A period of the periodic table ends with the highest energy level of an element's ators
having?"

the last period. i think has the largest energy level

Explanation:

it is to be noted that an alpha particle is an helium nucleus. so it contains 2 protons 2 neutrons and has a mass number of 4. when alpha decay particles are ejected from the nucleus it gets transformed into an atom which has atomic number less than 2 and a mass number less than 4.

an equation that could represent alpha decay is radium 226 decay of alpha particle forms radon 222.

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if you mean hydrogen and oxygen then water is the product