Year 8 Science · Unit 2 · Lesson 12
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Learning Goals
Match each feature to its job
Write the matching letter in the middle column. Each letter is used once.
| Feature | Letter | What it does |
|---|---|---|
| Atomic number | A. Gives the average mass of the element’s atoms, so it is usually not a whole number. | |
| Chemical symbol | B. Marked a place for an element that had not yet been discovered. | |
| Relative atomic mass | C. Is the full word for the element, which changes from one language to another. | |
| Element name | D. Holds elements with similar properties, one above another. | |
| Column of the table | E. Counts the protons in each atom, so it identifies the element. | |
| Gap in Mendeleev’s table | F. Lets a chemist in any country write the element in one or two letters. |
1. Give two reasons scientists find the periodic table more useful than a plain list of element names.
Classify with a reason
Decide whether each clue describes the atomic number or the relative atomic mass. Give a reason.
| Clue | Atomic number or relative atomic mass? | Reason |
|---|---|---|
| 22.99 in the sodium box | ||
| Is always a whole number | ||
| Decides where an element sits in the modern table | ||
| Is worked out across all the atoms in a normal sample | ||
| 6 in the carbon box |
Read the data
A student claims
“In the periodic table, every element is heavier than the one before it, so the table is really arranged by mass.”
| Element | Symbol | Atomic number | Relative atomic mass |
|---|---|---|---|
| sulfur | S | 16 | 32.06 |
| chlorine | Cl | 17 | 35.45 |
| cobalt | Co | 27 | 58.93 |
| nickel | Ni | 28 | 58.69 |
| tellurium | Te | 52 | 127.60 |
| iodine | I | 53 | 126.90 |
Source: periodic table data, to two decimal places.
2. Use Table 1 to test the claim. Name the two pairs of neighbouring elements where it fails, and give the mass difference for each pair.
Pair 1 Pair 2
3. Iodine behaves like chlorine and bromine, while tellurium behaves like sulfur and selenium. Explain what would go wrong if the table were arranged strictly by relative atomic mass, and why arranging by atomic number fixes it.
4. Compare Mendeleev’s 1869 table with the modern periodic table. Explain why changing from atomic mass to atomic number was an improvement to the system, not a sign that Mendeleev’s table had failed.
Match each feature to its job
Atomic number → E; Chemical symbol → F; Relative atomic mass → A; Element name → C; Column of the table → D; Gap in Mendeleev’s table → B.
1. Any two: it organises every element in one system (by atomic number); similar elements sit together, so they are easy to compare; its patterns can be used to predict properties; it gives scientists everywhere a shared way to communicate. [2 marks: 1 each]
Classify with a reason
22.99: relative atomic mass (a decimal, because it is an average). Always a whole number: atomic number (it counts protons). Decides where an element sits: atomic number (the modern table is arranged by proton count). Worked out across a sample: relative atomic mass (an average for the element’s atoms). 6 in the carbon box: atomic number (every carbon atom has 6 protons).
Read the data
2. Cobalt and nickel: 58.93 − 58.69 = 0.24 (nickel is lighter but comes after cobalt). Tellurium and iodine: 127.60 − 126.90 = 0.70 (iodine is lighter but comes after tellurium). So the claim is false. [3 marks: 1 each pair with difference, 1 conclusion]
3. By mass, iodine would come before tellurium, landing in the column with sulfur and selenium, and tellurium with chlorine and bromine: each would sit with elements it does not behave like. By atomic number (52, then 53) each lands with the elements it behaves like, so the columns keep their pattern. [3 marks: 1 swap by mass, 1 wrong columns, 1 atomic number keeps similar elements together]
Show What You Know
4. Marking guide (points expected): 1 Mendeleev: by atomic mass; modern: by atomic number; 1 another difference: about 63 elements then, 118 now, or all gaps now filled; 1 Mendeleev left gaps and predicted missing elements, and gallium and germanium later fitted; 1 what stayed: similar elements grouped together and patterns used to predict; 1 Table 1: cobalt/nickel and tellurium/iodine break the mass pattern, but atomic number (the proton count, which defines the element) puts them with the elements they behave like; 1 keeping the core idea and improving it with better evidence is how science refines a system. [6 marks]
Common error to watch for: in Q2, comparing atomic numbers instead of relative atomic masses.