(a) Define first ionisation energy of an element - Leaving Cert Chemistry - Question 5 - 2009

To properly plot the graph, follow these steps:

1. Gather Data: Collect the ionisation energy values for atomic numbers 10 to 20 from the Mathematics Tables.
2. Set Up Graph Paper: Label the x-axis with atomic numbers (10 to 20) and the y-axis with corresponding ionisation energy values. Ensure the axes are correctly scaled.
3. Plot Points: Mark each point corresponding to the atomic number and its first ionisation energy. Ensure accuracy in plotting.
4. Draw the Graph: Connect the plotted points using straight lines for clarity, indicating trends in ionisation energy across these elements.

The general increase in ionisation energy across the third period can be attributed to the following factors:

• Increase in Nuclear Charge: As atomic number increases, protons are added to the nucleus, leading to a higher positive charge that attracts electrons more strongly.
• Decreased Atomic Radius: With additional protons, electrons are drawn closer to the nucleus, reducing the size of the atomic radius, which in turn increases the effective nuclear charge experienced by the outermost electrons.
• Effective Nuclear Charge: The effective nuclear charge increases, making it harder to remove an electron, hence raising the ionisation energy.

The peaks at elements 12 (Magnesium) and 15 (Phosphorus) can be explained as follows:

• Element 12 (Magnesium): At this point in the periodic table, the added electron is entering a new subshell (the 3s subshell), which is at a higher energy level than the previous 3p subshell of Neon (element 10). This increase in energy level causes a peak as ionisation energy is relatively low at this point due to the positioning in the subshell.

• Element 15 (Phosphorus): Here, the additional electron is added to the half-filled 3p subshell. Half-filled subshells are particularly stable due to symmetry and exchange energy, which explains the increase in ionisation energy observed at this point.

The sharp decrease in ionisation energy between elements 18 (Argon) and 19 (Potassium) occurs due to:

• Change in Shell: Moving from Argon to Potassium involves removing an electron from a new, higher energy level (the 4s shell) rather than the fully filled 3p subshell. Electrons in higher energy levels are generally less tightly bound to the nucleus.
• Shielding Effect: The increased distance from the nucleus and the inner electrons provide a shielding effect, making it easier to remove the outermost electron in Potassium, hence leading to a lower ionisation energy.

The electron configuration for potassium (K) is:

$\text{K: } 1s^2 2s^2 2p^6 3s^1$

In total, potassium has 4 energy levels (or shells).

(i) Potassium has 6 energy sub-levels.

(ii) There are 10 individual orbitals occupied by electrons in potassium.

The presence of electrons in the fourth energy level of potassium can be explained as:

• Order of Filling: Electrons fill levels and sub-levels in order of increasing energy. Even though the 3rd level is not completely filled, the next available energy level (4s) can accommodate electrons and is filled before the 3p level is completed.
• Stability Factors: The 4s sub-level has lower energy than the 3d sub-level, allowing it to fill first in accordance with Aufbau's principle.