INTRODUÇÃO à RADIOATIVIDADE: Leis, Famílias, Fissão, Fusão, Equações Nucleares | Aula 05 (Química I)

Main ideas and concepts covered

1) Quick review needed for nuclear/atomic calculations

Atomic structure regions

• Nucleus
  • Contains protons (positive charge) and neutrons (no charge).
• Electron cloud
  • Contains electrons (negative charge).

Number relationships (example used: Uranium-238, (A=238), (Z=92))

• Mass number (A) = protons + neutrons.
• Atomic number (Z) = number of protons.
• Neutrons (N):
  • (N = A - Z)

Isotopes and related terms

• Isotopes
  • Same protons (same (Z)) and different neutrons (different (N)), so they have different mass numbers (A).
  • The subtitles mention cases like “same mass number / same number of neutrons,” but the examples clarify the intended use for the equations and typical exam classification.
• Isotones
  • Same number of neutrons.
• Isoelectronic atoms
  • Same number of electrons.

2) History context (brief)

• The teacher states he will not explain radioactivity history in detail and directs viewers to read about it through websites linked in the video description.

---

3) Radioactive emissions experiment (to identify particle types)

Apparatus and purpose

• Lead box in vacuum
  • Goal: prevent external interference (“remove influence of lead”).
• Small opening
  • Allows emissions to escape.
• Two charged metal plates
  • One plate becomes positive, the other negative.
• Photographic plate
  • Records where emissions land.

Observations

• Plates OFF
  • Emissions go straight in one direction.
• Plates ON
  • Emissions split into three behaviors:
    • Deflects toward the positive plate → particles are negative.
    • Deflects toward the negative plate → particles are positive.
    • No deflection → particles have no charge.

Identified radiation/particles

• Alpha (α)
  • Positive charge
  • Typical nuclear form:
    • mass number 4, atomic number 2
  • Interpreted as the helium nucleus: 2 protons + 2 neutrons
• Beta (β)
  • Negative charge
  • Typical nuclear form:
    • mass number 0, atomic number −1
  • Associated with an electron-like emission in the model used.
• Gamma (γ)
  • No charge, no mass number
  • Described as an electromagnetic wave
  • Passes straight through (no deflection)

---

4) Laws of radioactivity (core methodology for nuclear equations)

First law (α emission)

Rule

When a nucleus emits an alpha particle:

• Atomic number decreases by 2
• Mass number decreases by 4

Nuclear equation structure (emission direction)

• Use an emission form where the emitted particle is “coming out” from the parent nucleus.
• Apply subtraction to the parent nucleus:
  • (A_{\text{daughter}} = A_{\text{parent}} - 4)
  • (Z_{\text{daughter}} = Z_{\text{parent}} - 2)

Examples and exam-check tips

• Example: Uranium-235 ((A=235, Z=92)) emitting one α:
  • (A: 235 - 4 = 231)
  • (Z: 92 - 2 = 90)
  • Daughter atomic number 90 corresponds to thorium (Th) (as stated).
• Multiple α emissions:
  • If emitting 2 alpha particles, subtract:
    • mass: (2 \times 4 = 8)
    • atomic number: (2 \times 2 = 4)
• Consistency check emphasized
  • Mass number totals must match on both sides.
  • Atomic number totals must match on both sides.

Second law (β emission)

Rule

When a nucleus emits a beta particle:

• Mass number does not change
• Atomic number increases by 1

Reasoning/check (as taught)

• Beta changes the proton count (since (Z) changes), while the mass number stays fixed.
• Example described:
  • A nucleus like bismuth with (Z=83), (A=210) emitting β:
    • (A) stays 210
    • (Z) becomes 84
• Multiple beta emissions:
  • Each β increases Z by +1
  • Mass number remains the same

Gamma note

• The subtitles indicate there isn’t a separate “third law”:
  • γ is included in equations as no change in A or Z since it’s electromagnetic radiation.

---

5) How entrance exams test the rules (problem-solving patterns)

A) “Read transitions between A → B → C”

Use the pattern of changes to identify the emission:

• Mass −4 and atomic −2 → alpha
• Mass unchanged and atomic +1 → beta
• Mass and atomic unchanged → gamma

B) “Work forwards or backwards; verify by reversing if stuck”

• If forward reasoning is confusing:
  • start from the end nucleus and work backwards to verify.

