The Nuclear Atom

You will learn

⚛️
​Atomic models
​Rutherford Scattering Experiment
​Nuclear atom : Atomic number; Atomic Mass; Isotopes
​Nuclear Fission
​Nuclear Fusion

Note: Topics given an asterisk sign * means its additional information for your
knowledge.

Over the years, scientists have proposed several atomic models to explain the
structure and behavior of atoms. Here’s a brief overview of the most significant
ones:
1. Dalton’s Atomic Model
Date: 1807
Key Concepts:
- John Dalton proposed that atoms are indivisible particles that make up elements.
- Atoms of the same element are identical in mass and properties, while atoms of
different elements vary in these aspects.
- Atoms combine in whole-number ratios to form compounds.
2. Thomson's Plum Pudding Model
Date: 1897
Key Concepts:
- J.J. Thomson discovered electrons and proposed that atoms consist of negatively
charged electrons embedded within a positively charged "soup," similar to plums in
a pudding.
- This was the first model to show the existence of internal structure within
atoms.

3. Rutherford’s Nuclear Model

Date: 1911
Key Concepts:
- Ernest Rutherford’s gold foil experiment led him to propose that atoms have a
small, dense, positively charged nucleus at the center.
- Electrons orbit this nucleus, but most of the atom is empty space.
- This model introduced the concept of the nucleus.

4. Bohr’s Model*
Date: 1913
Key Concepts:
- Niels Bohr suggested that electrons orbit the nucleus in specific energy levels or
shells.
- Electrons can jump between these levels by absorbing or emitting energy.
- This model explained the stability of atoms and the discrete lines seen in atomic
spectra.

5. Quantum Mechanical Model*

Date: 1926 onwards
Key Concepts:
- Developed by scientists including Erwin Schrödinger and Werner Heisenberg.
- This model introduces the concept of electron clouds or orbitals, where electrons
are found with a certain probability rather than specific orbits.
- It incorporates the principles of quantum mechanics to provide a more accurate
and complex description of electron behavior.

These models have evolved as new discoveries were made, each contributing to our
understanding of atomic structure.

Rutherford Scattering Experiment

Setup of the Experiment

- A thin sheet of gold foil (about 0.00004 cm thick) was used as the target.
- Alpha particles (helium nuclei) were emitted from a radioactive source (radium),
directed at the gold foil.
- A fluorescent screen coated with zinc sulfide was placed around the foil to
detect scattered alpha particles. When alpha particles struck the screen, they
produced tiny flashes of light.

Observations
- Most of the alpha particles passed through the gold foil with little to no
deflection.
- Some alpha particles were deflected at small angles.
- A few alpha particles (around 1 in 8000) were deflected at very large angles,
some even bouncing back toward the source.

Conclusion
These observations were groundbreaking and led Rutherford to deduce the
following:

1. Atom is Mostly Empty Space: The fact that most alpha particles passed
through the foil without deflection indicated that the atom is largely empty space.

2. Dense Central Nucleus: The large-angle deflections suggested that the alpha
particles were encountering a dense, positively charged region within the
atom—this was later termed the nucleus. This region was incredibly small compared
to the overall size of the atom.

3. Electrons Orbit the Nucleus: The electrons were thought to orbit the nucleus,
similar to planets around the sun, with the remainder of the volume of the atom
being empty space.

Impact and Importance*

- Disproved Thomson’s Plum Pudding Model: The experiment showed that the “plum
pudding” model, which suggested electrons were distributed within a diffuse
positive charge, was incorrect.
- Nuclear Model of the Atom: Rutherford’s findings led to the nuclear model of the
atom, where a tiny, dense nucleus is surrounded by electrons.
- Foundation of Modern Physics: The nuclear model laid the groundwork for further
developments in atomic theory, including the Bohr model and eventually quantum
mechanics.

Visualization
Imagine shooting a bullet at a tissue paper and having it bounce back. This
unexpected result is akin to what Rutherford and his team observed when alpha
particles were deflected by the nucleus, showcasing the concentrated mass and
charge of the nucleus.

The Rutherford Scattering Experiment remains one of the most significant

experiments in physics, revealing the true structure of the atom and deeply
influencing our understanding of atomic physics.

Nucleus of atom

The nucleus is the central core of an atom, containing the majority of its mass and
all of its positive charge. Discovered by Ernest Rutherford in 1911, it’s a tiny,
dense region at the atom’s center. Here’s a detailed breakdown:

Components of the Nucleus

1. Protons:
- Positively charged particles.
- The number of protons (atomic number) defines the element. For example, all
carbon atoms have 6 protons.
 - Each proton has a charge of +1 and a relative mass of 1 atomic mass unit (amu).

