Development of the periodic table — AQA GCSE Combined Science
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Development of the periodic table explained
Before protons, neutrons and electrons were discovered, scientists had no concept of atomic number.
Read the full explanation
They tried to classify elements by arranging them in order of their atomic weights, which they could measure from chemical reactions. Early attempts, such as Newlands' law of octaves, forced elements into groups of eight and left no gaps, so they failed when new elements were found. Mendeleev's periodic table was more successful because he arranged elements mainly by atomic weight but left gaps for undiscovered elements and placed some elements out of strict weight order to keep similar properties together. For example, he predicted the properties of gallium and germanium, which were later discovered and matched his predictions. The modern periodic table orders elements by atomic number, which explains the patterns that early scientists could only observe.
The early periodic tables were incomplete and some elements were placed in inappropriate groups if the strict order of atomic weights was followed.
Before Mendeleev, chemists arranged elements by atomic weight, but their tables had two linked weaknesses. First, they were incomplete: only known elements could be included, leaving no gaps for undiscovered ones. Second, forcing every element into strict atomic weight order put some elements into groups where their properties did not match. For example, iodine (Ar ≈ 127) is lighter than tellurium (Ar ≈ 128). Strict weight order places iodine before tellurium, but iodine's properties match chlorine and bromine, while tellurium resembles sulfur and selenium. Early tables broke down because mass order and chemical behaviour disagreed.
Mendeleev overcame some of the problems by leaving gaps for elements that he thought had not been discovered and in some places changed the order based on atomic weights.
Mendeleev improved on earlier periodic tables by prioritising chemical and physical properties over strict atomic weight order. Where the mass order would have placed an element in a group whose properties did not fit, he swapped the order. He also left gaps in his table for elements he predicted were undiscovered, and used the gaps and trends to predict their properties. When gallium, scandium and germanium were later discovered, their properties matched his predictions closely, which gave strong support to his table. This shows how a model can be tested by prediction and subsequent evidence, and why Mendeleev's arrangement became the basis of the modern periodic table.
Elements with properties predicted by Mendeleev were discovered and filled the gaps. Knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct.
Mendeleev arranged elements mainly by increasing atomic weight but left gaps where no element fitted the pattern. He predicted properties of the missing elements, such as a metal he called eka-silicon, and when gallium and germanium were later discovered their properties matched his predictions, so his table gained acceptance. However, a few pairs of elements appeared in the wrong order when arranged strictly by atomic weight, for example tellurium before iodine. After isotopes were discovered, scientists understood that atoms of the same element can have different masses, so atomic weight is an average and is not the fundamental ordering property. Ordering by atomic (proton) number removes the anomalies and explains the pattern.
Students should be able to describe these steps in the development of the periodic table.
To describe the steps in the development of the periodic table, start with the early grouping of elements by similar properties, then describe Newlands' law of octaves, which arranged elements by atomic weight but was not accepted because the pattern broke down for heavier elements. Then describe Mendeleev's table, arranged mainly by atomic weight with gaps left for undiscovered elements and with properties predicted for those gaps. Later, the discovery of gallium and germanium matched his predictions, and the discovery of isotopes explained why a strict atomic-weight order sometimes placed elements incorrectly. Finally, the modern table orders elements by atomic (proton) number, which explains the pattern without anomalies.
Your focus
- Describe how early scientists attempted to classify elements by arranging them in order of atomic weight.
- Explain the strengths of Mendeleev's periodic table, including leaving gaps and predicting properties of undiscovered elements.
- Explain how the discovery of protons, neutrons and electrons led to the modern periodic table being ordered by atomic number.
Show all 15 objectives
- Describe why early periodic tables were incomplete.
- Explain how strict atomic weight order could place elements in inappropriate groups.
- Use a specific element pair to illustrate the conflict between mass order and property patterns.
- Explain why Mendeleev left gaps in his periodic table.
- Describe how and why Mendeleev changed the order based on atomic weights.
- Explain how later discoveries supported Mendeleev's predictions.
- Describe how Mendeleev used atomic weight and gaps to arrange the elements.
- Explain how the discovery of predicted elements supported Mendeleev's table.
- Explain why isotopes mean atomic weight is not always the correct basis for ordering elements.
- Sequence the main steps in the development of the periodic table.
- Describe how Mendeleev's predictions were tested by later discoveries.
- Explain how isotopes and atomic number led to the modern ordering of the periodic table.
Development of the periodic table exam tips
Marking Points
- State that before protons, neutrons and electrons were discovered, scientists had no knowledge of atomic number.
- Describe how early classification attempts arranged elements in order of their atomic weights.
- Explain that early tables, such as Newlands' law of octaves, forced elements into groups and left no gaps, causing problems when new elements were discovered.
- Describe Mendeleev's approach: arranging elements mainly by atomic weight but leaving gaps for undiscovered elements and sometimes reversing the order to keep similar properties together.
- Explain that Mendeleev's predictions of the properties of missing elements, such as gallium and germanium, were later confirmed, which supported his table.
- Recognise that the modern periodic table orders elements by atomic number, which resolved the inconsistencies in early weight-ordered tables.
- Early tables were built using atomic weights rather than atomic number.
- The tables were incomplete because only known elements could be placed, leaving no gaps for undiscovered elements.
- Following strict atomic weight order sometimes placed an element in a group with different chemical properties.
- The mismatch between mass order and property patterns showed that atomic weight alone was not fully reliable.
