Fiche de révision : Fundamentals of Atomic Structure and Models

📋 Course Outline

1. Origins of atomic theory
2. Thomson’s plum pudding model
3. Rutherford’s gold foil experiment
4. Bohr’s atomic model
5. Atomic mass and the neutron
6. Atomic number and mass number
7. Electronic configuration and valency
8. Isotopes and their uses

📖 1. Origins of atomic theory

🔑 Key Concepts & Definitions

• Parmanus : Parmanus are smallest, endlessly divisible particles proposed in ancient Indian thought, said to be too small for the senses.
• Atomos : Atomos are indivisible particles proposed by ancient Greek thinkers as the basic constituents of matter.
• Dalton’s atomic theory : Dalton’s atomic theory states that all matter is made of indivisible atoms that act as the fundamental building blocks.

📝 Essential Points

• Ancient Indian thinkers (in the Vaisesika Sutras) proposed parmanus as infinitely small particles that cannot be perceived by the senses.
• Ancient Greek philosophers used atomos as the name for indivisible particles, forming a similar idea to parmanus.
• The word atom began as an imaginary concept rather than something confirmed by experiments.
• In 1808, John Dalton proposed a scientific atomic theory based on experiments available at that time.
• Dalton’s model treated atoms as indivisible particles that cannot be broken into smaller parts. (i)

💡 Memory Hook

Parmanus (India) and atomos (Greece) are “imagination roots,” while Dalton (1808) is “experiment roots” of atomic theory.

📖 2. Thomson’s plum pudding model

🔑 Key Concepts & Definitions

• Cathode rays : Cathode rays are beams of negatively charged particles produced when electric discharge passes through low-pressure gas in a cathode ray tube.
• Electron : An electron is a tiny negatively charged subatomic particle found in every atom, identified through cathode-ray experiments.
• Plum pudding model : Plum pudding model is an atomic model where an atom is a uniform sphere of positive charge with electrons embedded throughout it.
• Watermelon analogy : Watermelon analogy is a picture-based comparison where the positively charged pulp represents positive charge and the seeds represent embedded electrons.

📝 Essential Points

• Thomson’s 1897 cathode-ray studies showed rays travel from cathode to anode and are streams of particles with much smaller mass than atoms.
• Thomson took the electron charge as −1 by convention, even though the measured value is −1.602×10−19 C.
• Because atoms are electrically neutral, Thomson placed positive charge in the atom and distributed electrons within it so total positive and negative charges balance.
• Thomson’s plum pudding model predicts that an incoming α-particle would pass straight through a gold foil with only slight deflection if positive charge is spread evenly.
• Thomson received the Nobel Prize in Physics in 1906 for his work on electrical conductivity of gases that led to the discovery of electrons.

💡 Memory Hook

Plum pudding atom: positive “pudding” fills the sphere, and negative “plums” (electrons) are scattered inside to keep the atom neutral.

📖 3. Rutherford’s gold foil experiment

🔑 Key Concepts & Definitions

• Alpha scattering experiment : An alpha scattering experiment sends a narrow alpha-particle beam at a thin metal foil to test how atoms deflect charged particles.
• Scattering of alpha particles : Scattering is the deviation of alpha particles from a straight path after they interact with charges in the foil’s atoms.
• Nuclear (planetary) model : The nuclear (planetary) model states that an atom has a tiny dense nucleus containing positive charge while electrons move around it.

📝 Essential Points

• In 1911 Geiger and Marsden, working with Rutherford, tested Thomson’s model by firing alpha particles at an extremely thin gold foil.
• Thomson’s model predicted most alpha particles would pass straight through the foil with only slight deflection.
• Rutherford’s team observed that most alpha particles passed undeflected, but some were deflected through large angles and a few bounced back.
• The deflection results led Rutherford to conclude that positive charge is concentrated in a very small nucleus rather than spread evenly.
• Rutherford estimated the atom’s diameter as ≈10−10 m and the nucleus diameter as ≈10−15 m, so the nucleus is about 105 times smaller than the atom.

💡 Memory Hook

Thin gold foil: mostly empty passes, a few big hits—only a tiny dense nucleus can cause sharp deflections or backscattering.

