What is a substance and what are the two types of substances?

The two types of substances are elements and compounds.

Explanation:

A pure substance is one in which the composition is definitive and constant throughout, and that has distinct chemical properties. These substances are comprised of only one entity, not a mixture of two or more.

Elements are pure substances, assuming they are free of imperfections, because they are made of only one type of atom.

Compounds are also pure substances, like water, table salt, table sugar, etc. because they are comprised of one type of compound (H2O, NaCL, C6H1206,etc)

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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a corroding iron, if that's one of your choices