2. Neutrons:
- Neutrally charged particles (no charge).
- Contribute to the atomic mass but not to the charge.
- Similar in mass to protons, roughly 1 amu.
- The number of neutrons can vary within atoms of the same element, leading to
different isotopes.

Properties of the Nucleus

- Size: Extremely small compared to the overall size of the atom. If the atom were
the size of a football stadium, the nucleus would be about the size of a small
marble.
- Density: Highly dense, containing nearly all the mass of the atom in a very small
volume.
- Charge: Positively charged due to the presence of protons. The charge is
balanced by the negatively charged electrons orbiting the nucleus.

Role of the Nucleus*

- Chemical Identity: Determines the chemical identity of the element through its
number of protons.
- Stability and Radioactivity: Neutrons contribute to stability. A balanced ratio of
protons to neutrons leads to stable nuclei. An imbalance can result in radioactive
decay, where the nucleus emits particles or radiation to achieve stability.
- Nuclear Reactions: Nuclei can undergo reactions, such as fusion (joining of nuclei)
and fission (splitting of nuclei), which release immense amounts of energy.

Forces within the Nucleus*

- Strong Nuclear Force: The fundamental force that holds protons and neutrons
together, overcoming the electrostatic repulsion between the positively charged
protons.
- Weak Nuclear Force: Plays a role in certain types of nuclear decay, such as beta
decay, where a neutron converts to a proton or vice versa.
 Atomic mass (A) and Atomic number (Z). XzA
Atomic Number (Z)
The atomic number represents the number of protons in the nucleus of an atom.
It defines the element and its position in the periodic table.
Here's why it's important:
- Identity of the Element: The atomic number determines what element an atom is.
For example:
- Hydrogen (H): Atomic number 1. Every hydrogen atom has 1 proton.
- Carbon (C): Atomic number 6. Every carbon atom has 6 protons.
- Oxygen (O): Atomic number 8. Every oxygen atom has 8 protons.
- Chemical Properties: Since the chemical behavior of an atom is largely dictated
by its electrons, and electrons equal protons in a neutral atom, the atomic number
also influences the chemical properties of the element.

Atomic Mass (A)

The atomic mass (also called atomic weight or mass number) is the total number
of protons and neutrons in an atom's nucleus. Here’s how it's calculated:
- Protons and Neutrons: Both carry approximately 1 atomic mass unit (amu).
- Electrons: Their mass is so small that it's often negligible in the calculation of
atomic mass.
- Isotopes: Atoms of the same element can have different numbers of
neutrons, resulting in different atomic masses. These variations are called
isotopes.

Examples:
Hydrogen (H)

- Atomic Number: 1
- Atomic Mass:
- Protium (¹H): No neutrons, atomic mass = 1 amu (1 proton + 0 neutrons)
- Deuterium (²H): 1 neutron, atomic mass = 2 amu (1 proton + 1 neutron)
- Tritium (³H): 2 neutrons, atomic mass = 3 amu (1 proton + 2 neutrons)

Carbon (C)

- Atomic Number: 6
- Atomic Mass:
- Carbon-12 (¹²C): 6 neutrons, atomic mass = 12 amu (6 protons + 6 neutrons)
- Carbon-13 (¹³C): 7 neutrons, atomic mass = 13 amu (6 protons + 7 neutrons)
- Carbon-14 (¹⁴C): 8 neutrons, atomic mass = 14 amu (6 protons + 8 neutrons)

Oxygen (O)
- Atomic Number: 8
- Atomic Mass:
- Oxygen-16 (¹⁶O): 8 neutrons, atomic mass = 16 amu (8 protons + 8 neutrons)
- Oxygen-17 (¹⁷O): 9 neutrons, atomic mass = 17 amu (8 protons + 9 neutrons)
- Oxygen-18 (¹⁸O): 10 neutrons, atomic mass = 18 amu (8 protons + 10 neutrons)

Understanding the Periodic Table*

In the periodic table, elements are arranged in order of increasing atomic number.
Each element’s box typically includes:
- Symbol: The one- or two-letter abbreviation of the element.
- Atomic Number: Shown at the top of the box.
- Atomic Mass: Usually displayed below the symbol, often as an average of all
isotopes weighted by their abundance.

Summary
- Atomic Number (Z): The number of protons in the nucleus, defining the element.
- Atomic Mass (A): The total number of protons and neutrons in the nucleus,
reflecting the isotope's mass.

