2.4
Ions and Ionic Charges
In ordinary chemical reactions, the nucleus — which contains the protons and neutrons of each atom and thus identifies the element — remains unchanged…
Atoms are neutral particles with equal numbers of protons and electrons. An element is defined by the number of protons, which does not change; however, many atoms can gain or lose one or more electrons to become a charged particle.
When an atom or molecule gains or loses electrons, it is called an ion. Electrons, being much smaller in mass than protons, do not contribute significantly to the atomic mass. Therefore, the atomic mass of an ion is essentially the same as the neutral atom’s atomic mass.
When atoms or molecules lose electrons, they are called cations. Cations have a positive charge because they have more protons, which have a positive charge, than electrons, which have a negative charge. Cations are given the same name as their element, followed by the word ion.
A calcium atom has 20 protons and 20 electrons. It loses two electrons to become a calcium ion with 20 protons and 18 electrons. Because it now has two more positive charges than negative charges, it has an overall charge of positive two.
The chemical symbol for an ion is written as the chemical symbol of the atom with the charge written on the right superscript position. The chemical symbol for a calcium ion is therefore written as Ca2+.
When atoms or molecules gain electrons, they are called anions. Anions have a negative charge because they have more electrons than protons. Anions are named by adding the suffix '-ide' to their element name.
A fluorine atom has 9 protons and 9 electrons. Fluorine gains one electron to become a fluoride ion with 9 protons and 10 electrons. Because it now has one more negative charge than positive charges, it has an overall charge of negative one.
The chemical symbol for fluoride is written as F−. The one is usually omitted for ions with a positive or negative one charge.
Atoms will lose or gain specific numbers of electrons when forming an ion. Many of the charges for ions formed by the main group elements can be predicted using the periodic table.
The metals, on the left side of the periodic table, will lose electrons. Group 1 alkali metals all lose one electron and have a positive one charge when they become an ion. Group 2 alkaline earth metals lose two electrons and have a positive two charge.
The nonmetals, on the right side of the periodic table, will gain electrons. Group 17 halogens gain one electron and have a negative one charge. Groups 16 elements gain two electrons and will have a negative two charge.
Many transition metals can form cations with different charges. Group 18 noble gases do not generally form ions.
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Q1: What is an ion and how does it form?
An ion is a charged particle formed when an atom or molecule gains or loses one or more electrons. Atoms are normally neutral with equal numbers of protons and electrons. When electrons are transferred, the balance shifts, creating a net positive or negative charge. The element identity remains unchanged because the number of protons stays constant.
Q2: What is the difference between a cation and an anion?
Cations form when atoms lose electrons, resulting in more protons than electrons and a positive charge. Anions form when atoms gain electrons, resulting in more electrons than protons and a negative charge. Cations are named the same as their element followed by 'ion,' while anions use the suffix '-ide' added to the element name.
Q3: How is the charge of an ion written in chemical notation?
The charge of an ion is written as a superscript on the right side of the chemical symbol. For example, a calcium ion that loses two electrons is written as Ca2+, and a fluoride ion that gains one electron is written as F−. The number one is typically omitted for ions with a single positive or negative charge.
Q4: How does the periodic table predict ion formation and charge?
The periodic table reveals predictable patterns for main-group elements. Metals on the left lose electrons to form cations: Group 1 metals lose one electron (1+ charge), Group 2 metals lose two electrons (2+ charge). Nonmetals on the right gain electrons to form anions: Group 17 elements gain one electron (1− charge), Group 16 elements gain two electrons (2− charge). Noble gases rarely form ions.
Q5: Why does atomic mass remain essentially unchanged when an ion forms?
Atomic mass is determined primarily by protons and neutrons in the nucleus. Electrons contribute negligibly to atomic mass because they are much smaller in mass than protons. When an atom loses or gains electrons to form an ion, the nucleus remains unchanged, so the atomic mass of the ion is essentially the same as the neutral atom's atomic mass.
Q6: What happens with transition metals when forming ions?
Unlike main-group elements, transition metals often form cations with variable charges that cannot be easily predicted from their periodic table position. For example, copper can form ions with either a 1+ or 2+ charge, and iron can form ions with either a 2+ or 3+ charge. This variability makes transition metal ion charges less predictable than those of main-group elements.
Q7: How do atoms achieve stable electron configurations when forming ions?
Main-group atoms gain or lose electrons to match the electron configuration of the nearest noble gas. Metals lose enough electrons to have the same electron count as the preceding noble gas, while nonmetals gain enough electrons to match the next noble gas. This electron transfer creates ions with stable, filled electron shells similar to noble gas configurations.