How Do You Know if Something Is Ionic or Covalent

Learning Outcomes

• Define ionic and molecular (covalent) compounds
• Predict the blazon of chemical compound formed from elements based on their location within the periodic table
• Determine formulas for unproblematic ionic compounds

In ordinary chemical reactions, the nucleus of each atom (and thus the identity of the chemical element) remains unchanged. Electrons, however, tin exist added to atoms past transfer from other atoms, lost by transfer to other atoms, or shared with other atoms. The transfer and sharing of electrons among atoms govern the chemical science of the elements. During the formation of some compounds, atoms proceeds or lose electrons, and form electrically charged particles called ions (Figure 1).

Figure 1. (a) A sodium atom (Na) has equal numbers of protons and electrons (11) and is uncharged. (b) A sodium cation (Na+) has lost an electron, so it has one more proton (11) than electrons (10), giving information technology an overall positive charge, signified past a superscripted plus sign.

You tin use the periodic table to predict whether an atom will form an anion or a cation, and you can often predict the charge of the resulting ion. Atoms of many main-grouping metals lose enough electrons to leave them with the aforementioned number of electrons as an cantlet of the preceding noble gas. To illustrate, an atom of an alkali metal (grouping 1) loses 1 electron and forms a cation with a one+ charge; an element of group i globe metal (group 2) loses two electrons and forms a cation with a 2+ accuse, and so on. For example, a neutral calcium atom, with 20 protons and 20 electrons, readily loses 2 electrons. This results in a cation with 20 protons, 18 electrons, and a 2+ charge. It has the same number of electrons every bit atoms of the preceding noble gas, argon, and is symbolized [latex]\text{Ca}^{2+}[/latex]. The name of a metallic ion is the aforementioned every bit the name of the metal cantlet from which it forms, then [latex]\text{Ca}^{2+}[/latex] is called a calcium ion.

When atoms of nonmetal elements grade ions, they more often than not gain plenty electrons to give them the same number of electrons every bit an atom of the next element of group 0 in the periodic table. Atoms of grouping 17 proceeds one electron and form anions with a ane− charge; atoms of grouping 16 gain ii electrons and form ions with a 2− charge, and so on. For example, the neutral bromine cantlet, with 35 protons and 35 electrons, can gain one electron to provide information technology with 36 electrons. This results in an anion with 35 protons, 36 electrons, and a i− charge. Information technology has the same number of electrons equally atoms of the next noble gas, krypton, and is symbolized [latex]\text{Br}^{-}[/latex]. (A discussion of the theory supporting the favored status of noble gas electron numbers reflected in these predictive rules for ion formation is provided in a afterwards module of this text.)

Note the usefulness of the periodic table in predicting likely ion formation and charge (Figure ii). Moving from the far left to the right on the periodic table, main-group elements tend to course cations with a charge equal to the group number. That is, grouping 1 elements class i+ ions; group 2 elements form 2+ ions, then on. Moving from the far right to the left on the periodic tabular array, elements often form anions with a negative accuse equal to the number of groups moved left from the noble gases. For example, grouping 17 elements (1 group left of the noble gases) form 1− ions; group 16 elements (2 groups left) form 2− ions, and so on. This trend can exist used as a guide in many cases, simply its predictive value decreases when moving toward the center of the periodic table. In fact, transition metals and some other metals often exhibit variable charges that are not anticipated past their location in the table. For example, copper tin form ions with a 1+ or 2+ charge, and iron can form ions with a 2+ or 3+ charge.

Figure two. Some elements showroom a regular design of ionic charge when they form ions.

Example one:Composition of Ions

An ion found in some compounds used as antiperspirants contains xiii protons and 10 electrons. What is its symbol?

Evidence Solution

Because the number of protons remains unchanged when an atom forms an ion, the atomic number of the element must be thirteen. Knowing this lets united states use the periodic table to identify the chemical element as Al (aluminum). The Al atom has lost three electrons and thus has three more positive charges (13) than it has electrons (10). This is the aluminum cation, [latex]\text{Al}^{iii+}[/latex].

Bank check Your Learning

Give the symbol and name for the ion with 34 protons and 36 electrons.

