30.2 Discovery of the Parts of the Atom: Electrons and Nuclei
3 min read•june 18, 2024
The discovery of electrons revolutionized our understanding of atoms. Thomson's cathode ray experiments and revealed these tiny, negatively charged particles. These findings challenged the idea of atoms as indivisible units and paved the way for further atomic exploration.
Rutherford's led to a groundbreaking atomic model. By firing at gold foil, he discovered the dense, positively charged . This replaced earlier concepts, showing atoms were mostly empty space with a tiny central core.
Discovery of Electrons
Thomson's cathode ray experiments
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- Investigated properties of produced by applying high voltage between electrodes in vacuum tube
- Observed cathode rays deflected by electric and magnetic fields indicating negatively charged particles with mass
- Measured charge-to-mass ratio () of cathode rays found much larger than hydrogen ions (lightest known charged particles at the time)
- Concluded cathode rays consisted of negatively charged particles smaller than atoms called "corpuscles" (later named electrons) challenging prevailing idea of indivisible atoms as smallest unit of matter
Millikan's oil drop experiment
- Measured charge of an using apparatus with parallel metal plates, oil droplets, and source to ionize air molecules attaching to droplets giving negative charge
- Observed behavior of individual oil droplets in electric field between plates with upward force on negatively charged droplets and downward gravitational pull
- Adjusted electric field strength to make droplets hover or rise and fall at constant velocity with electric force balancing gravitational force at equilibrium
- Calculated charge on each droplet using known electric field strength, mass of oil droplet (determined by size and density), and velocity of droplet
- Found charges on droplets always multiples of fundamental value determined to be charge of single electron ( C) providing first accurate measurement and confirming quantized nature
Discovery of the Atomic Nucleus
Rutherford's gold foil experiment
- Investigated atomic structure by firing alpha particles (positively charged helium nuclei) at thin gold foil
- Prevailing "plum pudding" atomic model proposed by with positively charged "pudding" and negatively charged electrons embedded throughout
- Expected alpha particles to pass through gold foil with minimal deflection due to evenly distributed positive charge
- Observed most alpha particles passed through undeflected or minimally deflected, small fraction experienced large deflections (>90°), and even smaller fraction bounced straight back
- Concluded large deflections and backscattering explained only if positive charge and most mass concentrated in tiny, dense region at center of atom (nucleus) with mostly empty space allowing most alpha particles to pass undeflected
- Led to development of Rutherford (nuclear) model of atom replacing
Rutherford's model vs previous atomic models
- (early 19th century): Atoms smallest indivisible units of matter with different properties and masses for different elements, cannot be created, divided, or destroyed
- (early 20th century): Atoms positively charged spheres with negatively charged electrons embedded throughout, balanced charges making atom electrically neutral
- (1911):
- Atom consists of small, dense, positively charged nucleus at center with electrons orbiting in large region of empty space
- Nucleus contains most of atom's mass but occupies very small fraction of volume
- Electron orbits similar to planets orbiting sun in solar system
- Key features of Rutherford's model:
- Explained gold foil experiment results including large deflections and backscattering of alpha particles
- Introduced concept of compact, positively charged nucleus
- Established atoms mostly empty space with electrons orbiting far from nucleus
- Limitations of Rutherford's model:
- Did not explain stability of atoms (orbiting electrons should radiate energy and spiral into nucleus)
- Did not account for discrete energy levels of electrons later explained by quantum mechanics
Subatomic Particles and Nuclear Structure
- : Positively charged particle found in the nucleus, determining the atomic number of an element
- : Electrically neutral particle in the nucleus, contributing to atomic mass but not affecting atomic number
- : Atoms of the same element with the same number of protons but different numbers of neutrons
- Atomic number: The number of protons in an atom's nucleus, defining the element's identity
- Radioactivity: Spontaneous emission of particles or energy from unstable atomic nuclei, leading to the discovery of nuclear structure
Key Terms to Review (26)
Alpha particles: Alpha particles are positively charged particles that consist of two protons and two neutrons, making them identical to helium nuclei. They are emitted during certain types of radioactive decay processes, specifically alpha decay, which is a key feature in understanding the structure of atomic nuclei and the nature of nuclear reactions.
Antielectron: An antielectron, also known as a positron, is the antimatter counterpart of an electron. It has the same mass as an electron but carries a positive charge.
Cathode Ray Tube: A cathode ray tube (CRT) is a vacuum-sealed glass or metal container that produces images by using a beam of electrons to strike a phosphorescent screen. This technology was widely used in early television sets and computer monitors before the advent of flat-panel displays.
Cathode Rays: Cathode rays are streams of electrons emitted from the negative electrode, or cathode, of a vacuum tube or similar device. They were a crucial discovery in the early understanding of the structure of atoms and the nature of electricity.
Dalton's model: Dalton's model, proposed by John Dalton in the early 19th century, describes the atom as a solid, indivisible sphere that is the fundamental building block of matter. This model laid the groundwork for modern atomic theory by introducing key concepts such as the idea that each element consists of unique atoms and that atoms combine in specific ratios to form compounds. Dalton's model was significant because it shifted the understanding of matter from classical elements to atomic structures.
Electron: An electron is a fundamental subatomic particle that carries a negative electric charge and is found in all atoms, playing a crucial role in various physical and chemical phenomena. Electrons are responsible for the flow of electric current, the formation of chemical bonds, and the behavior of matter at the atomic and molecular levels.
