A-Level · Chemistry · AQA · Mark scheme decoded
AQA A-Level Chemistry: Ionic Bonding and Formulas of Ionic Compounds — mark scheme explained
The short answer
Understanding ionic bonding is fundamental to chemistry, as it explains how atoms combine to form stable compounds through the transfer of electrons. This section delves into the nature of ionic bonding, the formation of ions, and the construction of formulas for ionic compounds.
The question
Predict the charge on a sodium ion and explain why it forms this charge. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the element and its group in the Periodic Table.
- S2
Sodium (Na) is in Group 1.
- S3
Step 2: Determine the typical charge for elements in Group 1.
- S4
Group 1 elements lose one electron to form +1 ions.
- S5
Step 3: Explain why sodium forms a +1 ion.
- S6
Sodium loses one electron to achieve a stable electron configuration, resembling the nearest noble gas (neon).
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the element and its group in the Periodic Table.
- S2
Sodium (Na) is in Group 1.
- S3
Step 2: Determine the typical charge for elements in Group 1.
- S4
Group 1 elements lose one electron to form +1 ions.
- S5
Step 3: Explain why sodium forms a +1 ion.
- S6
Sodium loses one electron to achieve a stable electron configuration, resembling the nearest noble gas (neon).
Final answer: +1
Common mistakes
- Misinterpreting the charges on ions based on their group in the Periodic Table. — Always refer to the periodic table and remember the common charges for each group. For transition metals, consider the specific compound formed.
- Forgetting to balance the charges when constructing formulas for ionic compounds. — Always check that the charges are balanced by finding the lowest common multiple (LCM) of the charges and adjusting the number of each ion accordingly.
- Incorrectly writing polyatomic ions in formulas. — Enclose polyatomic ions in parentheses and use subscripts to indicate the number of ions. For example, write (SO 4 ) 3 instead of SO 12 .
- Confusing the order of cations and anions in formulas. — Always write the cation (positive ion) first followed by the anion (negative ion).
- Failing to explain the formation of ionic bonds clearly. — Practice explaining the process step-by-step. Emphasize that metals lose electrons to form cations, non-metals gain electrons to form anions, and the resulting electrostatic attraction forms a stable ionic bond.
- Incorrectly identifying common polyatomic ions. — Memorize the formulas and charges of common polyatomic ions such as sulfate (SO 4 2- ), hydroxide (OH - ), nitrate (NO 3 - ), carbonate (CO 3 2- ), and ammonium (NH 4 + ).
Where the marks go
- Full worked solution (all marking points)3 marks