Write the chemical formula for an ionic compound formed from the following elements. The formula Mg 2Cl 4 has balanced charges with the ions in a 1:2 ratio, but is not correct because it is not the lowest whole number ratio. Rather, it exists as two individual chloride ions.) By convention, the lowest whole number ratio is used in the formulas of ionic compounds. Chlorine does not exist as a diatomic element in this compound. (Do not read the “Cl 2” part of the formula as a molecule of the diatomic elemental chlorine. This chemical formula says that there are one magnesium ion and two chloride ions in this formula. We could write the chemical formula for this ionic compound as MgClCl, but the convention is to use a numerical subscript when there is more than one ion of a given type-MgCl 2. Now the positive and negative charges are balanced. The easiest way to balance these charges is to assume the presence of two chloride ions for each magnesium ion: A magnesium ion has a 2 charge, while a chlorine ion has a 1− charge:Ĭombining one ion of each does not completely balance the positive and negative charges. Now consider the ionic compound formed by magnesium and chlorine. Therefore, the proper formula for this ionic compound is MgO. Although both of these ions have higher charges than the ions in lithium bromide, they still balance each other in a one-to-one ratio. When an ionic compound is formed from magnesium and oxygen, the magnesium ion has a 2 charge, and the oxygen atom has a 2− charge. Only one ion of each is needed to balance these charges. If we look at the ionic compound consisting of lithium ions and bromide ions, we see that the lithium ion has a 1 charge and the bromide ion has a 1− charge. This rule is ultimately based on the fact that matter is, overall, electrically neutral.īy convention, assume that there is only one atom if a subscript is not present. To determine the proper formula of any combination of ions, determine how many of each ion is needed to balance the total positive and negative charges in the compound. Finally, the proper formula for an ionic compound always obeys the following rule: the total positive charge must equal the total negative charge. Remember that in an ionic compound, the component species are ions, not neutral atoms, even though the formula does not contain charges. Second, charges are not written in a formula. Because most metals form cations and most nonmetals form anions, formulas typically list the metal first and then the nonmetal. First, the cation is written before the anion. The formula for an ionic compound follows several conventions. A crystal of sodium chloride, shown here, is a collection of alternating sodium and chlorine ions. The precise pattern depends on the compound. A crystal contains a three-dimensional array of alternating positive and negative ions. to make the number of charges the same, we need one Ca 2 ion and two OH - ionsĭeduce the formula for lead(II) sulfate.\) A Sodium Chloride Crystal.this is two positive charges and one negative charge.If the formula of an ionic compound needs more than one polyatomic ion, the formula of this ion is written inside brackets.Ĭalcium hydroxide contains Ca 2 and OH - ions: Polyatomic ions are formed from groups of two or more atoms. to make the number of charges the same, we need two Al 3 ions and three O 2- ions.this is three positive charges and two negative charges.this is two positive charges and two negative chargesĪluminium oxide contains Al 3 and O 2- ions:.Magnesium oxide contains Mg2 and O2 - ions: the number of charges are already the same.this is one positive charge and one negative charge. Sodium chloride contains Na and Cl - ions: The formula for an ionic compound must give the same number of positive and negative charges. Positive ionsĪlthough ionic compounds contain electrically charged ions, they are neutral overall. Similarly, the non-metal elements in groups 6 (IUPAC group 16) and 7 (IUPAC group 17), the ionic charge can be deduced by working out how many electrons must be gained to fill the outer shell. Remember that for the metal elements in groups 1, 2, and 3 the charge on the ion can be deduced by how many outer shell electrons there were in the neutral atom. The formula of an ionic compound can be deduced from the formulae of its ions. They almost always contain at least one metal element and at least one non-metal element. Ionic compounds are made up of oppositely charged ions joined together by ionic bonds.
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