The atomic number of an atom represents the number of protons in its nucleus. In a neutral atom, the number of protons is equal to the number of electrons. However, if an atom has lost or gained an electron at its outer energy level, it becomes an ion.
To determine if an atom has lost or gained an electron, you need to compare its atomic number with the number of electrons in the ion. Here's how you can do it:
Determine the atomic number: Look up the atomic number of the element on the periodic table. The atomic number tells you the number of protons in the nucleus and also the number of electrons in a neutral atom.
Check the charge of the ion: If the atom has lost electrons, it becomes a positively charged ion called a cation. The number of electrons in a cation is less than the atomic number. The positive charge on the cation indicates the number of electrons lost. For example, if the atomic number is 11 and the ion has a +1 charge, it means the atom has lost one electron.
If the atom has gained electrons, it becomes a negatively charged ion called an anion. The number of electrons in an anion is greater than the atomic number. The negative charge on the anion indicates the number of electrons gained. For example, if the atomic number is 17 and the ion has a -1 charge, it means the atom has gained one electron.
It's important to note that the number of electrons in an ion is not always equal to the atomic number. Ions can have different numbers of electrons due to the gain or loss of electrons during chemical reactions.