Chlorine atoms tend to form ions by taking in one electron. This means the atom has a charge of -1. How does the additional electron affect the atomic number and the
mass number for this atom?

The additional electron does not affect the atomic number nor the mass number. The atomic mass is the sum of neutrons and protons. Electrons are not accounted in atomic mass as their mass is so small, approximately 1/1,836 of a protons mass. Atomic number is the total number of protons of an atom. Chlorine taking in another electron will not change its atomic number. In an ion, the atomic number and atomic mass do not change from the original.

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explanation:

covalent bonds

consist of pairs of electrons shared by two atoms, and bind the atoms to fixed orientation and requre high energies to break them

ionic bonds

when two atoms differ considerably in their electronegativity, then one of the atoms will lose its electron to the other atom. this results in a positively charged ion(cation) and a negatively charged ion (anion)

while cooking a piece of 1/2 pound beef steak, a cast iron pan (2.4kg in weight) was heated 260 celsius.