1. Overview
The atom is the fundamental building block of all matter. Understanding its structure is essential for explaining how electricity flows, how chemical reactions occur, and how radioactive decay works. This topic explores the internal components of the atom and the historical evidence that led to our current nuclear model.
Key Definitions
- Atom: The smallest particle of an element, consisting of a central nucleus surrounded by orbiting electrons.
- Nucleus: The small, dense, positively charged center of an atom containing protons and neutrons.
- Electron: A negatively charged subatomic particle that orbits the nucleus in specific energy levels or shells.
- Proton: A positively charged subatomic particle found within the nucleus.
- Neutron: An uncharged (neutral) subatomic particle found within the nucleus.
- Ion: An atom that has become electrically charged by gaining or losing one or more electrons.
Core Content
The Structure of the Atom
An atom consists of two main regions:
- The Nucleus: Located at the center. It contains protons (positive charge) and neutrons (no charge). Because it contains protons, the nucleus itself is always positively charged.
- The Electrons: These are much smaller than protons and neutrons. They carry a negative charge and orbit the nucleus at high speeds in "shells" or orbits.
A central cluster of circles labeled "Nucleus" (containing '+' and 'n' symbols) with much smaller '-' circles orbiting on circular paths around it, showing vast empty space between the nucleus and the orbits.
Formation of Ions
In a neutral atom, the number of negative electrons equals the number of positive protons, so the charges cancel out. However, atoms can become charged by moving electrons:
- Positive Ions: Formed when an atom loses one or more electrons. It now has more positive protons than negative electrons.
- Negative Ions: Formed when an atom gains one or more electrons. It now has more negative electrons than positive protons.
Worked Example: A neutral Lithium atom has 3 protons and 3 electrons.
- If it loses 1 electron, it has 3(+) and 2(-). Total charge = +1 (Positive Ion).
- If it gains 1 electron, it has 3(+) and 4(-). Total charge = -1 (Negative Ion).
Extended Content (Extended Curriculum Only)
The Alpha (α) Particle Scattering Experiment
In the early 1900s, Ernest Rutherford fired alpha particles (which are positively charged) at a very thin sheet of gold foil. The results provided evidence for the Nuclear Model:
- Observation: Most alpha particles passed straight through the foil without changing direction.
- Conclusion: The atom is mostly empty space.
- Observation: Some alpha particles were deflected at large angles.
- Conclusion: The nucleus is positively charged, causing it to repel the positive alpha particles.
- Observation: A very small number of alpha particles bounced straight back.
- Conclusion: The nucleus is very small and contains most of the mass of the atom (it is extremely dense).
The Gold Foil Experiment showing a source firing alpha particles at a gold leaf. Most arrows go through, some curve away from the center of atoms, and one or two bounce back.
Key Equations & Data
| Particle | Relative Charge | Relative Mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Inside Nucleus |
| Neutron | 0 | 1 | Inside Nucleus |
| Electron | -1 | 1/1840 (negligible) | Orbiting Nucleus |
- Net Charge of an Ion = (Number of Protons) – (Number of Electrons)
Common Mistakes to Avoid
- ❌ Wrong: The nucleus is negatively charged and electrons are positive.
- ✓ Right: The nucleus is positive (due to protons) and electrons are negative.
- ❌ Wrong: Neutrons have a positive or negative charge.
- ✓ Right: Neutrons are neutral (zero charge).
- ❌ Wrong: Atoms become ions by changing the number of protons in the nucleus.
- ✓ Right: Atoms only gain or lose electrons to become ions; the nucleus never changes during ion formation.
- ❌ Wrong: Mistaking isotopes for ions.
- ✓ Right: Isotopes have different numbers of neutrons; ions have different numbers of electrons compared to the neutral atom.
Exam Tips
- Be precise with alpha scattering: If a question asks what evidence shows the nucleus is small, specifically mention that "only a very small minority of alpha particles were deflected back."
- Check the charge: When calculating the charge of an ion, always compare the total number of negative electrons to positive protons. Do not add the two numbers together; find the difference between them.
- The "Empty Space" point: This is the most common answer for why most alpha particles pass through. Always use the phrase "mostly empty space" in your description.