The number of electrons in an atom is equal to the number of protons in the nucleus. This is because atoms are electrically neutral, meaning that they have an equal number of positive and negative charges. Protons are positive, while electrons are negative. So, if an atom has 10 protons, it will also have 10 electrons.

You can also determine the number of electrons in an atom by looking at its atomic number. The atomic number of an element is the number of protons in its nucleus. So, if an element has an atomic number of 10, it will also have 10 electrons.

Here is a table showing the number of protons, electrons, and neutrons in the first 20 elements:

| Element | Protons | Electrons | Neutrons |

|---|---|---|---|

| Hydrogen | 1 | 1 | 0 |

| Helium | 2 | 2 | 2 |

| Lithium | 3 | 3 | 4 |

| Beryllium | 4 | 4 | 5 |

| Boron | 5 | 5 | 6 |

| Carbon | 6 | 6 | 6 |

| Nitrogen | 7 | 7 | 7 |

| Oxygen | 8 | 8 | 8 |

| Fluorine | 9 | 9 | 10 |

| Neon | 10 | 10 | 10 |

| Sodium | 11 | 11 | 12 |

| Magnesium | 12 | 12 | 12 |

| Aluminum | 13 | 13 | 14 |

| Silicon | 14 | 14 | 14 |

| Phosphorus | 15 | 15 | 16 |

| Sulfur | 16 | 16 | 16 |

| Chlorine | 17 | 17 | 18 |

| Argon | 18 | 18 | 18 |

| Potassium | 19 | 19 | 20 |

| Calcium | 20 | 20 | 20 |