Development of the periodic table
Lajoy Tucker
Teacher
Contents
Early Attempts to Classify Elements
Before the discovery of protons, neutrons and electrons, scientists tried to arrange elements in order of their atomic weights (now known as relative atomic masses).
They noticed that elements with similar properties appeared at regular intervals, but early versions of the periodic table were incomplete.
Because only a few elements were known, many were placed in inappropriate groups when arranged strictly by atomic weight
Newlands' Octaves
H | Li | Ga | B | C | N | O |
F | Na | Mg | Al | Si | P | S |
Cl | K | Ca | Cr | Ti | Mn | Fe |
Co, Ni | Cu | Zn | Y | In | As | Se |
Br | Rb | Sr | Ce, La | Zr | Di, Mo | Ro, Ru |
Pd | Ag | Cd | U | Sn | Sb | Te |
I | Cs | Ba, V | Ta | W | Nb | Au |
Pt, Ir | Tl | Pb | Th | Hg | Bi | Cs |
Mendeleev’s Contributions
In 1869, Russian chemist Dmitri Mendeleev created the first useful version of the periodic table.
He arranged elements mainly by increasing atomic weight, but unlike earlier scientists, he also considered chemical properties.
When properties did not fit the pattern, Mendeleev changed the order of some elements so that those with similar properties were in the same vertical groups.
He left gaps in his table for undiscovered elements, predicting their atomic weights and properties.
When these missing elements were later discovered (e.g. gallium and germanium), their properties closely matched Mendeleev’s predictions.
This provided strong evidence that his arrangement was correct and meaningful, and his work became the foundation for the modern periodic table.
Series | Group I | Group II | Group III | Group IV | Group V | Group VI | Group VII | Group VIII |
1 | H | |||||||
2 | Li | Be | B | C | N | O | F | |
3 | Na | Mg | Al | Si | P | S | Cl | |
4 | K | Ca | ? | Ti | V | Cr | Mn | Fe, Co, Ni, Cu |
5 | Cu | Zn | ? | ? | As | Se | Br | |
6 | Rb | Sr | ?Yt | Zr | Nb | Mo | ? | Ru, Rh, Pd, Ag |
7 | Ag | Cd | In | Sn | Sb | Te | I | |
8 | Cs | Ba | ?Di | ?Ce | - | - | - | --- |
9 | - | - | - | - | - | - | - | |
10 | - | - | ?Er | ?La | Ta | W | - | Os, Ir , Pt, Au |
11 | Au | Hg | Tl | Pb | Bi | - | - | |
12 | - | - | - | Th | - | U | - | --- |
Mendeleev's 1871 table
Later Developments and Modern Understanding
With the discovery of subatomic particles, scientists gained a clearer understanding of atomic structure.
The elements were later rearranged in order of atomic (proton) number, not atomic weight.
The discovery of isotopes explained why ordering purely by atomic weight sometimes caused elements to be in the wrong place.
For example, argon has a higher atomic mass than potassium, but it appears before potassium in the modern table because its atomic number is smaller (18 vs 19).
Feature | Mendeleev’s Periodic Table | Modern Periodic Table |
|---|---|---|
Basis of arrangement | Increasing atomic weight (relative atomic mass) | Increasing atomic (proton) number |
Grouping by properties | Grouped elements with similar properties together | Grouped elements with similar properties together |
Gaps for undiscovered elements | Left gaps and predicted their properties | No gaps – all known elements are included |
Check for Understanding
Recall
a) Before the discovery of subatomic particles, what property did scientists use to arrange elements in early periodic tables?
Answer
Atomic weight (relative atomic mass).
b) Which scientist created the first widely accepted periodic table in 1869?
Answer
Dmitri Mendeleev.
c) What did Mendeleev leave in his periodic table for elements that had not yet been discovered?
Answer
Gaps for undiscovered elements.
Application
d) Mendeleev sometimes changed the order of elements when arranging them. Explain why he did this.
Answer
He changed the order so that elements with similar chemical properties were placed in the same groups.
e) Argon has a higher atomic mass than potassium but appears before potassium in the modern periodic table. Explain why this is the case.
Answer
The modern periodic table is arranged by atomic number rather than atomic mass. Argon has a lower atomic number than potassium, so it appears before it.
f) In what key way is the modern periodic table similar to Mendeleev’s version?
Answer
Both tables group elements with similar chemical properties in the same vertical columns (groups).
Challenge
g) Explain why Mendeleev’s periodic table was considered successful even though some elements had not yet been discovered.
Answer
Mendeleev predicted the existence and properties of undiscovered elements. When these elements were later discovered, their properties matched his predictions, supporting the accuracy of his table.
h) How did the discovery of isotopes help explain problems with Mendeleev’s table?
Answer
It showed that elements could have different atomic masses but the same atomic number, explaining why some seemed out of order by weight.
Summary
Early periodic tables were based on atomic weight and often had errors.
Mendeleev corrected many issues by grouping elements by properties and leaving gaps for undiscovered ones.
The modern periodic table is arranged by atomic number, supported by discoveries about atomic structure and isotopes.
Both systems group elements with similar properties together, showing a repeating, periodic pattern in the elements’ behaviour.
No answer provided.