2.7 Ions and also Ionic Compounds


The cell core of an atom is unchanged by plain chemical processes, however atoms have the right to readily gain or lose electrons. If electrons are eliminated from or included to a neutral atom, a charged particle referred to as an ion is formed. An ion through a optimistic charge is referred to as a cation (pronounced CAT-ion); a negatively fee ion is dubbed an anion (AN-ion). The salt atom, because that example, which has 11 protons and 11 electrons, easily loses one electron. The result cation has 11 protons and also 10 electrons, so it has actually a net charge of

*
The net charge on an ion is represented by a superscript;
*
*
and also
*
median a net charge resulting indigenous the ns of one, two, or three electrons, respectively. The superscripts
*
*
and
*
stand for net charges resulting from the get of one, two, or 3 electrons, respectively. The formation of the
*
ion native an Na atom is displayed schematically together follows:


*


Chlorine, v 17 protons and 17 electrons, frequently gains one electron in chemistry reactions, developing the

*
ion.

You are watching: Predict the charge of the most stable ion of bromine


*


In general, steel atoms tend to shed electrons to kind cations, whereas nonmetal atoms often tend to get electrons to form anions.


SAMPLE exercise 2.7

Give the chemical symbols, including mass numbers, because that the complying with ions: (a) The ion v 22 protons, 26 neutrons, and 19 electrons; (b) the ion the sulfur that has 16 neutrons and 18 electrons.

Solution

(a) The variety of protons (22) is the atomic variety of the element, therefore the facet is Ti (titanium). The mass variety of this isotope is

*
(the amount of the protons and neutrons). Since the ion has three an ext protons 보다 electrons, it has actually a net charge of
*
Thus, the symbol because that the ion is
*

(b) By referring to a periodic table or table that elements, we watch that sulfur (symbol S) has actually an atomic variety of 16. Thus, every atom or ion of sulfur has 16 protons. We space told the the ion additionally has 16 neutrons, so the mass number of the ion is

*
since the ion has actually 16 protons and 18 electrons, that is net charge is
*
Thus, the finish symbol because that the ion is
*

In general, us will focus on the network charges of ions and also ignore your mass numbers unless the circumstances dictate that we specify a details isotope.

PRACTICE EXERCISE

How numerous protons and also electrons go the

*
ion possess?

Answer: 34 protons and 36 electrons


In addition to straightforward ions such as Na+ and also Cl-, there are polyatomic ions such as NO3- (nitrate ion) and SO42- (sulfate ion). These ion consist of atom joined as in a molecule, however they have a net confident or an adverse charge. We will think about further instances of polyatomic ion in ar 2.8.

The chemical properties of ion are really different from those that the atoms from which they space derived. The difference is choose the change from Dr. Jekyll come Mr. Hyde: return the body might be essentially the same (plus or minus a few electrons), the actions is much different.

Predicting Ionic Charges

Many atoms acquire or shed electrons so as to end up through the same variety of electrons as the noble gas closest come them in the regular table. The members of the noble-gas family are chemically very nonreactive and type very couple of compounds. We could deduce that this is because their electron species are really stable. Surrounding elements can obtain these very same stable arrangements by losing or gaining electrons. Because that example, ns of one electron native an atom that sodium pipeline it with the same number of electrons as the neutral neon atom (atomic number 10). Similarly, as soon as chlorine gains an electron, it end up through 18, the very same as argon (atomic number 18). Us will usage this simple observation to describe the development of ions in chapter 8, wherein we discuss chemical bonding.

The periodic table is an extremely useful because that remembering the dues of ions, especially those the the facets on the left and right sides of the table. As figure 2.22 shows,


*

Figure 2.22 fees of some usual ions found in ionic compounds. An alert that the steplike line the divides steels from nonmetals additionally separates cations native anions.


SAMPLE exercise 2.8

Predict the charges supposed for the most stable ions of barium and also oxygen.

Solution

We will assume the these elements type ions that have the same variety of electrons as the nearest noble-gas atom. Native the routine table, barium has actually atomic number 56. The nearest noble gas is xenon, atom number 54. Barium can attain a stable plan of 54 electrons by shedding two of its electrons, forming the

*
cation.

