The reactivity of an atom arises from

The number of outer shell electrons in the atom, outer shell distance from nucleus etc.

An atom can have one to eight outer shell electrons. Group 1 elements are extremely reactive because they only need to donate one outer shell electron to achieve noble gas configuration. Group 2 elements with 2 electrons in their atoms are also quite reactive. group 16 and 17 receive 2 and 1 electrons respectively. Non metals can also share electrons in covalent bonding. group 18 elements rarely react because they have stable noble gas configuration, thus they dont share or donate/receive electrons.

Potassium is more reactive than lithium because electron configuration of potassium is 1s22s22p63s23p64s1 while electron configuration of lithium is 1s22s1. The potassium has more electron shells than lithium thus the outer shell is farther from the nucleus, so its easier to donate the outer electron. Conversely chlorine has a smaller atomic radius than iodine so although they all have 7 outer electrons the chlorine can attract other electrons more easily than the iodine thus its more reactive.

                   hope it helps u

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definition

                    entropy is a measure of the randomness or the extent of disorder of a chemical process.

                      enthalpy is a measure of the heat change of a reaction occurring at a constant pressure.

measurement units:

                        entropy is measured in joule per kelvin (jk⁻¹)   whereas enthalpy is measured in joule per mole (jmol⁻¹)

requirements:

                            entropy has no requirements or limits, and its change is measured by the division between the heat change of the chemical process and the temperature.

                              in contrast, enthalpy can only be related to the heat change of the reaction under standard conditions.