Dmitri Mendeleev was a Russian chemist who created one of science's most useful organizing tools, the periodic table. In 1869, he arranged known elements so that patterns in their chemical behavior became clear. His table mattered because it turned a long list of substances into a system that could explain relationships and predict discoveries.
It showed that chemistry has an underlying order rather than being a collection of isolated facts.
Mendeleev mainly ordered elements by increasing atomic mass, but he also grouped elements with similar properties into columns. When the pattern did not fit, he left gaps and predicted that undiscovered elements would fill them. Some of his predictions, such as eka-aluminum later named gallium, matched measured properties closely.
Modern periodic tables use atomic number instead of atomic mass, but they still reflect the periodic patterns that Mendeleev recognized.
Key Facts
- Mendeleev published his periodic table in 1869.
- He arranged elements mostly by increasing atomic mass and recurring chemical properties.
- Periodic law in modern form: properties of elements are periodic functions of atomic number.
- Atomic number equals the number of protons: Z = number of protons.
- Average atomic mass can be found by isotope abundance: average mass = sum(isotope mass x fractional abundance).
- Mendeleev left gaps for undiscovered elements and predicted their masses, densities, and chemical behavior.
Vocabulary
- Periodic table
- A chart that organizes elements by atomic number and repeating patterns in their properties.
- Periodic law
- The principle that element properties repeat in a regular pattern when elements are ordered by atomic number.
- Atomic mass
- The weighted average mass of an element's atoms based on its naturally occurring isotopes.
- Group
- A vertical column of the periodic table whose elements often have similar chemical properties.
- Prediction
- A scientific statement about an unknown result based on patterns, evidence, and reasoning.
Common Mistakes to Avoid
- Saying Mendeleev arranged elements only by atomic mass is wrong because he also used chemical properties to keep similar elements together.
- Assuming Mendeleev knew about protons is wrong because atomic number was discovered later, after his original table was made.
- Filling Mendeleev's gaps with random elements is wrong because the gaps were based on recurring patterns in valence, formulas, mass, and density.
- Thinking periodicity means every property increases smoothly is wrong because periodicity means properties repeat in cycles across rows and columns.
Practice Questions
- 1 Mendeleev noticed a missing element between aluminum with atomic mass about 27 and indium with atomic mass about 115 in a group pattern. If a simple estimate uses the average of 27 and 115, what atomic mass would be predicted?
- 2 An element has two common isotopes: 60.0 percent with mass 68.9 amu and 40.0 percent with mass 70.9 amu. Calculate the average atomic mass using average mass = sum(isotope mass x fractional abundance).
- 3 Mendeleev placed tellurium before iodine even though tellurium has a slightly greater atomic mass. Explain why grouping by chemical properties helped him make this choice.