Chemistry a Molecular Approach Chapter 6 and 7 Review

Learning Objectives

By the cease of this section, you volition be able to:

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

In ordinary chemical reactions, the nucleus of each atom (and thus the identity of the element) remains unchanged. Electrons, however, tin be 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 chemistry of the elements. During the formation of some compounds, atoms gain or lose electrons, and grade electrically charged particles chosen ions (Figure 2.28).

Figure two.28 (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, then it has 1 more proton (11) than electrons (10), giving information technology an overall positive charge, signified by a superscripted plus sign.

You tin utilise the periodic table to predict whether an atom will form an anion or a cation, and yous can often predict the accuse of the resulting ion. Atoms of many master-group metals lose enough electrons to exit them with the same number of electrons equally an atom of the preceding element of group 0. To illustrate, an atom of an alkaline (group 1) loses one electron and forms a cation with a 1+ accuse; an element of group ii (grouping 2) loses 2 electrons and forms a cation with a two+ charge, and so on. For example, a neutral calcium cantlet, with 20 protons and 20 electrons, readily loses ii electrons. This results in a cation with 20 protons, 18 electrons, and a 2+ charge. It has the aforementioned number of electrons every bit atoms of the preceding noble gas, argon, and is symbolized Ca²⁺. The name of a metal ion is the same as the proper noun of the metallic atom from which information technology forms, and so Ca²⁺ is called a calcium ion.

When atoms of nonmetal elements course ions, they generally gain enough electrons to give them the same number of electrons as an atom of the next noble gas in the periodic table. Atoms of group 17 gain one electron and course anions with a 1− accuse; atoms of group 16 gain two electrons and grade ions with a 2− charge, and so on. For instance, the neutral bromine atom, with 35 protons and 35 electrons, tin gain one electron to provide information technology with 36 electrons. This results in an anion with 35 protons, 36 electrons, and a i− charge. It has the same number of electrons as atoms of the next element of group 0, krypton, and is symbolized Br⁻. (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 later chapter of this text.)

Note the usefulness of the periodic table in predicting likely ion formation and accuse (Figure 2.29). Moving from the far left to the correct on the periodic table, main-grouping elements tend to form cations with a charge equal to the grouping number. That is, grouping one elements class ane+ ions; group two elements form 2+ ions, and so on. Moving from the far right to the left on the periodic table, elements oftentimes form anions with a negative charge equal to the number of groups moved left from the noble gases. For example, group 17 elements (one group left of the noble gases) form 1− ions; group 16 elements (two groups left) course 2− ions, and so on. This trend can be used every bit a guide in many cases, but its predictive value decreases when moving toward the eye of the periodic table. In fact, transition metals and some other metals often showroom variable charges that are not predictable past their location in the table. For example, copper can form ions with a 1+ or two+ charge, and atomic number 26 can course ions with a two+ or 3+ charge.

Effigy 2.29 Some elements exhibit a regular design of ionic charge when they course ions.

Case 2.8

Limerick of Ions

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

Solution

Because the number of protons remains unchanged when an cantlet forms an ion, the atomic number of the element must be 13. Knowing this lets us employ the periodic tabular array to identify the element as Al (aluminum). The Al atom has lost 3 electrons and thus has 3 more positive charges (13) than it has electrons (10). This is the aluminum cation, Al^three+.

Check Your Learning

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

Reply:

Se^two−, the selenide ion

Example 2.9

Formation of Ions

Magnesium and nitrogen 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 proper noun them.

Solution

Magnesium'south position in the periodic table (group 2) tells us that it is a metal. Metals form positive ions (cations). A magnesium atom must lose ii electrons to have the same number electrons as an cantlet of the previous element of group 0, neon. Thus, a magnesium atom volition form a cation with 2 fewer electrons than protons and a accuse of 2+. The symbol for the ion is Mg²⁺, and it is called a magnesium ion.