Evidence Solution

[latex]\text{Se}^{two-}[/latex], the selenide ion

Example 2:Formation of Ions

Magnesium and nitrogen react to course an ionic compound. Predict which forms an anion, which forms a cation, and the charges of each ion. Write the symbol for each ion and proper noun them.

Check Your Learning

Aluminum and carbon react to grade an ionic compound. Predict which forms an anion, which forms a cation, and the charges of each ion. Write the symbol for each ion and name them.

Bear witness Solution

Al will form a cation with a charge of 3+: Al³⁺, an aluminum ion. Carbon will form an anion with a accuse of 4−: C^iv−, a carbide ion.

The ions that we accept discussed and then far are called monatomic ions, that is, they are ions formed from only one atom. We also observe many polyatomic ions. These ions, which act equally discrete units, are electrically charged molecules (a group of bonded atoms with an overall accuse). Some of the more than important polyatomic ions are listed in Table 1. Oxyanions are polyatomic ions that contain one or more oxygen atoms. At this point in your study of chemical science, you should memorize the names, formulas, and charges of the most mutual polyatomic ions. Because you will use them repeatedly, they volition before long go familiar.

Table i. Common Polyatomic Ions
Accuse	Proper noun	Formula		Charge	Name	Formula
ane+	ammonium	[latex]{\text{NH}}_{4}{}^{+}[/latex]		1−	permanganate	[latex]{\text{MnO}}_{4}{}^{-}[/latex]
1−	acetate	[latex]{\text{C}}_{2}{\text{H}}_{3}{\text{O}}_{two}{}^{-}[/latex]		1−	hydrogen carbonate, or bicarbonate	[latex]{\text{HCO}}_{three}{}^{-}[/latex]
1−	cyanide	[latex]\text{CN}^-[/latex]		2−	carbonate	[latex]{\text{CO}}_{3}{}^{2-}[/latex]
one−	hydroxide	[latex]\text{OH}^-[/latex]		2−	peroxide	[latex]{\text{O}}_{2}{}^{2-}[/latex]
1−	nitrate	[latex]{\text{NO}}_{3}{}^{-}[/latex]		1−	hydrogen sulfate, or bisulfate	[latex]{\text{HSO}}_{iv}{}^{-}[/latex]
ane−	nitrite	[latex]{\text{NO}}_{two}{}^{-}[/latex]		2−	sulfate	[latex]{\text{SO}}_{iv}{}^{ii-}[/latex]
ane−	perchlorate	[latex]{\text{ClO}}_{4}{}^{-}[/latex]		ii−	sulfite	[latex]{\text{SO}}_{3}{}^{2-}[/latex]
one−	chlorate	[latex]{\text{ClO}}_{3}{}^{-}[/latex]		1−	dihydrogen phosphate	[latex]{\text{H}}_{2}{\text{PO}}_{4}{}^{-}[/latex]
1−	chlorite	[latex]{\text{ClO}}_{2}{}^{-}[/latex]		2−	hydrogen phosphate	[latex]{\text{HPO}}_{iv}{}^{2-}[/latex]
1−	hypochlorite	[latex]\text{ClO}^-[/latex]		iii−	phosphate	[latex]{\text{PO}}_{4}{}^{three-}[/latex]

Note that there is a organization for naming some polyatomic ions; -ate and -ite are suffixes designating polyatomic ions containing more or fewer oxygen atoms. Per- (short for "hyper") and hypo- (significant "under") are prefixes pregnant more oxygen atoms than -ate and fewer oxygen atoms than -ite, respectively. For example, perchlorate is [latex]{\text{ClO}}_{4}{}^{-}[/latex], chlorate is [latex]{\text{ClO}}_{3}{}^{-}[/latex], chlorite is [latex]{\text{ClO}}_{2}{}^{-}[/latex] and hypochlorite is [latex]{\text{ClO}^{-}}[/latex]. Unfortunately, the number of oxygen atoms corresponding to a given suffix or prefix is not consistent; for instance, nitrate is [latex]{\text{NO}}_{3}{}^{-}[/latex] while sulfate is [latex]{\text{SO}}_{4}{}^{ii-}.[/latex] This volition be covered in more detail in the next module on nomenclature.

