Structure and bonding · GCSE Chemistry
Mendeleev and the periodic table
GCSE Chemistry revision on Mendeleev and the periodic table: atomic number order, groups and periods, left gaps, and how electronic structure explains the modern table.
Mendeleev left gaps and predicted properties; those predictions came true. Today the table is atomic number order. Group is outer electrons; period is number of shells.
The important bits
What you need to know
- 1
The modern periodic table arranges elements in order of atomic number (proton number). Columns are groups; rows are periods.
- 2
Elements in the same group have the same number of outer electrons and similar chemical properties. Period number equals the number of occupied shells.
- 3
Early tables used atomic weight. Newlands’ octaves repeated properties every eighth element but broke down after calcium and were not accepted.
- 4
Mendeleev arranged by atomic weight but left gaps for undiscovered elements, swapped pairs to keep properties aligned (tellurium before iodine), and predicted properties of missing elements such as gallium (eka-aluminium).
- 5
When gallium, scandium and germanium were found with properties close to his predictions, scientists accepted his table. That is the evidence question.
- 6
Atomic number, found later, explained the swaps: iodine has a higher atomic number than tellurium even though its atomic weight is lower. The table is proton order, not weight order.
- 7
Noble gases were discovered after Mendeleev’s first table and formed a new group — extra evidence that properties are periodic.
- 8
Metals sit on the left and towards the bottom; non-metals on the right. A metal oxide is basic; many non-metal oxides are acidic (CO₂, SO₂). The zig-zag line marks the changeover.
Quotations worth analysing
Short evidence. Real method.
“Mendeleev left gaps and predicted the properties of missing elements.”
A scientific idea is trusted when it makes predictions that experiments later confirm. “He was famous” does not score. Name a prediction that came true.
“The periodic table is in order of atomic number.”
Atomic number is the number of protons and defines the element. That order also groups elements by electronic structure, which Mendeleev could not yet see.
“Group = outer electrons. Period = number of shells.”
Sodium 2,8,1 is Group 1, Period 3. This is how the table, bonding and reactivity trends become one idea rather than three topics to memorise.
Go deeper
Acceptance came from predictions, not from a tidy grid
Other chemists had patterns. Mendeleev’s advantage was leaving gaps instead of forcing every known element into a box, and then saying what the missing element should be like. When eka-aluminium turned out to be gallium, with a matching density and oxide, the table looked like a law of nature rather than a filing system. He also placed tellurium before iodine so that iodine stayed with the other halogens, against atomic weight. Later, atomic number showed he was right about the order. In the exam, give one gap/prediction and one pair swap. That is the six-mark history of science item.
Go deeper
Electronic structure is the reason the groups work
Mendeleev did not know about protons or electron shells. We do. Group 1 has one outer electron; Group 7 has seven; Group 0 is full. That is why sodium and potassium both make +1 hydroxides, and why chlorine and bromine both make 1− salts. Period 3 elements Na to Ar fill the third shell. Once you can write the structure, you can explain a property without memorising the whole table. Metals on the left lose electrons; non-metals on the right gain or share. The periodic table is a map of that behaviour, not a poster to colour in.
Go deeper
Metals, non-metals and the oxide test
A metal oxide such as Na₂O or MgO is ionic and basic: it reacts with acids to make a salt and water. Many non-metal oxides such as CO₂ and SO₂ are covalent and acidic: they dissolve to make acids and cause acid rain. Silicon dioxide is giant covalent and does not behave like a simple acid or a Group 1 oxide. If a question gives an unknown element in Group 2, Period 4, you can already say: two outer electrons, 2+ ion, metal, basic oxide, more reactive than the Group 2 element above it. That is using the table as a tool, which is what GCSE Chemistry is after.
See the idea in action
Mendeleev placed tellurium (heavier) before iodine so that iodine stayed in the halogen group. Explain why this is correct in the modern table, and why scientists accepted his arrangement. Iodine has atomic number 53 and tellurium 52, so iodine comes after tellurium when elements are in proton order, even though tellurium’s relative atomic mass is slightly higher. Scientists accepted the table because Mendeleev left gaps and predicted properties of missing elements; those elements were later discovered with matching properties.
Exam technique
Turn knowledge into marks
For “why was Mendeleev’s table accepted?”, write: left gaps, predicted properties, predictions confirmed. For the modern table, say atomic number, then group = outer electrons. Do not say he arranged by electronic structure — he did not know it.
Common mistakes
Do not give these marks away
- 01
Saying Mendeleev arranged elements by atomic number, or by electronic structure.
- 02
Explaining acceptance with “he was a famous scientist” rather than predictions that came true.
- 03
Mixing up group and period, or saying noble gases were in Mendeleev’s original table.
Why did scientists accept Mendeleev’s periodic table?
AHe arranged elements strictly by atomic number
BHe left gaps and predicted properties of undiscovered elements, which were later confirmed
CHe included the noble gases from the start
DHe knew about electron shells
Show the answer
He left gaps and predicted properties of undiscovered elements, which were later confirmed. Successful predictions are the evidence. Atomic number and noble gases came later and explained why his order worked so well.
Quick questions
If this is the bit you searched
How did Mendeleev arrange the periodic table?
Mainly by atomic weight, but he left gaps for missing elements, swapped some pairs to keep properties in groups, and predicted the properties of the gaps.
Why is the modern periodic table in atomic number order?
Atomic number is the number of protons and defines the element. That order groups elements by electronic structure and explains pairs Mendeleev swapped.
What is the difference between a group and a period?
A group is a column: same number of outer electrons, similar chemistry. A period is a row: same number of occupied electron shells.
Did Mendeleev know about protons and electrons?
No. Those were discovered later. His table still worked because atomic weight is roughly in the same order as atomic number, with a few exceptions he handled by swapping.