Oxygen has atomic number 8. The nearest noble gas is neon, atomic number 10. Oxygen can obtain this steady electron arrangement by gaining two electrons, thereby forming the

*
anion.

PRACTICE EXERCISE

Predict the charge of the many stable ion the aluminum.

Answer: 3+


The routine table is really useful for remembering the fees of ions, specifically those that the facets on the left and right sides of the table. As number 2.22 shows, the charges of these ion relate in a simple way to their positions in the table. On the left next of the table, for example, the team 1A elements (the alkali metals) kind 1+ ions, and the group 2A elements (the alkaline earths) form 2+ ions. On the various other side that the table the team 7A facets (the halogens) form 1- ions, and also the team 6A elements form 2- ions. Together we will see later on in the text, numerous of the other groups do no lend us to such straightforward rules.

Ionic Compounds

A an excellent deal of chemical activity involves the transfer of electrons between substances. Ions kind when one or an ext electrons move from one neutral atom come another. Figure 2.23 shows that once elemental sodium is permitted to react through elemental chlorine, one electron move from a neutral sodium atom to a neutral chlorine atom. We space left through an Na+ ion and a Cl- ion. Objects the opposite charge attract, however, for this reason the Na+ and the Cl- ion bind with each other to kind the compound sodium chloride (NaCl), which we know better as usual table salt. Sodium chloride is an example of an ionic compound, a link that consists of both positively and also negatively fee ions.


*

Figure 2.23 (a) The carry of an electron indigenous a neutral Na atom to a neutral Cl atom leads to the formation of one Na+ ion and also a Cl- ion. (b) arrangement of these ion in solid salt chloride (NaCl) is pictured at the right.


We can frequently tell even if it is a link is ionic (consisting of ions) or molecule (consisting the molecules) indigenous its composition. In general, cations are metal ions, conversely, anions are nonmetal ions. Consequently, ionic compound are generally combinations that metals and nonmetals, as in NaCl. In contrast, molecule compounds are usually composed that nonmetals only, together in H20.


SAMPLE exercise 2.9

Which the the complying with compounds would certainly you intend to be ionic:

*
*
*
*

Solution

We would certainly predict the

*
and
*
room ionic compounds because they space composed the a metal combined with a nonmetal. The other two compounds, composed entirely of nonmetals, room predicted (correctly) to be molecule compounds.

PRACTICE EXERCISE

Which the the following compounds are molecular:

*
FeS,
*
*

Answer: CBr4 and also P4O6


The ions in ionic compounds space arranged in three-dimensional structures. The arrangement of Na+ and Cl- ion in NaCl is presented in figure 2.23. Due to the fact that there is no discrete molecule that NaCl, we are able come write just an empirical formula for this substance. In fact, only empirical formulas can be written for many ionic compounds.

We have the right to readily compose the empirical formula because that an ionic compound if we understand the fees of the ion of which the is composed. Chemical compounds are always electrically neutral. Consequently, the ions in an ionic compound always occur in together a proportion that the complete positive charge equals the total an unfavorable charge. Thus, there is one Na+ to one Cl- (giving NaCl), one Ba2+ to 2 Cl- (giving BaCl2), and also so forth.

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As you consider these and other examples, friend will see that if the charges on the cation and anion space equal, the subscript on every ion will be 1. If the charges are not equal, the charge on one ion (without that is sign) will become the subscript top top the other ion. For example, the ionic compound created from Mg (which forms Mg2+ ions) and N (which develops N3- ions) is Mg3N2:

formula come come


SAMPLE exercise 2.10

What room the empirical recipe of the compounds formed by (a)

*
and also
*
ions; (b)
*
and
*
ions; (c)
*
and also
*
ions?

Solution

(a) 3

*
ion are forced to balance the charge of one
*
ion. Thus, the formula is
*

(b) two

*
ion are required to balance the charge of three
*
ion (that is, the total positive fee is
*
and the total negative charge is
*
). Thus, the formula is
*

(c) two

*
ion are needed to balance the fee of one
*
for this reason the formula is
*
In this case the formula because that the whole polyatomic ion
*
need to be attached in parentheses so that it is clear the the subscript 2 applies to all the atoms of that ion.

PRACTICE EXERCISE

Write the empirical formulas for the compounds created by the complying with ions: (a)

*
and also
*
(b)
*
and
*
(c)
*
and also
*