Answer:
The number of electrons in the outermost shell of an atom determines its reactivity. Noble gases have low reactivity because they have full electron shells. Halogens are highly reactive because they readily gain an electron to fill their outermost shell.
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I think the correct answer would be the third option. An example of a pure research would be creating synthetic elements to study their properties. Pure research is also known as fundamental or basic research. It has an exploratory nature wherein it is done without any end use in mind. It is mostly out of the curiosity or intuition of a researcher. Most of the study under this type is aiming to advance an existing knowledge by explaining or identifying the relationship of the variables being studied. It is aimed at improving theories in order to the understanding of a specific phenomenon.
Sr N(e⁻)=38
Sr²⁺ N(e⁻)=38-2=36
36e⁻