Nitrogen'due south position in the periodic table (group 15) reveals that information technology is a nonmetal. Nonmetals grade negative ions (anions). A nitrogen cantlet must gain three electrons to accept the same number of electrons every bit an atom of the following noble gas, neon. Thus, a nitrogen atom will course an anion with iii more electrons than protons and a charge of 3−. The symbol for the ion is N³⁻, and it is called a nitride ion.

Bank check Your Learning

Aluminum and carbon 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 name them.

Answer:

Al volition form a cation with a charge of three+: Al³⁺, an aluminum ion. Carbon will class an anion with a charge of 4−: C⁴⁻, a carbide ion.

The ions that nosotros accept discussed so far are chosen monatomic ions, that is, they are ions formed from only one atom. We also discover many polyatomic ions. These ions, which human action as discrete units, are electrically charged molecules (a group of bonded atoms with an overall charge). Some of the more than important polyatomic ions are listed in Table 2.v. Oxyanions are polyatomic ions that incorporate i or more oxygen atoms. At this betoken in your study of chemistry, you should memorize the names, formulas, and charges of the most common polyatomic ions. Considering yous volition use them repeatedly, they will soon become familiar.

Common Polyatomic Ions

Name	Formula	Related Acid	Formula
ammonium	${\text{NH}}_4{}^{+}$
hydronium	${\text{H}}_3{\text{O}}^{+}$
peroxide	${\text{O}}_{\mathrm{ii}}{}^{2-}$
hydroxide	${\text{OH}}^{-}$
acetate	${\text{CH}}_{\mathrm{iii}}{\text{COO}}^{\mathrm{-}}$	acetic acid	CH3COOH
cyanide	CN−	hydrocyanic acid	HCN
azide	${\text{North}}_3{}^{-}$	hydrazoic acid	HN3
carbonate	${\text{CO}}_3{}^{2-}$	carbonic acid	HtwoCOiii
bicarbonate	${\text{HCO}}_{\mathrm{iii}}{}^{-}$
nitrate	${\text{NO}}_3{}^{-}$	nitric acid	HNO3
nitrite	${\text{NO}}_2{}^{-}$	nitrous acid	HNO2
sulfate	${\text{SO}}_{\mathrm{four}}{}^{2-}$	sulfuric acid	HiiAnd theniv
hydrogen sulfate	${\text{HSO}}_4{}^{-}$
sulfite	${\text{So}}_3{}^{two-}$	sulfurous acid	H2SO3
hydrogen sulfite	${\text{HSO}}_3{}^{-}$
phosphate	${\text{PO}}_4{}^{\mathrm{3-}}$	phosphoric acrid	H3POiv
hydrogen phosphate	${\text{HPO}}_4{}^{\mathrm{ii-}}$
dihydrogen phosphate	${\text{H}}_{\mathrm{two}}{\text{PO}}_{\mathrm{iv}}{}^{-}$
perchlorate	${\text{ClO}}_4{}^{-}$	perchloric acid	HClOfour
chlorate	${\text{ClO}}_3{}^{-}$	chloric acid	HClO3
chlorite	${\text{ClO}}_2{}^{-}$	chlorous acrid	HClOii
hypochlorite	ClO−	hypochlorous acrid	HClO
chromate	${\text{CrO}}_{\mathrm{four}}{}^{2-}$	chromic acid	H2CrO4
dichromate	${\text{Cr}}_{\mathrm{ii}}{\text{O}}_7{}^{2-}$	dichromic acid	H2CriiOvii
permanganate	${\text{MnO}}_4{}^{-}$	permanganic acrid	HMnOiv

Table 2.5

Annotation that there is a system 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- (meaning "under") are prefixes meaning more oxygen atoms than -ate and fewer oxygen atoms than -ite, respectively. For example, perchlorate is ${\text{ClO}}_{\mathrm{iv}}{}^{-}\text{,}$ chlorate is ${\text{ClO}}_3{}^{-}\text{,}$ chlorite is ${\text{ClO}}_{\mathrm{ii}}{}^{-}$ and hypochlorite is ClO⁻. Unfortunately, the number of oxygen atoms respective to a given suffix or prefix is not consistent; for instance, nitrate is ${\text{NO}}_3{}^{-}$ while sulfate is ${\text{So}}_4{}^{\mathrm{ii-}}.$ This will be covered in more than 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 class, ionic bonds result. Ionic bonds are electrostatic forces of attraction, that is, the attractive forces experienced between objects of opposite electrical charge (in this example, cations and anions). When electrons are "shared" and molecules grade, covalent bonds result. Covalent bonds are the bonny forces between the positively charged nuclei of the bonded atoms and i or more than pairs of electrons that are located betwixt the atoms. Compounds are classified as ionic or molecular (covalent) on the basis of the bonds present in them.