Significance of Isotopes*
- Scientific Research: Isotopes are used in various scientific fields such as
archaeology (radiocarbon dating), medicine (radioisotopes in diagnostic imaging like
PET scans), and environmental science (tracing chemical pathways).
- Industrial Applications: Used in energy production (nuclear reactors), agriculture
(fertilizer efficiency studies), and many other industries.

Conclusion
Isotopes provide insight into the subtle nuances of atomic structure and behavior,
enriching both theoretical studies and practical applications.
Conservation of charge and mass in nuclear
reaction.
In nuclear reactions, both the conservation of charge and mass are fundamental
principles that must be maintained. This ensures that the physical laws of nature
hold true, even during these high-energy processes.

Conservation of Charge
The principle of conservation of charge states that the total electric charge in a
closed system remains constant over time. Here’s how it applies to nuclear
reactions:
- Charge Balance: The total charge before the reaction must equal the total charge
after the reaction.
- Protons and Electrons: Protons carry a positive charge, while electrons carry a
negative charge. Neutrons are neutral and do not affect charge conservation.
- Example: Alpha Decay
- Initial State: A nucleus of radium-226 (226Ra, atomic number 88) undergoes
alpha decay.

- Charge Check:
- Radium (Ra) has an atomic number of 88 (88 protons).
- Resulting radon (Rn) has an atomic number of 86 (86 protons) and the alpha
particle (He) has an atomic number of 2 (2 protons).
- Total charge before and after the reaction remains ( 88 = 86 + 2 ).
Conservation of Mass (Mass-Energy Equivalence)
While the mass is not conserved in the classical sense due to the conversion of
mass into energy (and vice versa) as described by Einstein's equation ( E=mc2), the
overall mass-energy is conserved. Here’s how:
- Mass Number Balance: The sum of the mass numbers (total protons and
neutrons) before and after the reaction remains the same.
- Example:
Nuclear Fission
Nuclear fission is a nuclear reaction in which a heavy & unstable atomic nucleus
splits into two or more smaller nuclei, along with a few neutrons and a significant
amount of energy. This process is fundamental to both energy production in nuclear
power plants and the explosive power of atomic bombs.

Nuclear Fission Works

1. Initiation: Fission typically starts when a heavy nucleus (often uranium-235 or
plutonium-239) captures a neutron.
2. Excitation: This capture makes the nucleus unstable and causing it to become
excited.
3. Splitting: The excited nucleus splits into two (or more) smaller nuclei, called
fission fragments.
4. Release of Neutrons: Additional neutrons are released during the fission. These
neutrons can then initiate fission in other nuclei, creating a chain reaction.
5. Energy Release: The mass of the resulting fragments and released neutrons is
slightly less than the original mass of the nucleus and the neutron. This "missing"
mass is converted into energy according to Einstein's equation (E=mc^2), releasing
a vast amount of energy. About 200 MeV (million electron volts) per fission event.

- Initial State: A nucleus of uranium-235 (235U) undergoes fission when struck

by a neutron.
- Reaction:
 - Mass Check:
- Uranium-235 has a mass number of 235.
- Total mass number before reaction: 235 + 1 (neutron) = 236.
- Resulting Barium-144 (144Ba), Krypton-89 (89Kr), and three neutrons (3n).
- Total mass number after reaction: ( 144 + 89 + 3 = 236 ).
- The difference in binding energy is released as energy.

Nuclear Fusion
Nuclear fusion is a process in which two light atomic nuclei combine to form a
heavier nucleus, releasing an enormous amount of energy. This is the process that
powers the sun and other stars, providing a limitless source of energy.

- Example: Fusion of deuterium and tritium.

- Reaction:

- Mass Check:
- Deuterium (2D) has 1 proton and 1 neutron.
- Tritium (3T) has 1 proton and 2 neutrons.
 - Resulting helium (4He) has 2 protons and 2 neutrons, and one neutron (n) left
over.
- Total mass number before: ( 2 + 3 = 5 ).
- Total mass number after: ( 4 (helium) + 1 (neutron) = 5 ).
- Charge Check: Protons before: 1 (deuterium) + 1 (tritium) = 2. Protons after: 2
(helium) = 2. Charge conserved.

Conditions for Fusion

- Extremely High Temperature: Millions of degrees Celsius, often achieved in stars
naturally by gravitational compression.
- High Pressure: To keep the nuclei close enough for long enough to fuse.
- Containment: Creating and maintaining such extreme conditions is a significant
challenge on Earth, requiring advanced technology like magnetic confinement
(tokamaks) or inertial confinement (lasers).

