Ions: Predict Charge

The Periodic Table


You use the periodic table to assign standard charges to ions. 


Main Group and Transition Metals


To assign charges to ions, you must recognize these rules apply to main group elements. The other elements correspond to transition metals and post transition metals. You can not transfer the same rules to transition metals.


The main group comprises IA(1)-IIA(2), and IIIA(13)-XVIIA(18).

Figure 1 shows the main group elements in white. The grayed out areas show the transition metals.


Periodic chart main group
Figure 1: Main group in white, transition elements grayed out

The ability to predict charges resides in the main group elements. Any elements between IB(3)-VIIIB(12) are transition metal elements. You can not predict the charge in the same simple straight forward way.


Metals and Nonmetals


In another way to see the periodic table, elements are divided into metals and nonmetals. Ionic compounds form from a metal and nonmetal (or nonmetal polyatomic ion). Both a metal and a nonmetal must be present.


You must also distinguish between metals and nonmetals. 


You can do this when you divide the periodic table into metal and nonmetal elements, shown in Figure 2.




periodic table metals and non meteals
Figure 2: Regions of periodic table separated into metals and nonmetals



On the left side of Figure 2 the gray shaded area shows which elements are metals. Next to the metals on the right, a zig-zag region with a marbled texture shows metalloids. The far right of the periodic table shows white elements which correspond to nonmetals.


Metalloids are elements between metals and nonmetals. These elements have properties of both metals and nonmetals, but are not completely either one.


The Crucial Difference: Metals and Nonmetals


The way an ion forms from an element, whether it loses electrons or gains electrons depends on whether an element is a metal or nonmetal, Figure 3.


Ions and the periodic chart
Figure 3: Metals form cations and nonmetals form anions



Octet Rule


How many electrons (negative charges) does an atom lose to become a cation or gain to become an anion? The answer depends on how many valence electrons an element has when it is neutral.


The number of charges an element gains or loses, must in the end, equal eight valence electrons in its outer most electron shell. This is the same number of electrons in the valence electron shell of  as the inert noble gases.


Whether an element gains or loses electrons in order to conform to the octet rules is a characteristic which makes an element a metal or nonmetal.


Figure 4 shows both a metal and nonmetal in a gray neutral state. Their path to obtain an octet of valence electrons are opposites. Nonmetals gain electrons to become anions. Metals lose electrons to reach a state where they mimic the valence electrons of the noble gases. 



elements are metals lose electrons, nonmetals gain electrons
Figure 4: Metals lose electrons and nonmetals gain electrons



Metals Make Cations


When a metal forms an ion, it gives up its valence electrons and becomes a positively charge atom, cation.


For instance sodium has one valence electron. It gives up its electron because it is a metal. That gives it a +1 charge. The electron shell below its outermost electron shell has a full octet. Sodium has sacrificed its outer electron to resemble the electron configuration of neon.


Nonmetals Make Anions


In contrast to metals, nonmetals gain electrons. When a neutral atom gains electrons it becomes negative, an anion.


For example sulfur has six valence electrons in its outer most electron shell. That’s two fewer electrons than argon. When sulfur gains two electrons to become an anion, it now has eight electrons in its valence shell. This makes the sulfide -2 anion have the same stable configuration as argon.


Charge and Columns


ion charge and periodic table
Figure 5: The charge of an ion can be predicted by its position on periodic table


Main group elements have predictable charges they assume when they become ions. The charge they most frequently assume depends on which column an element is found in. Figure 5 shows which elements assume which charge by the family of element. 


Notice carbon and group 14 (IVA) does not have a charge assigned to it. This is because carbon can take a +4 or -4 charge depending on who its bonding partner is. 


The other main group elements however follow predictable patterns when they form ions.


It Takes Two: Metal Plus Nonmetal


Bare in mind an element only becomes an ion under the influence of a near by complimentary element. Metals and nonmetals in their native state always have a charge of zero (or neutral).


For example sodium metal has a charge of 0. When sodium (Na) encounters a nonmetal like chlorine (Cl2) which also has a charge of zero,  the sodium loses and electron while chlorine gains an electron. That is the moment when an element takes on the identity of an ion.


In language you will often hear: “what charge does calcium have?” The element itself does not have a charge. The question stated more correctly would be: “What charge does calcium take when it becomes an ion due to its interaction with an element which can accept its electrons?”


The first question is the more common one, but it usually means the second question. Just keep the idea straight that elements do not have charges, but ions of the elements do have charges.