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

the enthalpy of formation of water is given as -285.8kj/mol, which states that the enthalpy of formation is negative.

negative enthalpy of formation indicates that the heat is released during the reaction process. the heat is released from reaction to the surroundings the enthalpy is expressed with a negative sign with the energy given out in the reaction process.

the formation of water takes place by giving out heat which is -285.8 kj/mol.