How Nuclear Fusion Works ?

1. High Temperature and Pressure: Fusion requires extremely high temperatures
(millions of degrees) and pressure to overcome the electrostatic repulsion between
the positively charged nuclei.
2. Combining Nuclei: Under these conditions, the kinetic energy of the nuclei is
high enough to bring them close together, allowing the strong nuclear force to take
over and bind them together.
3. Formation of a Heavier Nucleus: The fusion of the light nuclei forms a heavier
nucleus and releases a large amount of energy in the form of light and heat.
4. Release of Neutrons and Energy: The process also releases neutrons and
additional energy, which is significantly more than what we see in nuclear fission.

Applications of Nuclear Fusion

Potential as an Energy Source

- Sustainable Energy: Fusion fuel, like deuterium extracted from water, is
abundant and widely available.
- High Energy Output: Fusion reactions release significantly more energy than
fission reactions for the same amount of fuel.
- Safety and Environmental Benefits: Fusion produces minimal radioactive waste
compared to fission and has no risk of catastrophic meltdowns.

Current Research and Developments*

- ITER (International Thermonuclear Experimental Reactor): A major international
project aimed at demonstrating the feasibility of fusion as a large-scale and
carbon-free energy source.
- Tokamaks: Magnetic confinement devices designed to contain the hot plasma
needed for fusion.
- Laser Fusion: Uses powerful lasers to compress and heat the fuel to initiate
fusion, exemplified by the National Ignition Facility (NIF).

Environmental Impact
- Advantages:
- Virtually Unlimited Fuel: The primary fuel for fusion is deuterium, found in
seawater, and tritium, which can be bred from lithium.
- Minimal Waste: Fusion produces far less radioactive waste compared to fission.
- Challenges:
- Technological Hurdles: Achieving and maintaining the necessary temperature and
pressure for a sustained fusion reaction on Earth remains a major technological
challenge.
- Lengthy Development: Despite significant progress, practical and commercial
fusion energy is still likely a few decades away.

Summary
Nuclear fusion offers the promise of virtually limitless, clean energy by mimicking
the processes that power the sun and stars. While significant hurdles remain in
harnessing this energy on Earth, the potential benefits make it a highly
sought-after goal in scientific research.
Conclusion
The conservation of charge and mass, alongside energy considerations, governs the
behavior of particles in nuclear reactions, ensuring that these reactions adhere to
the fundamental principles of physics. This balance is crucial for the stability of
matter and the continuation of predictable natural laws.

Applications of Nuclear Fission

Nuclear Power Plants

- Energy Production: Fission of uranium or plutonium nuclei releases heat, which is
used to generate steam. This steam drives turbines connected to generators,
producing electricity.
- Controlled Reaction: The chain reaction is carefully controlled using control rods,
which absorb excess neutrons, preventing the reaction from becoming too rapid.

Nuclear Weapons
- Uncontrolled Reaction: Unlike power plants, nuclear weapons are designed to allow
an uncontrolled chain reaction, leading to a massive release of energy in a very
short time, resulting in an explosion.
- Example: The bombs dropped on Hiroshima (using uranium-235) and Nagasaki
(using plutonium-239) during World War II utilized fission reactions.

Environmental Impact
- Advantages:
- High Energy Density: A small amount of nuclear fuel produces a vast amount of
energy.
- Low Greenhouse Gas Emissions: Nuclear power plants contribute minimally to
GHG emissions compared to fossil fuels.
- Disadvantages:
- Radioactive Waste: Disposal and management of radioactive waste remain
significant challenges.
- Accident Risk: Events like the Chernobyl and Fukushima disasters highlight the
potential dangers.
Summary
Nuclear fission is a powerful process that has both beneficial applications, like
energy production, and potential dangers, like nuclear weapons. Its understanding
and application require careful management and technology to harness its power
safely and efficiently.

Examples of Nuclear Reactions

Beta Decay
- Initial State: A neutron-rich nucleus emits a beta particle (electron or positron).
- Reaction:

Note: Anti-neutrino is an elementary particle. It's not in your syllabus. But it is

always emitted in β- beta (minus) decay i.e when an electron is emitted.
- Charge Check:
- Carbon-14 (14C) has 6 protons.
- Resulting nitrogen-14 (14N) has 7 protons.
- Emitted beta particle (electron) has a charge of -1.
- Total charge before: 6 (protons).
Total charge after: ( 7 - 1 = 6 ) (nitrogen + electron).
- Mass Check: The mass number 14C (14) remains unchanged, but the new element
14N balances the total proton-neutron count.