Explanation:

The number of protons in an atom is equal to its atomic number. The atomic number of an element is unique and identifies the element. When an atom loses or gains electrons, it becomes an ion. The charge of an ion is determined by the number of electrons lost or gained. A negative charge indicates that the atom has gained electrons, while a positive charge indicates that the atom has lost electrons.

In this case, the ion has a -1 charge, which means that it has gained one electron. Therefore, the number of protons in the ion is equal to the atomic number of the element minus one. Since the ion has 10 electrons, the atomic number of the element is 10 + 1 = 11. The element with the atomic number 11 is sodium (Na). Therefore, the ion is Na-.