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A neutral atom with atomic number 20 will have 20 electrons.
The atomic number is, by definiton, the number of protons in an atom's nucleus but for a neutral atom it's also equal to the number of electrons.
Each element has a different, unique number of protons that determines its identity. Electrons are outside the nucleus and can be lost or gained in chemical reactions. An atom that has lost or gained electrons has a net electrical charge and is called an ion.
The use of the word atom rather than ion implies that no electrons were lost or gained so the number of electrons is equal to the number of protons. Electrons have a charge of -1 and protons have a charge of +1, so an atom is neutral when it has equal numbers of these two particles.
Element 20 is calcium, which loses two electrons to form a 2+ ion. So while a calcium atom has 20 electrons, a calcium ion has only 18.
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