19) the nuclear symbol of helium,

he, is also the symbol for designating a(n).

a) proton

Alpha particle

Explanation:

The helium symbol is also used to represent he alpha particles.

For example:

The americium with atomic wight  224 undergo alpha decay and produce

₉₃Np²³⁷ . The alpha particle emitted is also called helium nuclei. During this decay some gamma radiations also produce as a byproduct.

₉₅Am²²⁴   →    ₉₃Np²³⁷ + ₂He⁴

Properties of alpha radiation:

Alpha radiations are emitted as a result of radioactive decay. The atom emit the alpha particles consist of two proton and two neutrons. Which is also called helium nuclei. When atom undergoes the alpha emission the original atom convert into the atom having mass number 4 less than and atomic number 2 less than the starting atom.

Alpha radiations can travel in a short distance.

These radiations can not penetrate into the skin or clothes.

These radiations can be harmful for the human if these are inhaled.

These radiations can be stopped by a piece of paper.

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.