C) “Avoid memorizing; use the numerical technique”

Typical exam tasks include:

• identify emission sequence (α/β/γ),
• count how many α/β particles occurred,
• find unknown parent/final isotopes.

The teacher emphasizes:

• balancing the mass-number equation,
• balancing the atomic-number equation.

---

6) Natural radioactive families (natural series)

Concept

• A natural radioactive series is an unstable isotope that decays through multiple steps until reaching a more stable isotope.
• Uranium is described as decaying through a chain that alternates α and β emissions.

Key points

• At each step, apply:
  • α law: (A \to A-4), (Z \to Z-2)
  • β law: (A) unchanged, (Z \to Z+1)
• The series is said to take millions of years (example given: ~4.5 million years).
• Carbon-14 is mentioned as another naturally occurring radioactive element in this context.

---

7) Artificial transmutation reactions (nuclear equations in reactors)

Core idea

• Artificial transmutation uses bombardment to convert one nucleus into another.
• Exams may involve:
  • alpha bombardment
  • or other projectiles (protons, neutrons, etc.)

Method for writing/testing nuclear equations

For a reaction:

• Projectile + Target → Products (unknown emission/particle and/or daughter element)

Apply:

• Mass number balance
  • Sum of A’s on the left = sum of A’s on the right.
• Atomic number balance
  • Sum of Z’s on the left = sum of Z’s on the right.

New particles introduced in this topic (beyond α, β, γ)

• Proton
  • mass number 1
  • atomic number 1
• Neutron
  • mass number 1
  • atomic number 0 (“x = 0” leading to identification)
• Positron (appears in some exam equations)
  • Presented as “like beta but positive charge”
  • Subtitles refer to it in a convention consistent with calculations that resolve to “positron”:
    • mass number 0
    • atomic number +1 behavior (inferred from the equation convention)

Example-style logic shown

• If an equation implies a particle with:
  • (A=1, Z=1) → proton
  • (A=1, Z=0) → neutron
  • (A=0, Z=+1) (subtitles’ convention) → positron

Exam emphasis

• Bombardment type is not always the same:
  • could be α, protons, or neutrons depending on the equation.
• For unknown particles:
  • determine A and Z numerically,
  • then classify using the particle “dictionary” (proton/neutron/positron).

---

8) Nuclear fusion (final part) + fission vs fusion (with embedded external speaker)

Embedded video segment

• The main teacher shows an excerpt from Paulo Valle (another chemistry channel).
• Purpose: explain fusion/fission conceptually with animations and clarify positron representation.

Paulo Valle’s main teaching points

• Radioactivity is a nuclear phenomenon, not an electron-only chemical reaction.
• Radioactive decay: a nucleus transforms into another nucleus to reach stability.
• Uranium decay example (alpha decay):
  • Uranium-238 → thorium-234 + α (as described)

Two reaction types

• Nuclear fission
  • A large nucleus splits into smaller nuclei
  • Used in power plants and atomic bombs (as stated)
  • Produces additional neutrons → chain reaction
  • Energy release described as very large compared to coal (scale given)
  • Produces radioactive waste (noted as a major issue)
• Nuclear fusion
  • Small nuclei join to form a larger nucleus
  • Occurs in the Sun
  • Described pathway involving hydrogen isotopes:
    • hydrogen → deuterium → helium-3 → helium-4
  • Releases energy, producing gamma radiation at one step
  • Currently difficult to control on Earth, but potentially cleaner than fission
  • Products are mainly hydrogen/helium

Wrap-up emphasis by the main teacher

• Fusion: two nuclei join.
• Fission: one nucleus splits.
• The course goal is to train students to solve difficult nuclear equations, especially those involving fusion.

---

Sources / speakers featured

• Professor Marcelo (speaker/host of the main lesson)
  • Nicknames mentioned: “Dia (Day)” and “A Onda Química”.
• Paulo Valle (guest/external video excerpt)
  • Mentioned as having a YouTube chemistry channel, shown via an excerpt with animations.