- A named example such as iodine and tellurium illustrates how strict mass order places iodine before tellurium, conflicting with observed group similarity.
- Mendeleev left gaps in his table for elements he believed were yet to be discovered.
- He sometimes reversed the strict atomic weight order so that elements with similar properties fell into the same group.
- He used the gaps and the trends in his table to predict the properties of undiscovered elements.
- Later discovery of elements such as gallium, scandium and germanium supported his predictions and the value of his table.
- His approach showed that chemical properties, not mass alone, were the better basis for arranging elements.
- Mendeleev arranged elements mainly in order of increasing atomic weight and left gaps for elements not yet discovered.
- He predicted the properties of missing elements from the properties of their neighbours, and later discoveries such as gallium and germanium matched those predictions.
- The success of these predictions supported Mendeleev's table and helped it replace earlier arrangements.
- Some pairs of elements, such as tellurium and iodine, were placed out of strict atomic-weight order because their chemical properties fitted better that way.
- Isotopes are atoms of the same element with different masses, so relative atomic mass is an average and does not uniquely determine an element's position.
- Ordering by atomic (proton) number explains the periodic pattern and removes the anomalies seen with atomic weight.
- Early attempts grouped elements by similar chemical and physical properties.
- Newlands arranged elements in order of atomic weight and proposed a repeating pattern, but his ideas were not widely accepted because the pattern did not hold for all elements.
- Mendeleev arranged elements mainly by atomic weight, left gaps for undiscovered elements and predicted their properties.
- The later discovery of elements such as gallium and germanium, with properties close to Mendeleev's predictions, supported his table.
- Isotopes were discovered later, showing that atomic weight is an average and explaining why strict atomic-weight ordering sometimes failed.
- The modern periodic table orders elements by atomic (proton) number, which removes the anomalies and explains the repeating pattern.
Examiner Tips
- 💡Name the scientist and describe the specific feature of their table, such as Mendeleev leaving gaps, rather than making a general statement.
- 💡Explain why a feature was an advantage or disadvantage, for example gaps allowed predictions of new elements to be made and tested.
- 💡Use a named example, such as gallium or germanium, to show how Mendeleev's predictions were confirmed.
- 💡Link the development of the table to the discovery of subatomic particles, showing how atomic number replaced atomic weight as the ordering principle.
- 💡Distinguish between the two problems: incompleteness was due to undiscovered elements, while inappropriate grouping stemmed from strictly following atomic weight order.
- 💡Use the iodine and tellurium example to show how strict mass order places iodine before tellurium, conflicting with chemical similarity.
- 💡State clearly that Mendeleev changed the order in some places and left gaps, then explain why each action helped.
- 💡Use one named predicted element, such as gallium or germanium, to show how later discoveries supported his table.
- 💡Contrast Mendeleev's approach with earlier strict mass-order tables to show the improvement.
- 💡When describing Mendeleev's table, always mention both the gaps and the predictions, because these are the key evidence that made his table successful.
- 💡Use a named example such as gallium or germanium to show that predicted properties were confirmed.
- 💡If asked why atomic weight caused problems, refer to isotopes and to the fact that relative atomic mass is an average value.
- 💡Use a clear sequence with connectives such as first, then, later and finally to show the order of the steps.
- 💡Include one named element whose discovery supported Mendeleev's predictions, such as gallium or germanium.
- 💡Link the isotope explanation to the modern table by stating that atomic number, not atomic weight, gives the correct order.
Common Mistakes
- Thinking Mendeleev arranged elements strictly by atomic weight: correct this by stating that he left gaps and sometimes placed elements out of weight order to group similar properties together.
- Believing early scientists knew about protons and neutrons: correct this by explaining that these subatomic particles were discovered later, so early tables could not use atomic number.
- Assuming all early periodic tables were equally successful: correct this by contrasting Newlands' table, which left no gaps, with Mendeleev's table, which left gaps and made successful predictions.
- Confusing atomic weight with atomic number: correct this by defining atomic weight as the mass of an atom relative to a standard, while atomic number is the number of protons in the nucleus.
- Stating strict mass order places iodine after tellurium; correction: iodine is lighter, so strict mass order places it before tellurium.
- Claiming the tables were incomplete only because scientists had not tried hard enough; correction: explain that undiscovered elements were absent.
- Treating every early table as identical; correction: recognise that different chemists proposed different arrangements, but all shared the mass-order limitation.
- Saying Mendeleev left gaps because he had run out of space. Correction: he left gaps deliberately for undiscovered elements and predicted their properties.
- Believing Mendeleev ignored atomic weights completely. Correction: he used atomic weights but changed the order where properties demanded it.
- Confusing Mendeleev's predictions with the modern table's use of atomic number. Correction: Mendeleev worked with atomic weights; atomic number came later.
- Saying Mendeleev arranged elements by atomic number: correct this by stating he used atomic weight, and that atomic number was introduced later.
- Claiming Mendeleev predicted the discovery of elements: correct this by saying he predicted their properties, and the elements were discovered later.
- Stating that isotopes have different chemical properties: correct this by explaining that isotopes of an element have the same proton number and so similar chemical properties, but different masses.
- Describing the steps in the wrong order: correct this by sequencing early property grouping, Newlands, Mendeleev, isotopes and the modern atomic-number table.
- Saying Newlands' table was accepted immediately: correct this by stating that it was criticised and not widely accepted at the time.
- Confusing atomic weight with atomic number: correct this by stating that Mendeleev used atomic weight, while the modern table uses atomic number.