21 neutrons, 20 electrons, 20 protons

Answer: Ca

Explanation:atoms are made of 3 types of sub atomic particles; protons, neutrons and electrons

electrons are negatively charged, protons are positively charged and neutrons are neutrally charged.

atoms of elements are neutral with an equal number of protons and electrons.

Atomic number is the number of protons. atomic number is characteristic for the element.

to identify an element we have to know the number of protons. the given atom has 20 protons therefore the atomic number is 20. we can check the periodic table to find the element with the atomic number of 20

the element with an atomic number of 20 is Ca.

answer is Ca