Final answer:
The reactivity of an atom arises primarily from its electron configuration, which is determined by the arrangement of electrons in the outer energy level. Valence electrons, found in the outermost energy level, are responsible for an atom's chemical reactivity. Elements in the same group of the periodic table have similar chemical properties due to their similar electron configurations. Hence the correct answer is option A
Step-by-step explanation:
The reactivity of an atom arises primarily from its electron configuration. The electrons in the outer energy level, also known as valence electrons, are involved in chemical reactions and determine how easily an atom can gain, lose, or share electrons with other atoms. This electron configuration influences the chemical behavior and properties of an element.
For example, elements in the same group of the periodic table have similar chemical properties because they have the same number of valence electrons, which are responsible for their reactivity. Noble gases, like helium and neon, have stable electron configurations and are therefore unreactive.
Hence the correct answer is option A