Answer
(a) $^{78}_{38}$Sr.
(b) group 2A; also called the alkaline earth metals
(c) 2+
(d) The element we are looking for is magnesium: $^{24}_{12}$Mg.
Work Step by Step
(a) To write the element symbol for this atom, we need to find out what its symbol is, its atomic number, and its mass number. If the atom contains 38 protons, then its atomic number is 38. According to the periodic table, the element with atomic number 38 is strontium (Sr). To find the mass number of an element, we add the number of protons and neutrons together. We are given that the number of neutrons is 40 and the number of protons is 38; therefore, the mass number, which is the number of protons and number of neutrons in the atom, is 78. With all this information, we can now write the element symbol for strontium as $^{78}_{38}$Sr.
(b) Strontium is in group 2A of the periodic table. This group is also called the alkaline earth metals.
(c) The charge of strontium's monatomic ion is 2+, which is the charge of all ions formed from elements in group 2A.
(d) To find another element in group 2A that has only 12 neutrons, we need to look at group 2A on the periodic table, locate an element where the atomic number subtracted from the atomic mass is about 12. This element is magnesium. Magnesium has an element symbol Mg and has an atomic number of 12. To find its mass number, we add the atomic number (number of protons) to the number of neutrons, and we get 24 as the mass number. The chemical symbol for magnesium is $^{24}_{12}$Mg.
