In a neutral atom (no charge), the number of electrons equals the number of protons, so it will be 17 electrons also. However, the number of neutrons will vary depending on the isotope (atoms of the same element with different numbers of neutrons). There are three isotopes of Chlorine: 35Cl, 36Cl, and 37Cl having 18, 19, and 20 neutrons respectively, though 35Cl is most common (75.77%).
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