Early tables were ordered by atomic weight
Definition
Atomic weight
A historical term for the relative atomic mass of an element, which depends on the masses and relative abundances of its isotopes.
- Before protons and electrons were known, scientists could not use atomic number, so they ordered elements by atomic weight.
- John Newlands arranged the known elements in order of atomic weight and noticed that every eighth element seemed similar, which he called the law of octaves.
- His idea broke down because he left no gaps and forced elements into groups where they did not fit.
- Some elements with very different properties ended up in the same column, so his work was not accepted.
Note
Ordering by weight alone was not enough; the arrangement also had to match properties.
Mendeleev left gaps and changed the order
Definition
Chemical properties
The ways in which an element or substance behaves during chemical reactions.
- Dmitri Mendeleev also arranged elements largely in order of atomic weight, but he was guided by their chemical properties.
- He left gaps for elements he believed had not yet been discovered.
- He sometimes changed the order of neighbouring elements so that each one lined up with others of similar properties.
- Using the gaps, he predicted the properties of missing elements.
- When those elements were later discovered and matched his predictions, his table became widely accepted.
Key Idea
- Gaps turned a weakness into a strength: they let Mendeleev make testable predictions.
- His table was organised by properties, not weight alone.
Isotopes explained why the order sometimes swaps
Definition
Isotope
Atoms of the same element with the same number of protons but different numbers of neutrons.
- Mendeleev's swaps looked strange while elements were ordered by atomic weight.
- The discovery of isotopes explained the problem: atomic weight is an average of isotope masses.
- This average can make one element heavier on average than the element that should come after it.
- Modern tables order elements by atomic number (protons), which removes the problem completely.
- Ordering by atomic number also matches the pattern of electronic structures, so properties line up correctly.
Self review
- Why did Newlands' arrangement break down?
- Answer: he left no gaps and forced elements with different properties into the same groups.
- What two things did Mendeleev do that made his table work?
- Answer: he left gaps for undiscovered elements and changed the order to match properties.
- How did the gaps help his table become accepted?
- Answer: he predicted the missing elements' properties, and later discoveries matched.
- How did isotopes explain the swapped order?
- Answer: atomic weight is an average of isotopes, so it can put a heavier-average element first; atomic number fixes this.
