9.3
Ionic Bonding and Electron Transfer
Ions are atoms or molecules bearing an electrical charge. A cation (a positive ion) forms when a neutral atom loses one or more electrons from its val…
Ionic bonds are formed by the transfer of electrons from metals to nonmetals. In comparison to covalent bonds where electrons are shared, in ionic bonds metals tend to lose and nonmetals tend to accept electrons — but why?
The most stable electron configuration of an atom is with a full octet. Achieving this state in conjunction with lowering the potential energies is a major driving force for bond formation.
But how to predict when electrons will be transferred and not shared? For this, examine the ionization energies and electron affinities of atoms.
On the periodic table, s-block metals have low ionization energies. This facilitates easier loss of electrons to reach an octet while concurrently forming cations.
On the contrary, nonmetals from the p-block, except for noble gases, have high electron affinities and readily accept electrons to form anions. The oppositely charged cations and anions experience strong electrostatic interactions, attracting each other to form ionic bonds.
Ionic compounds often form hard and well-defined crystal structures with high melting points, due to strong electrostatic attraction between ions.
Consider lithium fluoride, an ionic compound made of lithium and fluorine. When lithium loses an electron to become a lithium cation with a duet, the electron is being taken up by fluorine to form the anion with an octet.
According to Coulomb's law, these two charged particles are attracted to each other to form lithium fluoride, a neutral compound.
The Lewis model can be used to describe and predict the chemical formula of ionic compounds. The Lewis symbol shows charges in the upper-right hand corner and depicts anions and cations with the symbol written within brackets.
Consider strontium chloride, a commonly used salt to get a bright red color in fireworks. The Lewis symbol for strontium has two, and chlorine has one unpaired electron.
Strontium must lose two electrons, while chlorine must gain one electron to reach an octet. Therefore, one strontium cation binds to two chloride anions, in the ratio of 1:2.
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Q1: Why do metals lose electrons and nonmetals gain electrons in ionic bonding?
Metals have low ionization energies, making electron loss easier to achieve a stable octet configuration. Nonmetals have high electron affinities and readily accept electrons to complete their valence shells. This electron transfer between oppositely charged ions creates strong electrostatic attractions that form ionic bonds, lowering the overall potential energy of the system.
Q2: How do you predict the chemical formula of an ionic compound?
Use the Lewis model to determine how many electrons each atom must lose or gain to reach a stable octet. The cation loses valence electrons while the anion gains them. The formula reflects the simplest ratio ensuring equal positive and negative charges. For example, strontium loses two electrons and chlorine gains one, creating the 1:2 ratio in strontium chloride.
Q3: What is the relationship between ionic bonding and crystal structure?
Ionic compounds form hard, well-defined three-dimensional crystal lattice structures due to strong electrostatic attraction between oppositely charged ions. The attractive forces are isotropic, meaning each ion is equally attracted to all nearby ions of opposite charge. This regular arrangement results in high melting points and brittleness characteristic of ionic solids.
Q4: How does the periodic table help predict which elements form cations and anions?
S-block metals on the left side of the periodic table have low ionization energies and readily form cations by losing electrons. P-block nonmetals in the upper-right corner have high electron affinities and readily form anions by gaining electrons. This periodic trend reflects the ease of electron transfer needed to achieve stable electron configurations.
Q5: Why are ionic compounds good conductors when melted or dissolved but poor conductors as solids?
In solid ionic compounds, strong electrostatic forces hold ions in fixed positions within the crystal lattice, preventing free movement. When melted or dissolved in water, ions can move freely through the liquid, allowing electrical current to flow. This mobility of charged particles makes molten and aqueous ionic solutions excellent conductors of electricity.
Q6: What does Coulomb's law explain about ionic bonding?
Coulomb's law describes the electrostatic force of attraction between oppositely charged ions. The force is proportional to the product of the charges and inversely proportional to the distance between them. In ionic compounds like lithium fluoride, this law explains why the positively charged lithium cation and negatively charged fluoride anion attract each other strongly to form a stable neutral compound.
Q7: How do transition metals differ from main group elements in forming cations?
Transition metal cations typically have 2+ or 3+ charges resulting from loss of outermost s electrons first, followed by d electrons from the next-to-outermost shell. Main group elements lose all valence electrons to achieve a noble gas configuration. This difference occurs because transition metals have accessible d orbitals, allowing variable oxidation states unlike main group elements.