Ionic Compounds

When an element equanimous of atoms that readily lose electrons (a metal) reacts with an element composed of atoms that readily gain electrons (a nonmetal), a transfer of electrons usually occurs, producing ions. The chemical compound formed by this transfer is stabilized by the electrostatic attractions (ionic bonds) between the ions of opposite charge nowadays in the compound. For instance, when each sodium atom in a sample of sodium metal (group 1) gives up one electron to course a sodium cation, Na⁺, and each chlorine cantlet in a sample of chlorine gas (group 17) accepts ane electron to form a chloride anion, Cl⁻, the resulting compound, NaCl, is equanimous of sodium ions and chloride ions in the ratio of 1 Na⁺ ion for each Cl⁻ ion. Similarly, each calcium atom (group ii) can surrender 2 electrons and transfer one to each of two chlorine atoms to form CaCl₂, which is composed of Ca^two+ and Cl⁻ ions in the ratio of one Caⁱⁱ⁺ 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 can assistance united states recognize many of the compounds that are ionic: When a metal is combined with i or more nonmetals, the compound is commonly ionic. This guideline works well for predicting ionic chemical compound germination for most of the compounds typically encountered in an introductory chemical science course. However, information technology is not always true (for example, aluminum chloride, AlCl₃, is not ionic).

You can often recognize ionic compounds considering of their properties. Ionic compounds are solids that typically cook at high temperatures and eddy at even higher temperatures. For example, sodium chloride melts at 801 °C and boils at 1413 °C. (As a comparison, the molecular chemical compound h2o melts at 0 °C and boils at 100 °C.) In solid form, an ionic compound is non electrically conductive considering its ions are unable to menstruation ("electricity" is the menses of charged particles). When molten, even so, it can conduct electricity because its ions are able to move freely through the liquid (Figure 2.30).

Figure 2.30 Sodium chloride melts at 801 °C and conducts electricity when molten. (credit: modification of piece of work by Mark Blaser and Matt Evans)

Link to Learning

Spotter this video to encounter a mixture of salts melt and bear electricity.

In every ionic chemical 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 comprise positive and negative ions. We can use this observation to help 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 two.10

Predicting the Formula of an Ionic Compound

The gemstone sapphire (Figure two.31) is mostly a chemical compound of aluminum and oxygen that contains aluminum cations, Al³⁺, and oxygen anions, O²⁻. What is the formula of this compound?

Effigy two.31 Although pure aluminum oxide is colorless, trace amounts of fe and titanium give blue sapphire its characteristic color. (credit: modification of work past Stanislav Doronenko)

Solution

Considering the ionic compound must be electrically neutral, it must accept the same number of positive and negative charges. Two aluminum ions, each with a charge of 3+, would give u.s.a. six positive charges, and three oxide ions, each with a charge of ii−, would give u.s.a. half-dozen negative charges. The formula would be Al_iiO₃.

Cheque 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 2.five) every bit the cation, the anion, or both. As with simple ionic compounds, these compounds must likewise exist electrically neutral, and then their formulas can be predicted past treating the polyatomic ions equally discrete units. We 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 Ca_iii(PO_iv)_ii. This formula indicates that in that location are 3 calcium ions (Caⁱⁱ⁺) for every two phosphate $({\text{PO}}_4{}^{\mathrm{3-}})$ groups. The ${\text{PO}}_4{}^{\mathrm{3-}}$ groups are discrete units, each consisting of one phosphorus cantlet and four oxygen atoms, and having an overall charge of three−. The compound is electrically neutral, and its formula shows a total count of three Ca, two P, and eight O atoms.

