Atomic Structure and Chemical Bonding — Question 9
Back to all questions(a) Orbit structure of NaCl is shown below:
(b) Orbit structure diagram of calcium oxide is shown below:
Chapter Overview: Atomic Structure and Chemical Bonding
This chapter explores the internal structure of atoms and how atoms bond to form molecules. An atom consists of a positively charged nucleus (containing protons and neutrons) surrounded by negatively charged electrons in energy levels (shells). The atomic number (Z) equals the number of protons, and the mass number (A) equals protons plus neutrons. Electrons are arranged in shells following the 2n2 rule (maximum electrons per shell: K=2, L=8, M=18, N=32), with the outermost shell holding a maximum of 8 electrons. Isotopes are atoms of the same element with different numbers of neutrons. The chapter introduces chemical bonding: atoms bond to achieve stable noble gas configurations (octet/duplet). Ionic bonds form through electron transfer between metals and non-metals, creating charged ions. Covalent bonds form through electron sharing between non-metals. Students learn to draw electron configurations, identify valence electrons, and predict bonding behaviour based on electronic structure.
Key Definitions
| Term | Definition |
|---|---|
| Atomic Number (Z) | Number of protons in the nucleus (equals number of electrons in neutral atom) |
| Mass Number (A) | Total number of protons and neutrons (A = Z + N) |
| Isotopes | Atoms of same element with same Z but different A (different neutrons) |
| Valence Electrons | Electrons in the outermost shell that participate in bonding |
| Octet Rule | Atoms tend to achieve 8 electrons in outermost shell for stability |
| Ion | Charged atom formed by gain or loss of electrons (cation = +, anion = −) |
| Shell Capacity | Maximum electrons = 2n2 (K=2, L=8, M=18) but outermost max = 8 |
Must-Know Concepts
- Subatomic particles: Proton (p+, mass 1, charge +1), Neutron (n0, mass 1, charge 0), Electron (e−, mass ~0, charge −1)
- Electron filling order: K(2) → L(8) → M(8) → N for the first 20 elements
- Na (Z=11): 2,8,1 → loses 1e− to form Na+ (2,8) - ionic bonding with Cl
- Cl (Z=17): 2,8,7 → gains 1e− to form Cl− (2,8,8) - achieves octet
- Isotopes of hydrogen: Protium (1H), Deuterium (2H), Tritium (3H)
- Isotopes have same chemical properties (same electron configuration) but different physical properties
- Covalent bond in H2: each H shares 1 electron to achieve duplet (2 electrons)
Cation vs Anion
| Feature | Cation | Anion |
|---|---|---|
| Charge | Positive (+) | Negative (−) |
| Formation | Loss of electrons | Gain of electrons |
| Formed by | Metals | Non-metals |
| Size vs Atom | Smaller than parent atom | Larger than parent atom |
Important Diagrams to Practice
- Bohr model diagrams for atoms of first 20 elements showing shells and electrons
- Electron transfer diagrams for NaCl and MgO formation
- Electron sharing diagrams for H2, O2, N2, H2O, CH4
Common Mistakes
- Putting more than 8 electrons in the outermost shell for elements 1-20
- Confusing atomic number with mass number
- Writing electron configuration of K as 2,8,9 instead of 2,8,8,1
- Forgetting that neutrons have no charge but contribute to mass
- Not showing the charge on ions after electron transfer diagrams
Scoring Tips
- Practice drawing Bohr models quickly for all elements Z=1 to Z=20
- For isotope questions, always state: same atomic number, different mass number
- When drawing ionic bond formation, show the electron transfer with arrows and final charges
- Remember: electrons and protons are equal in a neutral atom; different in ions
Frequently Asked Questions
Why don't electrons fall into the nucleus?
Electrons occupy specific energy levels (shells) at fixed distances from the nucleus. They have kinetic energy that balances the electrostatic attraction of the nucleus. As long as they remain in their designated shells, they do not lose energy or spiral into the nucleus (Bohr's postulate).
Why do isotopes have the same chemical properties?
Chemical properties depend on the number and arrangement of electrons (which equals the number of protons). Isotopes have the same atomic number (same number of electrons), so they undergo the same chemical reactions. The different number of neutrons only affects mass and nuclear properties.
Why does the M shell hold only 8 electrons in elements up to Z=20?
Although the M shell can theoretically hold 18 electrons (2n2 = 2×32), the outermost shell can hold a maximum of 8 electrons. For elements up to calcium (Z=20), the M shell acts as the outermost shell (for Z=11-18) and follows the octet limit. Once 8 electrons fill M, the next electron enters the N shell (as in K, Z=19).