Topic 2.2: Atomic Structure and the Periodic Table Revision Notes
1. Overview
Atomic structure is the foundation of chemistry, describing how subatomic particles are arranged within an atom. Understanding the distribution of protons, neutrons, and electrons allows us to predict how elements behave, how they bond, and their specific placement within the Periodic Table.
Key Definitions
- Proton Number (Atomic Number): The number of protons in the nucleus of an atom. This defines the identity of the element.
- Mass Number (Nucleon Number): The total number of protons and neutrons in the nucleus of an atom.
- Electronic Configuration: The arrangement of electrons in the shells (energy levels) around the nucleus of an atom.
- Nucleus: The dense, central part of an atom containing protons and neutrons.
- Shells (Energy Levels): Regions surrounding the nucleus where electrons are found.
Core Content
The Structure of the Atom
An atom consists of a central nucleus surrounded by electrons orbiting in shells.
- Nucleus: Contains protons and neutrons. It holds almost all the mass of the atom but occupies a tiny fraction of its volume.
- Shells: Electrons orbit the nucleus at specific distances.
Relative Masses and Charges
You must memorize the properties of the three subatomic particles:
| Particle | Relative Mass | Relative Charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | In the nucleus |
| Neutron | 1 | 0 (Neutral) | In the nucleus |
| Electron | 1/1840 (Negligible) | -1 | In the shells |
Determining Subatomic Particles
- Number of Protons = Proton Number.
- Number of Electrons = Proton Number (in a neutral atom).
- Number of Neutrons = Mass Number – Proton Number.
Worked Example: Determine the number of protons, neutrons, and electrons in an atom of Sodium ($\text{Na}$).
- Given: Proton number = 11, Mass number = 23.
- Protons: 11
- Electrons: 11 (atoms are neutral)
- Neutrons: $23 - 11 = 12$
Electronic Configuration (Proton numbers 1–20)
Electrons fill the shells closest to the nucleus first.
- 1st Shell: Holds a maximum of 2 electrons.
- 2nd Shell: Holds a maximum of 8 electrons.
- 3rd Shell: Holds a maximum of 8 electrons (for the first 20 elements).
Examples:
- Helium (2): 2
- Carbon (6): 2,4
- Sodium (11): 2,8,1
- Calcium (20): 2,8,8,2
Relationship with the Periodic Table
The electronic configuration of an atom is directly linked to its position in the Periodic Table:
- Group Number: The number of electrons in the outer shell is equal to the Group number (for Groups I to VII).
- Example: Magnesium (2,8,2) is in Group II.
- Period Number: The number of occupied electron shells is equal to the Period number.
- Example: Chlorine (2,8,7) has 3 shells, so it is in Period 3.
- Group VIII (Noble Gases): These elements (He, Ne, Ar) have a full outer shell, making them very stable and unreactive (unreactive).
Extended Content (Extended Only)
Note: Per the provided instructions, there is no additional supplement content required for this specific sub-topic (2.2).
Key Equations
While this topic is conceptual, the following "formulas" are essential for calculations:
- Mass Number ($A$) = Protons ($Z$) + Neutrons ($n$)
- Number of Neutrons = $A - Z$
Example Calculation: For an isotope of Carbon with a mass number of 14 and proton number 6: $\text{Neutrons} = 14 - 6 = 8$
Common Mistakes to Avoid
- ❌ Wrong: Confusing the Mass Number with the Proton Number.
- ✅ Right: Remember the Mass number is always the larger number (it includes both protons and neutrons).
- ❌ Wrong: Thinking electrons have no mass at all.
- ✅ Right: Electrons have a "negligible" mass (1/1840th of a proton), but it is not zero.
- ❌ Wrong: Forgetting that ions have a different number of electrons than protons.
- ✅ Right: In a neutral atom, Protons = Electrons. In an ion, you must add or subtract electrons based on the charge.
Exam Tips
- Command Words: If a question says "State the electronic configuration," simply write the numbers (e.g., 2,8,1). If it says "Give the number of neutrons," show the subtraction to ensure you get the mark even if you make a calculation error.
- Typical Values: Expect to work with elements up to Calcium (Proton number 20). Be very comfortable with the numbers 2, 8, 10, and 18, as these represent full shells or common atomic numbers.
- Real-World Contexts: Questions often use "nuclear" contexts, such as the use of isotopes in medical imaging or nuclear fuel. Even in these contexts, the rules for protons and electrons remain the same.
- Drawing: When drawing "dot and cross" diagrams for atoms, always check your total electron count against the proton number. For example, a neutral Oxygen atom must have exactly 8 electrons total ($O_{(g)}$).