When trying to identify an unknown element, a scientist determines what other elements the unknown element reacts with chemically. which property of the unknown element determines the other elements it reacts with?

The number of valence electrons and electronegativity of an element in the unknown element determines the other elements it reacts with.

Explanation:

The ability of an atom to attract the electrons for bonding is called electro negativity. When an atom has highjer electronegativity it can attract electrons stronger. When the atom's electronegativity is lower it can attract electrons weaker. This property is very important in the determination of how well an element can readily react with another element.  

The determination of an element's reactivity can also be done  by the number of valence electrons in the outermost shell. The elements that has valence electrons lesser than 4 will be giving its valence electron for getting 8 electrons in the next shell so that it can become an ion of stable nature. An element that has 4 electrons will form covelant bond. The elements that have greater than 4 electrons will be getting electrons from some other element.

they begin to vibrate harder causing the material that was solid to turn into a liquid i.e gallium or ice

a is the answer to this question

explanation: