C. added an electron to the outer electron shell.
Explanation:
Atoms consist of three particles:
- Protons: they are located in the nucleus, they have positive charge of [tex]+e[/tex], and mass of [tex]1.67\cdot 10^{-27}kg[/tex]
- Neutrons: they are also located in the nucleus, they have no electric charge, and mass similar to that of the proton
- Electrons: they orbit around the nucleus, they have negative charge of [tex]-e[/tex], and mass around 1800 smaller than the proton
Normally, atoms are neutral (no electric charge), because they have an equal number of protons and electrons.
However, sometimes atoms can give off or take electrons from other atoms. We have two cases:
Therefore, in order to form a negative ion, the atom must have
C. added an electron to the outer electron shell
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