## Chemistry: Principles and Practice (3rd Edition)

(a) $^{25}_{12}$Mg$^{2+}$ (b) $^{27}_{13}$Al$^{3+}$ (c) $^{29}_{14}$Si (d) $^{79}_{35}$Br$^{-}$
(a) The element with 12 protons in the nucleus, which means the atomic number is 12, is magnesium, chemical symbol Mg. The superscript to the left of the chemical symbol would be the mass number, which is the total number of protons and neutrons in the nucleus of that atom. We add the number of protons, 12, to the number of neutrons, 13, to get a mass number of 25. The left subscript is the atomic number, which is 12. The superscript to the right of the chemical symbol is the charge of the ion. We figure this out by adding up the negative and positive charges to get a net charge. There are 12 protons, each with a single positive charge for a total charge of +12. The number of electrons is 10, each with a single negative charge for a total charge of -10. We add +12 to -10 to get a net charge of +2. The symbol for this ion would be $^{25}_{12}$Mg$^{2+}$. (b) The element with 13 protons in the nucleus, which means the atomic number is 13, is aluminum, chemical symbol Al. The superscript to the left of the chemical symbol would be the mass number, which is the total number of protons and neutrons in the nucleus of that atom. We add the number of protons, 13, to the number of neutrons, 14, to get a mass number of 27. The left subscript is the atomic number, which is 13. The superscript to the right of the chemical symbol is the charge of the ion. We figure this out by adding up the negative and positive charges to get a net charge. There are 13 protons, each with a single positive charge for a total charge of +13. The number of electrons is 10, each with a single negative charge for a total charge of -10. We add +13 to -10 to get a net charge of +3. The symbol for this ion would be $^{27}_{13}$Al$^{3+}$. (c) The element with 14 protons in the nucleus, which means the atomic number is 14, is silicon, chemical symbol Si. The superscript to the left of the chemical symbol would be the mass number, which is the total number of protons and neutrons in the nucleus of that atom. We add the number of protons, 14, to the number of neutrons, 15, to get a mass number of 29. The left subscript is the atomic number, which is 14. The superscript to the right of the chemical symbol is the charge of the ion. We figure this out by adding up the negative and positive charges to get a net charge. There are 14 protons, each with a single positive charge for a total charge of +14. The number of electrons is 14, each with a single negative charge for a total charge of -14. We add +14 to -14 to get a net charge of 0. This is, therefore, a neutral atom. The symbol for this atom would be $^{29}_{14}$Si. (d) The element with 35 protons in the nucleus, which means the atomic number is 35, is bromine, chemical symbol Br. The superscript to the left of the chemical symbol would be the mass number, which is the total number of protons and neutrons in the nucleus of that atom. We add the number of protons, 35, to the number of neutrons, 44, to get a mass number of 79. The left subscript is the atomic number, which is 35. The superscript to the right of the chemical symbol is the charge of the ion. We figure this out by adding up the negative and positive charges to get a net charge. There are 35 protons, each with a single positive charge for a total charge of +35. The number of electrons is 36, each with a single negative charge for a total charge of -36. We add +35 to -36 to get a net charge of -1. The symbol for this ion would be $^{79}_{35}$Br$^{-}$.