Example 2.11

Predicting the Formula of a Compound with a Polyatomic Anion

Baking pulverization contains calcium dihydrogen phosphate, an ionic compound composed of the ions Caⁱⁱ⁺ and ${\text{H}}_{\mathrm{ii}}{\text{PO}}_{\mathrm{iv}}{}^{-}.$ What is the formula of this compound?

Solution

The positive and negative charges must residual, and this ionic compound must be electrically neutral. Thus, we must have two negative charges to balance the 2+ charge of the calcium ion. This requires a ratio of 1 Ca²⁺ ion to two ${\text{H}}_2{\text{PO}}_{\mathrm{iv}}{}^{-}$ ions. We designate this past 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 compound formed between the lithium ion and the peroxide ion, ${\text{O}}_2{}^{two-}$ (Hint: Use the periodic table to predict the sign and the charge on the lithium ion.)

Because an ionic chemical compound is not made upwards of unmarried, discrete molecules, it 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 ions. For compounds containing only monatomic ions (such as NaCl) and for many compounds containing polyatomic ions (such as CaSO₄), these formulas are merely the empirical formulas introduced before in this chapter. However, the formulas for some ionic compounds containing polyatomic ions are not empirical formulas. For example, the ionic compound sodium oxalate is comprised of Na⁺ and ${\text{C}}_2{\text{O}}_4{}^{\mathrm{two-}}$ ions combined in a 2:one ratio, and its formula is written as Na_twoC₂O₄. The subscripts in this formula are not the smallest-possible whole numbers, equally each can exist divided by 2 to yield the empirical formula, NaCO_two. This is not the accustomed formula for sodium oxalate, however, as information technology does not accurately stand for the compound'due south polyatomic anion, ${\text{C}}_2{\text{O}}_4{}^{\mathrm{2-}}.$

Molecular Compounds

Many compounds do not comprise ions simply 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 volition be treated in considerable detail in a later chapter of this text. We tin frequently identify molecular compounds on the basis of their physical properties. Under normal conditions, molecular compounds often exist equally gases, low-humid liquids, and depression-melting solids, although many of import exceptions be.

Whereas ionic compounds are commonly formed when a metal and a nonmetal combine, covalent compounds are unremarkably formed past a combination of nonmetals. Thus, the periodic table can help us recognize many of the compounds that are covalent. While we tin use the positions of a compound'southward elements in the periodic tabular array to predict whether it is ionic or covalent at this point in our written report of chemistry, you lot should be aware that this is a very simplistic approach that does not account for a number of interesting exceptions. Shades of gray exist between ionic and molecular compounds, and you'll learn more well-nigh those later.

Case 2.12

Predicting the Blazon of Bonding in Compounds

Predict whether the following compounds are ionic or molecular:

(a) KI, the chemical compound used as a source of iodine in table salt

(b) H₂O₂, the bleach and disinfectant hydrogen peroxide

(c) CHCl_iii, the anesthetic chloroform

(d) Li₂CO_three, a source of lithium in antidepressants

Solution

(a) Potassium (group 1) is a metal, and iodine (grouping 17) is a nonmetal; KI is predicted to be ionic.

(b) Hydrogen (group 1) is a nonmetal, and oxygen (grouping 16) is a nonmetal; H₂O₂ is predicted to exist molecular.

(c) Carbon (group 14) is a nonmetal, hydrogen (grouping 1) is a nonmetal, and chlorine (grouping 17) is a nonmetal; CHCl₃ is predicted to exist molecular.

(d) Lithium (grouping 1) is a metallic, and carbonate is a polyatomic ion; Li_twoCO₃ is predicted to exist ionic.

Cheque Your Learning

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

(a) SO₂

(b) CaF₂

(c) N₂H₄

(d) Al₂(And then₄)₃

Respond:

(a) molecular; (b) ionic; (c) molecular; (d) ionic

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Source: https://openstax.org/books/chemistry-2e/pages/2-6-ionic-and-molecular-compounds

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