The nature of the attractive forces that hold atoms or ions together within a compound is the basis for classifying chemical bonding. When electrons are transferred and ions form, ionic bonds upshot. Ionic bonds are electrostatic forces of attraction, that is, the attractive forces experienced between objects of opposite electrical charge (in this case, cations and anions). When electrons are "shared" and molecules form, covalent bonds issue. Covalent bonds are the attractive forces betwixt the positively charged nuclei of the bonded atoms and one or more pairs of electrons that are located betwixt the atoms. Compounds are classified as ionic or molecular (covalent) on the footing of the bonds present in them.

Ionic Compounds

When an chemical element composed of atoms that readily lose electrons (a metallic) reacts with an element equanimous of atoms that readily gain electrons (a nonmetal), a transfer of electrons usually occurs, producing ions. The compound formed past this transfer is stabilized by the electrostatic attractions (ionic bonds) between the ions of opposite accuse present in the compound. For example, when each sodium cantlet in a sample of sodium metal (grouping ane) gives upward one electron to course a sodium cation, Na⁺, and each chlorine atom in a sample of chlorine gas (group 17) accepts one electron to course a chloride anion, Cl⁻, the resulting chemical compound, NaCl, is equanimous of sodium ions and chloride ions in the ratio of ane Na⁺ ion for each Cl⁻ ion. Similarly, each calcium atom (grouping 2) can give up two electrons and transfer i to each of two chlorine atoms to form CaCl₂, which is composed of Ca²⁺ and Cl⁻ ions in the ratio of ane Ca²⁺ ion to two Cl⁻ ions.

A compound that contains ions and is held together by ionic bonds is called an ionic chemical compound. The periodic table tin can help us recognize many of the compounds that are ionic: When a metal is combined with one or more nonmetals, the compound is usually ionic. This guideline works well for predicting ionic compound formation for nearly of the compounds typically encountered in an introductory chemistry class. Withal, it is not always true (for example, aluminum chloride, AlCl_three, is non ionic).

Yous can often recognize ionic compounds because of their properties. Ionic compounds are solids that typically melt at high temperatures and boil at even higher temperatures. For case, sodium chloride melts at 801 °C and boils at 1413 °C. (As a comparison, the molecular compound water melts at 0 °C and boils at 100 °C.) In solid grade, an ionic chemical compound is not electrically conductive because its ions are unable to flow ("electricity" is the period of charged particles). When molten, nevertheless, it tin can conduct electricity because its ions are able to move freely through the liquid (Figure 3).

Figure 3. Sodium chloride melts at 801 °C and conducts electricity when molten. (credit: modification of work by Mark Blaser and Matt Evans)

Lookout man this video to see a mixture of salts melt and conduct electricity. (Annotation that the video has no narration. You lot can access the audio description using the widget below the video.)

You lot can view the transcript for the audio clarification of "Conductivity molten salt" hither (opens in new window).

In every ionic compound, the total number of positive charges of the cations equals the total number of negative charges of the anions. Thus, ionic compounds are electrically neutral overall, even though they contain positive and negative ions. We can apply this ascertainment to assistance us write the formula of an ionic compound. The formula of an ionic compound must have a ratio of ions such that the numbers of positive and negative charges are equal.

Example iii:Predicting the Formula of an Ionic Compound

The gemstone sapphire (Figure 4) is mostly a compound of aluminum and oxygen that contains aluminum cations, Alⁱⁱⁱ⁺, and oxygen anions, Oⁱⁱ⁻. What is the formula of this chemical compound?

Effigy 4. Although pure aluminum oxide is colorless, trace amounts of iron and titanium give blue sapphire its characteristic colour. (credit: modification of work past Stanislav Doronenko)

Bear witness Solution

Because the ionic compound must exist electrically neutral, it must accept the same number of positive and negative charges. Two aluminum ions, each with a charge of iii+, would give u.s.a. six positive charges, and three oxide ions, each with a accuse of ii−, would requite us 6 negative charges. The formula would be Al₂O₃.

Check Your Learning

Predict the formula of the ionic compound formed between the sodium cation, Na⁺, and the sulfide anion, S²⁻.

