Atomic Structure

Revision Notes for Atomic Structure

1. Quantum Model

  • Principal Quantum Number, n: Represents the energy levels (or electron shells) of an atom.

  • Sub-shells: The different types of orbitals within an energy level, identified as s, p, d, and f.

    • Orbitals in sub-shells:

    • s sub-shell: Contains 1 orbital.

    • p sub-shell: Contains 3 orbitals.

    • d sub-shell: Contains 5 orbitals.

    • f sub-shell: Contains 7 orbitals.

  • Trends:

    • As the principal quantum number (n) increases, the energy levels become closer together, leading to a decrease in the energy difference between successive levels.

    • For an electron shell with principal quantum number, n:

    • Number of sub-shells: Equal to n.

    • Number of orbitals: Equal to n2n^2.

    • Number of electrons: Equal to 2n22n^2 .

2. spdf Electronic Configuration

  • Understanding Electronic Configuration:

    • It is essential to know how to write the spdf electronic configuration for the first 20 elements, for both atoms and ions.

    • Shorthand Notation: Utilizes the electronic configurations of noble gases, e.g., [Ne] for Neon, [Ar] for Argon.

3. Ionisation Energy

  • Definition: Ionisation energy of an element is the energy required to remove one mole of electrons from its gaseous atom or ion in its ground state.

  • Equation Representation:

    • The process can be denoted as:

    • X(g)→X+(g)+e−X(g) \rightarrow X^+(g) + e^- (1st ionisation energy)

    • X+(g)→X2+(g)+e−X^+(g) \rightarrow X^{2+}(g) + e^- (2nd ionisation energy)

    • X2+(g)→X3+(g)+e−X^{2+}(g) \rightarrow X^{3+}(g) + e^- (3rd ionisation energy)

  • Context: Ionisation energy measures the ease with which an atom or ion can lose an electron.

  • Influencing Factors:

    • The ease of removing an electron is influenced by both nuclear charge and the shielding effect.

Trends in Ionisation Energy

  • Across the Period:

    • Increasing Nuclear Charge: As you move across a period, the nuclear charge increases due to an increase in the number of protons while the shielding effect remains constant.

    • Leads to an increased effective nuclear charge.

    • Higher attraction (E.S.F.O.A) between the nucleus and valence electrons.

    • This results in higher energy (IE) required to remove valence electrons.

  • Down the Group:

    • Increasing Number of Filled Principal Quantum Shells: Each subsequent element adds a filled electron shell, increasing the distance from nucleus to valence electrons.

    • Effect of Shielding: More inner electrons lead to a significant shield against the nucleus's pull on valence electrons.

    • Increased shielding, coupled with a greater distance, results in decreased effective nuclear attraction (E.S.F.O.A).

    • Although the nuclear charge increases, the effect is less significant than the shielding effect.

    • This results in a decrease in the energy (IE) required to remove valence electrons.

Successive Ionisation Energies

  • Successive ionisation energies of an element can be used to predict the:

    1. Group number of the element.

    2. Number of valence electrons in the outermost shell.

Explanation of Trends:

  • The largest increase in energy is observed between the last and the next ionisation energies:

    • This indicates a transition where an electron is removed from a tighter-bound inner principal quantum shell instead of the valence shell, signifying fewer electrons in the valence shell.

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  • Template for Completion:

    • The largest increase occurs between the and ionisation energies.

    • The electron is removed from the inner principal quantum shell whereas the electron is removed from the valence shell.

    • Therefore, there are _ electrons in the valence shell.