Question

✔NO SPAMMING

What Do You Mean By The Term "Chemical Reactivity"?

How does it change across a period from left to right and down a group from top to bottom In the Periodic Table?

✔Content Quality Answer Is Required !!

Answer 1

Chemical reactivity is the tendency of a substance to undergo chemical changes in a system. The chemical reactivity worksheet offers an excellent database of reactivity for over 4,000 common hazardous chemicals. 
As we go from left to right of the periodic table (across), nuclear charge increases and the additive electrons are added to the same shell, therefore ionization energy increases with increase in electronegativity lead to dynamic exchange of electrons.
DOWN a Group: In METALS reactivity INCREASES as you go DOWN a Group because the farther down a Group of metals you go, the easier it is for electrons to be given or taken away, resulting in higher reactivity.
ACROSS a Period : In METALS reactivity DECREASES as you go ACROSS a Period because though they still want to give away valence electrons they have more of them to get rid of, which requires more energy. Not as easy to blow off a little steam!
NON-METALS
UP a Group: In NON-METALS reactivity INCREASES as you go UP a Group because the higher up and to the right atoms are, the higher the electronegativity, resulting in a more vigorous exchange of electrons. Fluorine? A greedy, impatient beast when it comes to electron exchange manners.
ACROSS a Period : In NON-METALS reactivity INCREASES as you go ACROSS a Period because (notice how trends repeat?) the closer you get to fulling your s- and p- orbitals the more motivated you are to do so.

Answer 2

Hey.....!!!!

Chemical reactivity is the tendancy of an element to perform chemical reaction and undergo any chemical change....

Across a period, the chemical reactivity decreases because the new electrons are added to same shell, increasing the nuclear force of attraction between the nucleus and the outermost electrons...
while going down the group, new shells are added which brings the outermost electron away from the nucleus and increasing the chemical reactivity.....

Hope this helps you.... ☺☺