Many ionic compounds contain polyatomic ions (Table i) equally the cation, the anion, or both. As with uncomplicated ionic compounds, these compounds must likewise be electrically neutral, and then their formulas can be predicted by treating the polyatomic ions as detached units. Nosotros use parentheses in a formula to signal a group of atoms that behave as a unit. For example, the formula for calcium phosphate, one of the minerals in our basic, is [latex]\text{Ca}_{three}(\text{PO}_{4})_{2}[/latex]. This formula indicates that there are three calcium ions [latex](\text{Ca}^{two+})[/latex] for every two phosphate [latex]\left({\text{PO}}_{4}{}^{3-}\correct)[/latex] groups. The [latex]{\text{PO}}_{4}{}^{three-}[/latex] groups are discrete units, each consisting of one phosphorus atom and iv oxygen atoms, and having an overall charge of iii-. The compound is electrically neutral, and its formula shows a total count of iii Ca, two P, and eight O atoms.

Example iv:Predicting the Formula of a Chemical compound with a Polyatomic Anion

Baking powder contains calcium dihydrogen phosphate, an ionic chemical compound composed of the ions Ca^two+ and [latex]{\text{H}}_{two}{\text{PO}}_{4}{}^{-}[/latex]. What is the formula of this compound?

Evidence Solution

The positive and negative charges must balance, and this ionic compound must be electrically neutral. Thus, nosotros must take 2 negative charges to residual the 2+ charge of the calcium ion. This requires a ratio of 1 Caⁱⁱ⁺ ion to two [latex]{\text{H}}_{2}{\text{PO}}_{4}{}^{-}[/latex] ions. Nosotros designate this by enclosing the formula for the dihydrogen phosphate ion in parentheses and adding a subscript 2. The formula is Ca(H₂PO₄)_ii.

Check Your Learning

Predict the formula of the ionic chemical compound formed between the lithium ion and the peroxide ion, [latex]{\text{O}}_{2}{}^{2-}[/latex] (Hint: Use the periodic tabular array to predict the sign and the charge on the lithium ion.)

Because an ionic chemical compound is not made up of single, discrete molecules, information technology may not be properly symbolized using a molecular formula. Instead, ionic compounds must be symbolized by a formula indicating the relative numbers of its constituent cations. For compounds containing just monatomic ions (such as [latex]\text{NaCl}[/latex]) and for many compounds containing polyatomic ions (such as [latex]\text{CaSO}_{four})[/latex], these formulas are but the empirical formulas introduced earlier. However, the formulas for some ionic compounds containing polyatomic ions are not empirical formulas. For example, the ionic compound sodium oxalate is comprised of [latex]\text{Na}^{+}[/latex] and [latex]{\text{C}}_{2}{\text{O}}_{4}{}^{2-}[/latex] ions combined in a 2:1 ratio, and its formula is written as [latex]\text{Na}_{2}\text{C}_{ii}\text{O}_{4}[/latex]. The subscripts in this formula are not the smallest-possible whole numbers, as each can be divided by 2 to yield the empirical formula, [latex]\text{NaCO}_{2}[/latex]. This is not the accepted formula for sodium oxalate, however, every bit information technology does not accurately correspond the compound'due south polyatomic anion, [latex]{\text{C}}_{2}{\text{O}}_{four}{}^{2-}[/latex].

Molecular Compounds

Many compounds practise non contain ions but instead consist solely of discrete, neutral molecules. These molecular compounds (covalent compounds) result when atoms share, rather than transfer (gain or lose), electrons. Covalent bonding is an important and extensive concept in chemistry, and information technology will be treated in considerable item in a later module of this text. We can often identify molecular compounds on the basis of their physical properties. Nether normal conditions, molecular compounds frequently be every bit gases, low-boiling liquids, and depression-melting solids, although many of import exceptions exist.

Whereas ionic compounds are usually formed when a metallic and a nonmetal combine, covalent compounds are usually formed past a combination of nonmetals. Thus, the periodic table can help united states of america recognize many of the compounds that are covalent. While we can use the positions of a compound's elements in the periodic table to predict whether it is ionic or covalent at this point in our report of chemistry, you lot should be aware that this is a very simplistic approach that does non business relationship for a number of interesting exceptions. Shades of gray exist betwixt ionic and molecular compounds, and you'll larn more about those afterwards.

