Question #1740d

Yes, you were mostly correct. Reactivity or lack thereof are observations, and could not be known ahead of time without knowledge of an experiment already performed.

Periodicity, #(b)#, was proposed by Mendeleev. It started as "hey, what if I arranged the elements like this?", and then became an observation, i.e. "oh hey, there's a pattern here." However, I would say that it developed into a theory that was tested over time, and eventually became known as the periodic law.

The relationship of reactivity with electronic structure, #(d)#, was a theory when it was first proposed. It still is a theory, as for example, #"Kr"# reacts with #"F"# quite a bit more than #"Ar"# or #"Ne"#... even though it also has filled valence #p# orbitals like #"Ar"# or #"Ne"#.

A theory is a prediction that will have at some point been put to the test, and could be modified at some point in time.

A nice example is the geocentric theory... it has clearly been proven false that the sun revolved around the Earth for many years, so this theory has been disregarded and replaced with the heliocentric theory. (It's not to say that the heliocentric theory will stay forever, but it probably will...)

A law has repeatedly been proven true such that it is simply accepted, provided the conditions are right for the law to apply.

Examples are the laws of conservation of mass and conservation of energy, provided that the system being considered is closed to mass transfer and energy transfer.

An observation is a statement that can only be made in practice, i.e. by performing the experiment.

Examples are, "this thing is reactive", or "my god, this is hot!". Or, one could say, "hey, apparently, if I do this... I get this!"

A. Yes Chlorine is observed to be a highly reactive gas.
This could have been predicted by theories of the reactivity of electron configurations. The theories explain the observations.
Good theories make good predictions.

B. Yes this has become the periodic theory or law.
initially these observations were used to develop the theory of periodicity that became the Periodic law.

C. Yes this is an observation. Neon and all of the noble gases are non reactive.
These observations were made after the development of the Periodic Theory and helped to confirm the predictions of the Periodic Table.

D. Yes and No The theory explains why the reactivity depends on the electron configuration and electro negativity. The law describes what will happen.