The concept of the electron is central to understanding topics such as static electricity, electric fields, magnetic fields, the photoelectric effect, quantum mechanics, and the structure of atoms. Electrons are the building blocks of matter and are essential for understanding the fundamental nature of the universe.
Electron volt: An electron volt (eV) is a unit of energy equal to the amount of kinetic energy gained or lost by an electron when it moves through an electric potential difference of one volt. It is commonly used in atomic and particle physics.
Electron Volt: The electron volt (eV) is a unit of energy used in atomic and nuclear physics to measure the energy gained by a single electron when it is accelerated through a potential difference of one volt. It is a fundamental unit that connects the concepts of electric potential, energy, and the behavior of charged particles in various physics contexts.
Ernest Rutherford: Ernest Rutherford was a physicist known as the father of nuclear physics, recognized for his pioneering work in the early 20th century on the structure of the atom. His groundbreaking gold foil experiment led to the discovery of the atomic nucleus, transforming our understanding of atomic structure and laying the groundwork for future theories about atomic particles and their interactions.
Gold Foil Experiment: The gold foil experiment, also known as the Rutherford scattering experiment, was a landmark scientific investigation that provided crucial evidence for the existence of the atomic nucleus. It demonstrated that the majority of an atom's mass is concentrated in a tiny, dense central core, rather than being evenly distributed throughout the atom.
Isotopes: Isotopes are variants of a particular chemical element that have the same number of protons but different numbers of neutrons. This results in different atomic masses while maintaining the same chemical properties.
Isotopes: Isotopes are atoms of the same element that have the same number of protons in their nucleus but a different number of neutrons. This results in atoms with the same atomic number but different atomic masses, leading to unique properties and applications.
J.J. Thomson: J.J. Thomson was a British physicist best known for discovering the electron and proposing the plum pudding model of the atom. His work fundamentally changed the understanding of atomic structure and paved the way for future developments in atomic theory, emphasizing the presence of smaller particles within atoms.
Millikan's Oil Drop Experiment: Millikan's oil drop experiment was a groundbreaking scientific experiment conducted by American physicist Robert Andrews Millikan in the early 20th century. It played a crucial role in the discovery of the parts of the atom, specifically the electron, and provided evidence for the quantized nature of electric charge.
Neutron: A neutron is a subatomic particle found in the nucleus of an atom, possessing no electric charge and a mass slightly greater than that of a proton. Neutrons play a crucial role in the stability of atomic nuclei.
Neutron: A neutron is a subatomic particle that has no electric charge and is found in the nucleus of an atom, along with protons. Neutrons play a crucial role in the stability and properties of atomic nuclei, as well as in various physical and nuclear processes.
Nuclear model: The nuclear model describes the structure of the atom, where a small, dense nucleus containing protons and neutrons is surrounded by a cloud of electrons. This model highlights the arrangement of these subatomic particles and how they interact, fundamentally altering our understanding of atomic structure compared to earlier models that proposed uniform distribution of positive charge throughout the atom.
Nucleus: The nucleus is the central and most important part of an atom, containing the protons and neutrons that make up the atom's core. It is the defining feature of an atom and plays a crucial role in the structure and behavior of matter at the most fundamental level.
Planetary model of the atom: The planetary model of the atom, proposed by Niels Bohr in 1913, depicts electrons orbiting a central nucleus similar to how planets orbit the sun. It introduced quantized electron orbits to explain atomic stability and spectral lines.
Plum pudding model: The plum pudding model is an early 20th-century scientific theory that describes the atom as a uniform sphere of positively charged matter with negatively charged electrons embedded within it, resembling a pudding filled with plums. This model was proposed to explain the internal structure of atoms before the discovery of the atomic nucleus and was pivotal in the exploration of atomic theory during the discovery of the parts of the atom.
Proton: A proton is a subatomic particle that is the positively charged constituent of the nucleus of an atom, with a mass approximately 1,836 times that of an electron. Protons are fundamental to understanding various topics in physics, including static electricity, electric fields, magnetic fields, atomic structure, and nuclear physics.
Proton-proton cycle: The proton-proton cycle is a series of nuclear fusion reactions that convert hydrogen into helium, releasing energy. It is the dominant energy source in stars like the Sun.
Radius of a nucleus: The radius of a nucleus is the distance from the center of the nucleus to its outer edge. It is typically measured in femtometers (fm), where 1 fm = $10^{-15}$ meters.
Rutherford's Planetary Model: Rutherford's planetary model is a representation of the atom that was proposed by the physicist Ernest Rutherford in 1911. This model describes the atom as having a small, dense, positively charged nucleus at the center, with negatively charged electrons orbiting the nucleus in circular paths, similar to the way planets orbit the sun.
Thomson's Plum Pudding Model: The Thomson's plum pudding model was a proposed model of the atom developed by British physicist J.J. Thomson in 1904. It described the atom as a positively charged sphere with negatively charged electrons embedded within it, similar to the structure of a plum pudding dessert.
X-ray: X-rays are a type of electromagnetic radiation with wavelengths shorter than visible light, making them capable of penetrating various materials. They are widely used in fields such as physics, medicine, and industry, with applications ranging from medical imaging to materials analysis.