Two atoms are isoelectric if they each have the same number of electrons. In its ground state, oxygen has 8 electrons, equal to the number of protons in order to have a neutral atom. In order to become a stable ion, the oxygen atom must gain two electrons. This gives it a charge of negative two, but allows it to satisfy the octet rule, making it a very stable ion. With the ionization, the atom now has 10 electrons.

Neon has atomic number 10, meaning that it will also have 10 electrons in its ground state.

Two atoms are isoelectric if they each have the same number of electrons. In its ground state, bromine has 35 electrons, equal to the number of protons in order to have a neutral atom. In order to become a bromide ion, the bromine atom must gain one electron. This gives it a charge of negative one, but allows it to satisfy the octet rule, making it a very stable ion. With the ionization, the atom now has 36 electrons.

Krypton has atomic number 36, meaning that it will also have 36 electrons in its ground state.

Isoelectronic atoms are atoms or ions that contain the same number of total electrons.  is not isoelectronic with the rest of the ions and atoms listed because it contains a total of ten electrons, while the rest in the series contain thirty-six. 

Positively charged ions (cations) are formed when electrons are removed from the outer shell of the atom. Removing a negatively charged electron gives the ion a positive charge. For example, the element rubidium, , in its ground state contains thirty-seven electrons. The ion contains one less, thirty-six, making it isoelectronic with krypton and the other ions and atoms in the series.

Negatively charge ions (anions) are formed when electrons are added to the outer shell of the atom. Adding a negatively charged electron gives the ion a negative charge. For example, ground state selenium, , contains thirty-four electrons. The ion , has two added electrons giving it a total of thirty-six electrons. The ion is isoelectronic with the other ions and atoms in the series. 

The general isotopic notation is as follows: , where  is the mass number, which is equal to the number of neutrons plus protons, and  is the atomic number, which is equal to the number of protons. We know that the atomic number determines the identity of an element, thus an atom with 19 protons must be potassium . Following the isotopic notation mentioned above, our mass number is 39.

The general isotopic notation is as follows: , where  is the mass number, which is equal to the number of neutrons plus protons, and  is the atomic number, which is equal to the number of protons. We know that the atomic number determines the identity of an element, thus an atom with a mass number of 197 and 118 neutrons must have 79 protons. Thus the element is gold .

The atomic number of aluminum is 13, which means it has 13 protons. All atoms with 13 protons are called aluminum. The mass number of aluminum is 27 and is the sum of neutrons and protons. We simply subtract the number of protons from the mass number to get the number of neutrons. Normally there are an equal number of electrons and protons. But since this aluminum exists as an ion with a +3 charge, it has lost three electrons and has only 10 electrons.

Chlorine-35 and chlorine-37 are isotopes of each other. Isotopes are atoms of the same element that have different masses due to different numbers of neutrons.

We know that the element is iron and that it is missing two electrons. This means that the atom has two less electrons than it has protons resulting in a charge of 2+. This can be written as

If an ion is positively charged it is a cation.  If it is negatively charged (more electrons than protons) it is an anion.

The ion is a cation since it has a charge of 2+.

We should first remember the difference between sulfate, sulfite, and sulfide. Sulfate is , sulfite is ` and sulfide is .

The only answer choices that could be right must have  in them. We then need to see that barium usually has a charge of , as the periodic table shows us, and so we need a charge of  to cancel that out. The answer is .

Ionic chemical compounds are always named with the metal before the non-metal. The correct way to name sodium chloride would be , and not , because sodium is the metal and chloride is the non-metal.

All the other examples have the metal before the non-metal. The other answer choices correspond to potassium chloride, magnesium sulfate, hydrogen chloride (hydrochloric acid), and calcium fluoride.