AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › Atomic structure and the periodic table › The periodic table
Practise Development of the periodic table. 7 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How the periodic table developed: early tables ordered by atomic weight, the changes Mendeleev made (gaps and swapped elements), and how later discoveries showed he was right. It is usually a 2 to 6 mark 'describe' or 'explain' question about how scientific ideas change with new evidence.
Key facts
Early tables: ordered by atomic weight; incomplete; some elements in the wrong groups
Mendeleev left gaps for elements not yet discovered
Mendeleev changed the order of some elements to fit similar properties
Elements discovered later (e.g. gallium, germanium) matched his predictions
Isotopes explain why atomic weight order is not always right
Modern table: ordered by atomic (proton) number
Notes
Early periodic tables
Before protons, neutrons and electrons were discovered, scientists tried to classify the elements by arranging them in order of their atomic weights (what we now call relative atomic masses).
The early tables were incomplete, because many elements had not yet been discovered.
If the strict order of atomic weights was followed, some elements were placed in inappropriate groups, with elements whose properties were different from theirs.
What Mendeleev did
diagram
Dmitri Mendeleev overcame some of these problems in two ways.
He left gaps for elements that he thought had not been discovered yet, and used the patterns in his table to predict their properties.
In some places he changed the order based on atomic weights, so that elements were in groups with similar properties. For example, he put tellurium (atomic weight about 128) before iodine (about 127), so that iodine was in the same group as chlorine and bromine.
Mendeleev left gaps and swapped the order of some elements.
Evidence that he was right
Elements with properties that matched Mendeleev's predictions were later discovered and filled the gaps, e.g. gallium and germanium.
His predictions had been tested and found to be correct, so other scientists accepted his table.
After protons were discovered, scientists realised that the table is really in order of atomic (proton) number.
Mendeleev's table compared with the modern table
Mendeleev: ordered mainly by atomic weight, with some swaps; had gaps; had no group of noble gases (they had not been discovered).
Modern: ordered by atomic number; no gaps; includes Group 0 (the noble gases).
How to answer each type of question
Describe how Mendeleev's table differed from earlier tables
2 marksGrade 5
Gaps for undiscovered elements.
Some elements swapped out of atomic weight order so that they fitted groups with similar properties.
Example. Describe two ways in which Mendeleev's periodic table was different from earlier periodic tables.
Show the model answerHide the model answer
He left gaps for elements that had not been discovered yet (1). He changed the order of some elements (from atomic weight order) so that they were in groups with similar properties (1).
Don’t lose marks
Saying Mendeleev ordered the elements by atomic number. Protons had not been discovered.
Saying he guessed the gaps. He predicted properties from patterns, and the predictions were tested.
Saying he swapped elements because of isotopes. Isotopes were discovered later; they explained why his swaps were right.
Writing 'mass number' instead of atomic weight or relative atomic mass.
More tips
Memory tricks
Weights then, numbers now: early tables used atomic weight; the modern table uses atomic number.
Mendeleev's two changes: Gaps and Group swaps. The proof: gallium and germanium.
Pairs that break atomic weight order: tellurium before iodine, argon before potassium.
Exam technique
For 'why was it accepted', the key idea is that his predictions were tested and found to be correct.
In the 6-mark question, write in time order and use linking words such as 'because', 'so' and 'later'.
If the question gives data, quote the numbers in your answer.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State how early tables were orderedBy atomic weight, because protons, neutrons and electrons had not been discovered.
Grade 4
Describe problems with early tablesThey were incomplete, and strict atomic weight order put some elements in groups with the wrong properties.
Grade 5
Describe what Mendeleev didHe left gaps for undiscovered elements and changed the order of some elements to fit their properties.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Before protons, neutrons and electrons were discovered, scientists arranged the elements in order of one property. Name this property.
atomic weight
Complete the sentence. Knowledge of .......... made it possible to explain why the order based on atomic weights was not always correct.
isotopes
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Scientists tried to classify the elements long before the modern periodic table was developed.
(a) Before protons, neutrons and electrons were discovered, scientists arranged the elements in order of one property. Name this property.[1]
(b) Some elements were placed in inappropriate groups in early periodic tables. Suggest why.[1]
(c) Mendeleev left gaps in his periodic table. Why?[1]
(d) Give one reason why other scientists came to accept Mendeleev’s periodic table.[1]
Show the answer and mark scheme
(a)Answer: atomic weight
atomic weight / relative atomic mass
(b)Answer: The strict order of atomic weights was followed.
(because) the strict order of atomic weights was followed
(c)Answer: For elements that he thought had not yet been discovered.
for (undiscovered) elements / elements that had not been discovered (yet)
(d)Answer: Elements were discovered that fitted the gaps and had the properties he predicted.
elements were discovered that filled the gaps / had properties that matched his predictions
Question 2Medium2 marks
Mendeleev left a gap in his table for an element he called eka-aluminium, and predicted its properties. When gallium was discovered, its properties matched his predictions closely. A student says: ‘Mendeleev was just lucky that gallium turned out to have the properties he predicted.’
Evaluate the student’s statement.[2]
Show the answer and mark scheme
Answer: It was not luck: Mendeleev based his predictions on the patterns in the properties of the elements in the same group, and other elements he predicted were also discovered with the properties he predicted.
the predictions were based on patterns / trends in the properties of the elements in the same group, not guesses
other elements that Mendeleev predicted were also discovered with the properties he predicted, which is very unlikely to happen by chance