Introduction to Atomic Structure

Welcome to the world of the incredibly small! Everything you see around you—your phone, the air you breathe, and even you—is made of tiny building blocks called atoms. In this chapter, we are going to look inside the atom to see how it is built and what happens when an atom is "unstable." Understanding this is the first step to mastering radioactivity!

1. The Structure of the Atom

An atom consists of a central nucleus surrounded by electrons that orbit in shells. The nucleus is very small compared to the whole atom, but it contains almost all of the atom's mass.

There are three sub-atomic particles you need to know:

  • Protons: Found in the nucleus. They have a positive charge.
  • Neutrons: Found in the nucleus. They have no charge (they are neutral).
  • Electrons: Found orbiting the nucleus. They have a negative charge.

Summary of Particles

Proton: Relative Mass = \( 1 \), Relative Charge = \( +1 \)
Neutron: Relative Mass = \( 1 \), Relative Charge = \( 0 \)
Electron: Relative Mass = \( 1/1835 \) (essentially zero), Relative Charge = \( -1 \)

Memory Trick: Protons are Positive. Neutrons are Neutral.

2. Understanding Nuclide Notation

To describe a specific atom (a nuclide), we use a standard symbol format. It looks like this:

\( _{Z}^{A}X \)

  • \( X \): The chemical symbol for the element (e.g., \( C \) for Carbon, \( U \) for Uranium).
  • \( A \) (Mass Number / Nucleon Number): The total number of protons + neutrons in the nucleus.
  • \( Z \) (Atomic Number / Proton Number): The number of protons in the nucleus.

Important Note: The number of protons \( Z \) defines what the element is. If you change the number of protons, you change the element! In a neutral atom, the number of electrons is always equal to the number of protons.

How to find the number of neutrons:
Number of neutrons = \( A - Z \) (Top number minus bottom number).

3. What are Isotopes?

Sometimes, atoms of the same element have different numbers of neutrons. We call these isotopes.

Definition: Isotopes are atoms of the same element (same number of protons) but with different numbers of neutrons (different mass numbers).

Example: Carbon-12 (\( _{6}^{12}C \)) and Carbon-14 (\( _{6}^{14}C \)) are isotopes. Both have 6 protons, but Carbon-12 has 6 neutrons, while Carbon-14 has 8 neutrons.

4. Nuclear Radiation and Decay

Some nuclei are "unstable" because they have too much energy or an awkward ratio of protons to neutrons. To become stable, they randomly emit radiation. This is called radioactive decay.

There are four types of emission we focus on in this chapter:

  • Alpha (\( \alpha \)): A helium nucleus (\( _{2}^{4}He \)). It consists of 2 protons and 2 neutrons.
  • Beta-minus (\( \beta^- \)): A fast-moving electron (\( _{-1}^{0}e \)) emitted when a neutron turns into a proton.
  • Gamma (\( \gamma \)): High-energy electromagnetic radiation (\( _{0}^{0}\gamma \)). It has no mass and no charge.
  • Neutron (\( n \)): A single neutron (\( _{0}^{1}n \)) sometimes ejected from the nucleus.

Note: For more detail on how these behave (like how far they travel), see the chapter "Types of radiation and detection."

5. Balancing Nuclear Equations

Nuclear equations show what happens to an atom during radioactive decay. The "Golden Rule" is that the total mass number and total atomic number must be the same on both sides of the arrow.

Alpha Decay (\( \alpha \))

When an atom emits an alpha particle, it loses 4 from its mass number and 2 from its atomic number.

\( _{Z}^{A}X \rightarrow _{Z-2}^{A-4}Y + _{2}^{4}\alpha \)

Example: \( _{92}^{238}U \rightarrow _{90}^{234}Th + _{2}^{4}He \)

Beta-minus Decay (\( \beta^- \))

In beta decay, a neutron changes into a proton and an electron. The mass number stays the same, but the atomic number increases by 1.

\( _{Z}^{A}X \rightarrow _{Z+1}^{A}Y + _{-1}^{0}e \)

Example: \( _{6}^{14}C \rightarrow _{7}^{14}N + _{-1}^{0}e \)

Gamma Decay (\( \gamma \))

Gamma radiation is just energy. The atomic number and mass number do not change.

\( _{Z}^{A}X \rightarrow _{Z}^{A}X + _{0}^{0}\gamma \)

Neutron Emission (\( n \))

If a nucleus emits a neutron, the mass number decreases by 1, but the atomic number stays the same.

\( _{Z}^{A}X \rightarrow _{Z}^{A-1}X + _{0}^{1}n \)

Quick Review Table: Effect on the Nucleus

  • Alpha: Mass -4, Protons -2
  • Beta: Mass 0, Protons +1
  • Gamma: No change
  • Neutron: Mass -1, Protons No change

Common Mistakes to Avoid

1. Forgetting the Beta proton increase: In beta decay, the bottom number (atomic number) goes up by 1 because \( Z - (-1) = Z + 1 \). Don't let the minus sign on the electron confuse you!

2. Identifying elements: If the atomic number (bottom number) changes, you must change the chemical symbol (e.g., Uranium becomes Thorium).

3. Randomness: Remember that radioactive decay is a random process. We cannot predict exactly when a single nucleus will decay.

Key Takeaways

- Atoms have a nucleus (protons, neutrons) and shells (electrons).
- Isotopes have the same protons but different neutrons.
- Nuclide symbols show Mass (\( A \)) on top and Atomic number (\( Z \)) on bottom.
- Alpha, beta, and neutron emissions change the mass or identity of the nucleus.
- Gamma emission only changes the energy of the nucleus.