📖 4. Bohr’s atomic model

🔑 Key Concepts & Definitions

• Stationary states : Stationary states are Bohr’s postulated energy states where an electron’s energy stays constant while it moves around the nucleus.
• Energy levels K L M N : Energy levels are allowed electron shells in Bohr’s model, labelled K, L, M, N or by shell number $n=1,2,3,4,\dots$.
• Electron transitions by energy quanta : Electron transitions occur when an electron absorbs or releases a fixed energy equal to the difference between two shell energies.

📝 Essential Points

• Bohr’s model (1913) says electrons move in fixed circular paths called stationary states rather than randomly in all directions.
• Electrons can exist only in allowed shells, and while in a fixed shell they do not lose energy.
• The K-shell ($n=1$) is closest to the nucleus and has the least energy, with energy increasing for L ($n=2$), M ($n=3$), and so on.
• Shell labels K, L, M, N come from early X-ray line naming, with Charles Barkla calling the first observed line K.

💡 Memory Hook

Stability = fixed shells: no energy loss while orbiting; change shells only by absorbing or releasing exactly the energy gap.

📖 5. Atomic mass and the neutron

🔑 Key Concepts & Definitions

• Neutron : A neutron is a subatomic particle in the nucleus with zero charge that helps determine nuclear stability and atomic mass.
• Nuclear force : Nuclear force is the strong interaction that binds the particles inside the nucleus together, especially in heavier atoms.
• Neutrons in heavier nuclei : Heavier atoms contain many more neutrons than protons, unlike lighter atoms that often have nearly equal proton and neutron counts.
• James Chadwick discovery : James Chadwick discovered the neutron in 1932, explaining atomic mass and transforming research in atomic and nuclear physics.

📝 Essential Points

• Protons in a nucleus repel each other due to like positive charges, but neutrons reduce this repulsion by intervening between protons and increasing their separation.
• Neutrons strengthen the nuclear force, so heavier atoms need many more neutrons to keep the nucleus tightly bound.
• Iron has 26 protons and 30 neutrons, and uranium has 92 protons and 146 neutrons.
• James Chadwick discovered the neutron in 1932 and received the Nobel Prize in Physics in 1935 for this breakthrough.
• Because neutrons are uncharged, they can penetrate nuclei and helped enable artificial radioactive elements and the splitting of uranium atoms.

💡 Memory Hook

Protons push; neutrons intervene and hold—without neutrons, nuclei can’t stay tightly together.

📖 6. Atomic number and mass number

🔑 Key Concepts & Definitions

• Atomic number Z : Atomic number is the count of protons in an atom’s nucleus and uniquely identifies the element.
• Mass number A : Mass number is the total number of nucleons in an atom’s nucleus, given by protons plus neutrons.
• Nucleons : Nucleons are the particles in the nucleus consisting of protons and neutrons.

📝 Essential Points

• Mass number satisfies $A=\text{number of protons}+\text{number of neutrons}$ because proton and neutron masses are roughly equal while the electron mass is negligible.
• In standard nuclide notation, the atomic number is written as the lower left value $Z$ and mass number as the upper left value $A$, e.g. $^{12}_{6}\text{C}$.
• For hydrogen, helium, and lithium the (p+, n0, A) values are (1,0,1), (2,2,4), and (3,4,7) respectively.

💡 Memory Hook

Z counts protons (element ID), while A counts protons + neutrons (nucleus total).

📖 7. Electronic configuration and valency

🔑 Key Concepts & Definitions

• Valence shell : The valence shell is the outermost electron shell of an atom where electrons are located for bonding.
• Valence electrons : Valence electrons are the electrons present in an atom’s outermost valence shell that determine combining capacity.
• Octet rule : The octet rule states that atoms are most stable when the valence shell has a complete set of electrons, typically 8.

📝 Essential Points

• Atoms with a complete octet (8 electrons) in the outermost shell, or 2 electrons for helium, are largely unreactive and stable.
• Incomplete valence shells make atoms more reactive because they try to gain, lose, or share electrons to complete a stable outer shell.
• Valency is the number of electrons that an atom gains, loses, or shares to achieve an octet-based stable configuration.
• If an element has fewer than 4 valence electrons, it generally tends to lose electrons to complete its octet.
• If an element has more than 4 valence electrons, it generally tends to gain electrons to complete its octet.
• Carbon with configuration 2,4 has valency 4 by sharing four electrons to complete its octet.

