Very reactive elements are alkali metals and halogens.
The reactivity of iron is quite high, as it reacts by coming into contact with moisture in the air.
A substance with high reactivity has the tendency to undergo chemical reactions easily with other substances. This can lead to the release of energy, heat, light, or gas in a relatively short period of time. Examples include alkali metals like sodium and potassium.
A substance with high reactivity readily undergoes chemical reactions with other substances or even with itself, often releasing energy in the process. This high reactivity can lead to the substance easily forming new compounds or decomposing into simpler substances.
All halogens , carbon and oxygen.
Bromine, chlorine, and fluorine are examples of halogens, which are a group of reactive nonmetal elements located in group 17 of the periodic table. They are known for their high reactivity and ability to form salts with metals.
For metals high electronegativity mean low reactivity; for halogens, C, O, N, S, etc. the meaning is high reactivity.
Potassium is an example of a substance with high reactivity due to its tendency to readily react with water or oxygen. This reactivity is due to its outer electron configuration, which makes it highly reactive in forming bonds with other elements.
Oxygen stays as diatomic molecules in the atmosphere. These 2 oxygen atoms are bond with a double bond. So reactivity of oxygen is low.
Reactivity with water, oxidation, flammability
Examples: flammability, corrosivity, chemical reactivity.
Electronegativity - capacity to loss electrons - is representative for the reactivity of chemical elements; for metals low electronegativity is a high reactivity.
Some characteristics that indicate high chemical reactivity for one or more reagents include a high tendency to undergo chemical reactions, instability, high electronegativity, and high reactivity in the presence of other substances or conditions such as heat, light, or catalysts. Additionally, a high reactivity may be indicated by the presence of reactive functional groups such as alkyl halides, carbonyl compounds, or highly polarizable atoms.