PYTR Class

Learning Objectives

1. Explain the nature and formation of ionic bonds
2. Calculate effective nuclear charge
3. Explain why noble gasses are inert
4. Predict the charge of an ion from the electron configuration of the atom.

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Part 1: The Nature of Ionic Bond

• Definition of ionic bond: an electrostatic attraction between positively charged ion (cation) and negatively charged ion (anion).
• Mechanism: transfer of electron(s). Cation donates its electron(s) and anion receives the electron(s)

Explanation of the formation of ionic bond

• Electron can escape the nuclear attraction due to two main factors
  • Effective nuclear charge (Zeff​)
    • This is influenced by shielding effect
    • Low nuclear attraction will result in low ionisation energy to remove an electron. Therefore a low effective nuclear chrage will usually result in formation of positively charged ions (cations)
    • In contrast, high effective nuclear charge (with small atomic radius) will often result in the formation of negatively charged ions (anions)
  • Size of atom (atomic radius)
    • Larger atoms with low effective nuclear charge would usually donate their electrons. For example metals such as sodium, lithium, aluminium etc
    • Smaller atomic radius with high effective nuclear charge would result in electron being received for example non-metal in the right side of the periodic table

How to calculate effective nuclear charge

1. Identify which electron and its location in the energy level (shell)
2. Count how many electrons are there in the inner shell(s)
3. Subtract the number of proton (nuclear charge) with the number of electrons in the inner shell(s)

For example:

Lithium has 1 valence electron and 2 inner electron. The nuclear charge is +3. Therefore the effective nuclear charge experienced by the valence electron is +3 – 2 = +1

Part 2: Why Noble Gasses Are Inert?

• Noble gases have very high first ionisation energies, meaning their nuclei hold tightly to their outer electrons.
• These outer electrons are unavailable for chemical reactions due to the strong attraction from the nucleus.
• Noble gases possess complete outer electron shells with no vacancies to accept extra electrons.
• Any additional electron would have to enter a new, empty outer energy level, where it would experience almost zero effective nuclear charge.
• These factors make noble gases chemically un-reactive.
• The full outer energy level of noble gases represents the “ultimate goal” of electron configuration for all atoms.
• Metal atoms lose electrons to achieve the stable electron configuration of the preceding noble gas.
• Non-metal atoms gain electrons to achieve the stable electron configuration of the succeeding noble gas.

Example:

• Sodium loses one electron to achieve the electron configuration of neon.
• Chloride gains one electron to achieve the electron configuration of argon.

Part 3: Predicting the Charges of Ions

You can see jump to post Chemistry S3.1.8 Ionisation Energies to discover further about ionisation energy patterns and anomalies