Example 5:Predicting the Type of Bonding in Compounds

Predict whether the following compounds are ionic or molecular:

1. KI, the compound used as a source of iodine in tabular array salt
2. H_iiO₂, the bleach and disinfectant hydrogen peroxide
3. CHCl_iii, the anesthetic chloroform
4. Li₂CO₃, a source of lithium in antidepressants

Check Your Learning

Using the periodic table, predict whether the following compounds are ionic or covalent:

1. SO₂
2. CaF₂
3. N₂H₄
4. Al_two(SO_iv)₃

Cardinal Concepts and Summary

Metals (peculiarly those in groups 1 and two) tend to lose the number of electrons that would exit them with the aforementioned number of electrons every bit in the preceding noble gas in the periodic table. By this means, a positively charged ion is formed. Similarly, nonmetals (especially those in groups xvi and 17, and, to a lesser extent, those in Grouping 15) can gain the number of electrons needed to provide atoms with the aforementioned number of electrons equally in the next noble gas in the periodic table. Thus, nonmetals tend to form negative ions. Positively charged ions are called cations, and negatively charge ions are called anions. Ions can exist either monatomic (containing only one atom) or polyatomic (containing more than 1 atom).

Compounds that contain ions are chosen ionic compounds. Ionic compounds more often than not form from metals and nonmetals. Compounds that do not contain ions, simply instead consist of atoms bonded tightly together in molecules (uncharged groups of atoms that carry every bit a single unit), are called covalent compounds. Covalent compounds unremarkably form from ii nonmetals.

Endeavor It

1. Using the periodic table, predict whether the following chlorides are ionic or covalent: KCl, NCl_three, ICl, MgCl₂, PCl_v, and CCl₄.
2. Using the periodic table, predict whether the following chlorides are ionic or covalent: SiCl_four, PCl_iii, CaCl_ii, CsCl, CuCl₂, and CrCl_iii.
3. For each of the post-obit compounds, state whether it is ionic or covalent. If information technology is ionic, write the symbols for the ions involved:
   1. NF_three
   2. BaO,
   3. (NH₄)₂CO_three
   4. Sr(H_twoPO_iv)₂
   5. IBr
   6. Na₂O
4. For each of the following compounds, country whether it is ionic or covalent, and if it is ionic, write the symbols for the ions involved:
   1. KClO_four
   2. MgC₂H₃O_two
   3. H₂South
   4. Ag_iiDue south
   5. Due north_twoCl_four
   6. Co(NO_three)_two
5. For each of the following pairs of ions, write the symbol for the formula of the compound they will form:
   1. Ca^two+, Southward^2-
   2. [latex]{\text{NH}}_{4}{}^{+}[/latex], [latex]{\text{So}}_{4}{}^{two-}[/latex]
   3. Al³⁺, Br^–(d) Na⁺, [latex]{\text{HPO}}_{4}{}^{2-}[/latex]  (e) Mgⁱⁱ⁺, [latex]{\text{PO}}_{4}{}^{3-}[/latex]
6. For each of the following pairs of ions, write the symbol for the formula of the compound they will form:
   1. K⁺, O^ii-
   2. [latex]{\text{NH}}_{iv}{}^{+}[/latex], [latex]{\text{PO}}_{4}{}^{iii-}[/latex]
   3. Al³⁺, O^ii-
   4. Na⁺, [latex]{\text{CO}}_{3}{}^{2-}[/latex]
   5. Ba²⁺, [latex]{\text{PO}}_{four}{}^{3-}[/latex]

Glossary

covalent bond:bonny strength between the nuclei of a molecule'southward atoms and pairs of electrons between the atoms

covalent compound:(also, molecular compound) composed of molecules formed past atoms of ii or more unlike elements

ionic bail:electrostatic forces of attraction between the oppositely charged ions of an ionic chemical compound

ionic compound:chemical compound composed of cations and anions combined in ratios, yielding an electrically neutral substance

molecular compound:(as well, covalent compound) composed of molecules formed by atoms of two or more unlike elements

monatomic ion:ion composed of a single atom

oxyanion:polyatomic anion composed of a cardinal atom bonded to oxygen atoms

polyatomic ion:ion equanimous of more than than one atom

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