Nuclear Model of the Atom

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⚛️ NUCLEAR MODEL OF THE ATOM

19.1 THE ATOM

What is an Atom?

ATOM = The smallest unit of a chemical element

Structure of Atom

ATOM = NUCLEUS + ELECTRONS
Part Charge Location Mass
Nucleus Positive (+) Center Almost all mass
Electrons Negative (−) Orbit around nucleus Very small
KEY FACT: Most of atom is EMPTY SPACE!

Formation of IONS

Neutral Atom:
Number of Protons = Number of Electrons
LOSE electrons → POSITIVE ION (+)
GAIN electrons → NEGATIVE ION (−)
Example:
- Atom with 11 protons & 11 electrons = neutral
- Loses 1 electron → 11 protons & 10 electrons = positive ion (+1)
- Gains 1 electron → 11 protons & 12 electrons = negative ion (−1)

Gold Foil Experiment (1911)

Scientists: Geiger, Marsden, Rutherford
Method: Shot alpha (α) particles at thin gold foil

Results:

Observation Conclusion
Most α-particles passed through Atom is mostly empty space
Some deflected Nucleus is positive (repels α-particles)
Few bounced back Nucleus is very small & massive
Evidence for Nuclear Model:
✓ Small nucleus
✓ Mostly empty space
✓ Nucleus has most mass
✓ Nucleus is positive

19.2 THE NUCLEUS

Composition of Nucleus

NUCLEUS = PROTONS + NEUTRONS
Particle Symbol Relative Charge Mass (amu)
Proton p +1 1
Neutron n 0 1
Electron e⁻ −1 1/2000

Important Numbers

PROTON NUMBER (Z) = Atomic Number
= Number of protons in nucleus
= Defines the element
NUCLEON NUMBER (A) = Mass Number
= Total number of protons + neutrons
NUMBER OF NEUTRONS = A − Z

Nuclide Notation

A
ZX

A = Nucleon number (top)
Z = Proton number (bottom)
X = Element symbol
Example: ¹²₆C (Carbon-12)
- A = 12 (nucleon number)
- Z = 6 (proton number)
- Protons = 6
- Neutrons = 12 − 6 = 6
- Electrons (neutral) = 6

ISOTOPES

ISOTOPES = Atoms of same element with:
✓ Same proton number (Z)
✓ Different neutron number
✓ Different mass number (A)
Three Carbon Isotopes:
Isotope Protons Neutrons Electrons
¹²₆C 6 6 6
¹³₆C 6 7 6
¹⁴₆C 6 8 6
All have 6 protons = All are CARBON!

Worked Example

Question: Element ¹³¹₅₃X
(a) How many electrons in neutral atom?
(b) How many neutrons?

Solution:
(a) Z = 53 → 53 protons
Neutral atom: electrons = protons
Answer: 53 electrons

(b) A = 131, Z = 53
Neutrons = A − Z = 131 − 53
Answer: 78 neutrons

19.3 NUCLEAR FISSION & FUSION

NUCLEAR FISSION

FISSION = Large nucleus SPLITS into smaller nuclei + ENERGY
¹₀n + ²³⁵₉₂U → ¹⁴⁴₅₆Ba + ⁹⁰₃₆Kr + 2¹₀n + ENERGY

Steps:

  1. Neutron hits large nucleus (e.g., Uranium-235)
  2. Nucleus becomes unstable
  3. Nucleus SPLITS into 2 smaller nuclei
  4. More neutrons released
  5. HUGE energy released
Conservation Rules in Nuclear Reactions:
✓ Total nucleon number (A) SAME before & after
✓ Total charge (Z) SAME before & after
Check Conservation:
¹₀n + ²³⁵₉₂U → ¹⁴⁴₅₆Ba + ⁹⁰₃₆Kr + 2¹₀n

Nucleon number:
Before: 1 + 235 = 236
After: 144 + 90 + 2(1) = 236 ✓

Charge:
Before: 0 + 92 = +92
After: 56 + 36 + 0 = +92 ✓

Worked Example - Fission

Find A and Z:
²³³₉₂U + ¹₀n → ¹³⁷₅₄Xe + AZSr + 3¹₀n

Find A (nucleon number):
Before: 233 + 1 = 234
After: 137 + A + 3 = 140 + A
234 = 140 + A
A = 94

Find Z (proton number):
Before: 92 + 0 = 92
After: 54 + Z + 0 = 54 + Z
92 = 54 + Z
Z = 38

Answer: ⁹⁴₃₈Sr

Chain Reaction

1 neutron → 1 fission → 2-3 neutrons
2-3 neutrons → 2-3 fissions → 4-9 neutrons
= CHAIN REACTION!

Controlled = Nuclear power plant
Uncontrolled = Nuclear bomb

NUCLEAR FUSION

FUSION = Small nuclei COMBINE into larger nucleus + ENERGY
²₁H + ³₁H → ⁴₂He + ¹₀n + ENERGY

Process:

  1. Two light nuclei (e.g., hydrogen isotopes)
  2. Combine to form heavier nucleus (helium)
  3. Release neutron
  4. Release ENORMOUS energy
Check Conservation:
²₁H + ³₁H → ⁴₂He + ¹₀n

Nucleon number:
Before: 2 + 3 = 5
After: 4 + 1 = 5 ✓

Charge:
Before: 1 + 1 = +2
After: 2 + 0 = +2 ✓

Why is Fusion Difficult?

Both nuclei are POSITIVE → They REPEL!

Need:
- Very high temperature (millions of degrees)
- Very high pressure

Where it happens: THE SUN! ☀️

Comparing Fission vs Fusion

Feature Fission Fusion
Process Split large nucleus Combine small nuclei
Fuel Uranium-235 Hydrogen isotopes
Conditions Normal temp Millions of degrees
Use Today Power plants Only in Sun/bombs
Energy Huge Even bigger!

Where Does Nuclear Energy Come From?

Einstein's Equation:
E = mc²
E = Energy
m = Mass
c = Speed of light

Mass can be converted to ENERGY!
Mass-Energy Conversion:
In nuclear reactions:
- Total mass BEFORE > Total mass AFTER
- "Missing" mass → converted to ENERGY
- Even tiny mass = HUGE energy (because c² is very large!)

Applications

FISSION - Nuclear Power Plants:
✓ Huge energy from small fuel
✓ No greenhouse gases
✗ Radioactive waste
✗ Risk of accidents

FUSION - Future Energy (not yet!):
✓ Fuel from seawater (abundant)
✓ Little radioactive waste
✓ Safe (no chain reaction)
✗ Very difficult to control

📝 QUICK SUMMARY

Key Formulas:

1. Number of Neutrons = A − Z

2. Neutral Atom: Electrons = Protons = Z

3. Conservation in Nuclear Reactions:
• Total A before = Total A after
• Total Z before = Total Z after

4. E = mc² (Mass-energy conversion)
Key Particles:
- Proton: charge +1, mass 1 amu
- Neutron: charge 0, mass 1 amu
- Electron: charge −1, mass 1/2000 amu
Nuclide Notation:
A
ZX
A = mass number (top)
Z = atomic number (bottom)
X = element symbol
Remember:
✓ Isotopes = same Z, different A
✓ Fission = split large nucleus
✓ Fusion = combine small nuclei
✓ Both release HUGE energy!

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