Atomic Structure and The Periodic Table
2.2 Atomic Structure and the Periodic Table
1. Structure of the Atom
1.1 Basic Model
- Atom: The smallest particle of an element that retains its chemical properties.
- Central nucleus:
- Contains protons (positively charged) and neutrons (neutral).
- Responsible for most of the atom’s mass.
- Very small compared to overall atom size but extremely dense.
- Electrons:
- Negatively charged particles.
- Move around the nucleus in energy levels or shells.
- Occupy most of the atom’s volume.
1.2 Relative Sizes
- Diameter of an atom ≈ 1 × 10⁻¹⁰ m.
- Diameter of nucleus ≈ 1 × 10⁻¹⁵ m → about 100,000 times smaller than atom.
- Most of atom is empty space.
1.3 Sub-atomic Particles Overview Table
| Particle | Relative Charge | Relative Mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | -1 | 1/1836 | Electron shells |
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course
2. Relative Charges and Relative Masses
2.1 Protons
- Positive charge: +1.
- Relative mass: 1.
- Determines element identity (proton number = atomic number).
- Example: Hydrogen atom → 1 proton.
2.2 Neutrons
- Neutral charge: 0.
- Relative mass: 1.
- Provide stability to the nucleus.
- Different numbers of neutrons in the same element → isotopes.
2.3 Electrons
- Negative charge: -1.
- Relative mass: 1/1836 (almost negligible in mass calculations).
- Found in energy levels around the nucleus.
- Responsible for chemical bonding and reactions.
3. Proton Number (Atomic Number)
3.1 Definition
- Proton number = number of protons in the nucleus of an atom.
- Symbol: Z.
- Equal to number of electrons in a neutral atom.
- Determines the element in the Periodic Table.
- Example: Oxygen → Z = 8 → 8 protons, 8 electrons (neutral atom).
3.2 Importance
- All atoms of the same element have the same proton number.
- Changing the proton number → changes the element entirely.
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course
4. Mass Number (Nucleon Number)
4.1 Definition
- Mass number = total number of protons + neutrons in the nucleus.
- Symbol: A.
- A = Z + number of neutrons.
- Example: Carbon-12:
- Z = 6 (protons)
- Neutrons = 6
- A = 12.
4.2 Notation
- Atoms often written as:
A
Z X
Example:
¹²₆C → Carbon with mass number 12 and proton number 6.
4.3 Isotopes
- Atoms of the same element (same Z) but different numbers of neutrons (different A).
- Example: Carbon-12, Carbon-14.
5. Electronic Configuration
5.1 Shells and Energy Levels
- Electrons arranged in shells around the nucleus.
- Shells filled in order of increasing energy.
- Maximum electrons in each shell:
1st shell: 2
2nd shell: 8
3rd shell: 8 (for first 20 elements in syllabus).
5.2 Rules for Writing Electronic Configurations
- Fill the lowest energy shell first.
- Continue filling until total equals the number of electrons.
- Write configuration as numbers separated by commas.
5.3 Examples (Proton numbers 1–20)
- H (Z=1) → 1
- He (Z=2) → 2
- Li (Z=3) → 2,1
- O (Z=8) → 2,6
- Na (Z=11) → 2,8,1
- Ca (Z=20) → 2,8,8,2
5.4 Ions
- Positive ions (cations): Formed by losing electrons.
- Na⁺: 2,8
- Negative ions (anions): Formed by gaining electrons.
- Cl⁻: 2,8,8
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course
6. Group and Period Relationships
6.1 Noble Gases (Group VIII / Group 0)
- Have full outer shells.
- Stable and unreactive.
- Example: Neon → 2,8.
6.2 Group Number
- For Groups I–VII:
- Group number = number of electrons in the outer shell.
- Example: Group I → 1 electron in outer shell (Na → 2,8,1).
6.3 Period Number
- Period number = number of occupied shells.
- Example: Na → 2,8,1 → 3 shells → Period 3.
7. Atomic Structure Summary Table
| Term | Definition | Example |
|---|---|---|
| Proton number | Number of protons in nucleus (Z) | Oxygen: Z = 8 |
| Mass number | Total number of protons and neutrons (A) | Carbon-12: A = 12 |
| Isotope | Same Z, different A | C-12, C-14 |
| Electronic config | Arrangement of electrons in shells | Na: 2,8,1 |
| Group number | Number of electrons in outer shell (Groups I–VII) | Mg: Group 2 |
| Period number | Number of occupied shells | Mg: Period 3 |
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course
8. Exam Tips
- Always define proton number and mass number correctly — mixing them up is a common mistake.
- Remember number of electrons = proton number in a neutral atom.
- For electron configurations, stick to 2,8,8 rule for first 20 elements.
- When asked about group number, relate it to outer shell electrons.
- When asked about period number, relate it to number of shells.
- Use nuclear notation correctly in answers.