted at 10.0 µm gives frequency equals to 3.00 x 10^8 m/s / 10 x 10^-6 =3 x 10^13, so, v = 10
• Addis Ababa Fana FM broadcasts at 98.1 MHz gives: wavelength equals 3.00 x 10^8 m/s / 98.1 x 10^6 =3.06m, so, = c/v = 3.00 x 10^8 / 98.1 x 10^6 = 3.06

Quantum Theory

• Max Planck (1900) proposed energy is discontinuous.
• Atoms and molecules emit or absorb energy in discrete bundles (quanta), which the energy equal to the Planck's constant times frequency, so, E = hv (Planck's constant is 6.63 x 10^-34 J.s)
• Energy emitted or absorbed equals to the Planck's constant times the wave speed divoided by wavelength, so, E = hc/λ.
• Energy absorbed or emitted in multiples of hv.
• System transfers energy in whole quanta, showing particulate properties.
• The blue emitted by heating copper(I) chloride (CuCl) equaled the Planck's constant times the wave speed divoided by a wavelength of 600 nm or 6 x 10^-7, so, E = hv = 6.63 x 10^-34 J. s × 0.50 × 10^15 s-1 = 3.315 × 10^-19 J.

Photoelectric Effect

• Albert Einstein (1905)
• Photoelectric effect: Electrons ejected from surface of certain metals exposed to light exceeding a minimum frequency (threshold frequency, vo ).
• EMR is quantized, consisting of photons.
• Minimum energy removes electron from metal surface: (Eo = hvo )
• Photon energy less than Eo (v vo ) produces no electron removal.
• Light has (energy=v > vo), excess energy equals an electrons kinetic energy, which can be calculated by: (KEe = ½ mv2 = hv - hvo )
• Light intensity: Photons in beam, radiation is long as radiation.
• Mass and energy derived Einstein's equation: E = mc2 is equivalent when rearranged, E m c = 2,
• For electromagnetic radiation has a wavelength, apparent mass of photon is: E= h= ( )==22 photon
• Energy is quantized.
• EMR has particle-like characteristics (wave-particle duality).

Wave Properties

• . Electron (mass = 9.11 x 10^-31 kg), travels at 1.00 x 10^7 m/s, wavelength: = h =6.63 x 10 – 3""J.s/9. mve

""11 x 10 kg x 1.00x 10 m/s) = 728 nm.

• Ball (mass = ""7""2.8 0.10 kg), travels at 35 m/s h/=6.63 ""h
• Photoelectrons emitted by light on metal equals approximately 1.5 x 10" -20 " J, while the metal
• Light has 750 nm, thus Solve for wavelength and v
• Light frequency: 3 x 10^8 m/s/()
• Threshold energy has to be lower than or equal to h.

Explore the relationship between frequency, wavelength, and energy of electromagnetic radiation. Understand Maxwell's theory, wave characteristics, and frequency units. Learn about the properties of electromagnetic waves and related calculations.