💡 Memory Hook

Octet = “8 is the goal”: stable outer shell comes from losing, gaining, or sharing electrons to reach 8 (or 2 for He).

📖 8. Isotopes and their uses

🔑 Key Concepts & Definitions

• Isotopes : Isotopes are atoms of the same element that share the same atomic number but have different mass numbers.
• Weighted average atomic mass : Weighted average atomic mass is the average mass of an element found by multiplying each isotope’s mass by its natural fractional abundance and adding the results.
• Isobars : Isobars are atoms of different elements that have the same mass number but different atomic numbers.

📝 Essential Points

• Isotopes do not mean any single atom has a fractional atomic mass; the fractional result is only for the element’s average over a large number of atoms.
• For chlorine with ³⁵Cl ≈ 75% and ³⁷Cl ≈ 25%, the weighted average atomic mass is 35.5 u.
• The weighted average atomic mass is computed as $\sum (\text{isotope mass} \times \text{fractional abundance})$ rather than as a simple mean.
• For bromine with ⁷⁹Br = 49.7% and ⁸¹Br = 50.3%, the average atomic mass is about 80.0 u.
• Atoms with the same mass number but different atomic numbers are called isobars.

💡 Memory Hook

Weighted average = mass × abundance; simple average ignores abundance and can misrepresent what exists naturally.

📅 Key Dates

📊 Synthesis Tables

Atomic models (idea to evidence)

⚠️ Common Pitfalls & Confusions

1. Confusing “atomos/parmanus” (imagined/smallest particles) with Dalton’s “experiment roots” in 1808.
2. Thinking Thomson’s model predicted electrons would fall into the nucleus; the stability issue is the later problem Rutherford/Bohr address.
3. Saying Rutherford concluded electrons move around the nucleus because gold foil shows electron motion; the deflection results establish a concentrated nucleus/positive charge and emptiness.
4. Mixing up atomic number Z (protons) with mass number A (protons + neutrons).
5. Assuming isotopes have different chemical properties; the source says chemical properties are similar because electrons/valence electrons are the same.
6. Using a simple arithmetic mean to compute atomic mass when the source specifies weighted average by natural abundance.
7. Believing Bohr electrons can have any energy or any position “anywhere”; the source says electrons can only be in allowed shells/stationary states and energy changes only between levels.

✅ Exam Checklist

1. State what parmanus and atomos mean and explain how the word “atom” originated as an imaginary idea versus Dalton’s 1808 scientific atomic theory.
2. Describe cathode rays, what Thomson concluded about them being negatively charged particles, and why he needed to place positive charge in his model to keep atoms neutral.
3. Explain what the gold foil (α-ray scattering) experiment involved and what results contradicted Thomson’s evenly spread positive charge model.
4. Using Rutherford’s conclusions, state what was inferred about the nucleus (size, concentrated positive charge, most of the atom being empty space).
5. Explain the stability limitation of Rutherford’s model (accelerating electrons should lose energy and spiral inward) and why Bohr’s stationary states address this.
6. List Bohr’s key postulates: fixed shells/energy levels (K, L, M, N / n), no energy loss in a shell, and energy change only by absorbing/releasing the difference between shell energies.
7. Define neutron and use the source’s explanation for why neutrons affect nuclear repulsion and why heavier atoms need more neutrons.
8. Given an atom, compute atomic number Z from protons and mass number A as protons + neutrons (and infer neutrons when enough information is given).
9. Use standard nuclide notation to interpret which value is Z and which is A, and correctly apply that electrons equal protons for a neutral atom.
10. Apply electron-shell rules for maximum electrons in shells (2n²) and describe the stepwise filling order K, L, M, N.
11. Use the octet rule/valence idea: identify stable configurations (8 electrons, or 2 for helium) and determine valency as the number of electrons gained/lost/shared to reach an octet.
12. Differentiate isotopes (same Z, different A) and isobars (same A, different Z) and compute weighted average atomic mass using relative abundances as in the chlorine and bromine examples.