You find the number of electrons in an atom or ion by considering its atomic number and charge.
Here's a breakdown of how to determine the number of electrons:
1. Neutral Atoms
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In a neutral atom (an atom with no charge), the number of electrons is equal to the number of protons.
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The number of protons is also known as the atomic number (Z), which can be found on the periodic table.
Example:
- Carbon (C) has an atomic number of 6. Therefore, a neutral carbon atom has 6 electrons.
2. Ions
An ion is an atom or molecule that has gained or lost electrons, giving it an electrical charge.
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Cations (Positive Ions): Cations have lost electrons and have a positive charge. To find the number of electrons, subtract the magnitude of the positive charge from the number of protons (atomic number).
Example:
- Sodium ion (Na+) has an atomic number of 11. Because it has a +1 charge, it has lost one electron. Therefore, Na+ has 11 - 1 = 10 electrons.
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Anions (Negative Ions): Anions have gained electrons and have a negative charge. To find the number of electrons, add the magnitude of the negative charge to the number of protons (atomic number).
Example:
- Chloride ion (Cl-) has an atomic number of 17. Because it has a -1 charge, it has gained one electron. Therefore, Cl- has 17 + 1 = 18 electrons.
Summary Table
| Particle | Charge | How to find # of electrons |
|---|---|---|
| Neutral Atom | 0 | Number of electrons = Atomic Number (Number of protons) |
| Cation | Positive (+) | Number of electrons = Atomic Number - (Magnitude of charge) |
| Anion | Negative (-) | Number of electrons = Atomic Number + (Magnitude of charge) |
