Question

Draw a Lewis structure for each of the following ionic compounds. What chemical formula does Lewis theory predict? a. sodium fluoride b. calcium chloride c. calcium oxide d. aluminum chloride

Short answer

Sodium fluoride (NaF), Calcium chloride (CaCl2), and Calcium oxide (CaO) all have their cations transferring their valence electrons to achieve full octets for the anions. Aluminum chloride (AlCl3) does the same with aluminum providing 3 electrons to 3 chlorines.

Step-by-step solution

Determine Valence Electrons for Sodium Fluoride

Identify the valence electrons for sodium (Na) and fluorine (F). Na, a group 1 element, has 1 valence electron. F, a group 17 element, has 7 valence electrons.

Draw Lewis Structure for Sodium Fluoride

Sodium will transfer its 1 valence electron to fluorine, completing fluorine's octet. This results in Na+ and F- ions, which are held together by ionic bonds.

Determine Valence Electrons for Calcium Chloride

Identify the valence electrons for calcium (Ca) and chlorine (Cl). Ca, a group 2 element, has 2 valence electrons. Cl, a group 17 element, has 7 valence electrons.

Draw Lewis Structure for Calcium Chloride

Calcium will transfer its 2 valence electrons to two chlorine atoms, one electron to each, forming Ca2+ and two Cl- ions.

Determine Valence Electrons for Calcium Oxide

Identify the valence electrons for calcium (Ca) and oxygen (O). Ca, a group 2 element, has 2 valence electrons. O, a group 16 element, has 6 valence electrons.

Draw Lewis Structure for Calcium Oxide

Calcium will transfer its 2 valence electrons to one oxygen atom, completing oxygen's octet and forming Ca2+ and O2- ions.

Determine Valence Electrons for Aluminum Chloride

Identify the valence electrons for aluminum (Al) and chlorine (Cl). Al, a group 13 element, has 3 valence electrons. Cl, as before, has 7 valence electrons.

Draw Lewis Structure for Aluminum Chloride

Aluminum will transfer its 3 valence electrons to three chlorine atoms, one electron to each, forming Al3+ and three Cl- ions.

Key concepts

Valence Electrons

Valence electrons play a pivotal role in chemical bonding because they are the outermost electrons of an atom and are primarily responsible for the chemical properties of the element. To understand how ionic bonds form, it is essential to know the number of valence electrons in an atom. For instance, in sodium (Na), which is located in group 1 of the periodic table, there is one valence electron, and for fluorine (F), in group 17, there are seven valence electrons.

The valence electrons are significant when predicting how atoms will bond. Atoms tend to either lose, gain, or share electrons to achieve a full outer shell, commonly an octet, as seen in noble gases. The 'octet rule' is a central concept in understanding how elements interact to form compounds.

Ionic Bonds

Ionic bonds are formed when there is a complete transfer of valence electrons between atoms, resulting in a positive ion (cation) and a negative ion (anion). This type of bond typically occurs between metals and non-metals. The metal, with fewer valence electrons, tends to lose them, becoming positively charged, while the non-metal, with more valence electrons, gains them, becoming negatively charged.

For example, in calcium chloride, calcium loses two electrons to achieve a stable electron configuration, forming a Ca²⁺ ion, while each of the two chlorine atoms gains one electron to form two Cl^- ions. The electrostatic attraction between these oppositely charged ions constitutes the ionic bond, which is strong and holds the ions together in a lattice structure in solid form.

Electron Transfer

Electron transfer is the mechanism that leads to the formation of ionic bonds. It involves the movement of electrons from an atom that has a relatively low electron affinity to one with a higher electron affinity – from a metal to a non-metal in most cases. This transfer allows the atoms to reach a more stable electronic state. In Lewis structures, this is illustrated by arrows showing the transfer of electrons.

As seen in the reaction between aluminum and chlorine, the aluminum atom transfers its three valence electrons, forming an Al³⁺ cation, and each chlorine atom accepts one electron, forming Cl^- anions. By losing or gaining electrons, the ions attain a noble gas configuration, which is energetically favorable.

Chemical Formula Prediction

Using Lewis theory to predict chemical formulas involves determining the ionic charges and balancing them to achieve a neutral compound. The total number of positive charges must equal the total number of negative charges. This involves some simple arithmetic. For example, sodium with one positive charge (Na⁺) will combine with fluorine with one negative charge (F^-) to form the compound sodium fluoride (NaF). Similarly, calcium with two positive charges (Ca²⁺) combines with two chlorides (2Cl^-) to form calcium chloride (CaCl₂).

The chemical formulas are determined by the ratio of ions needed to balance the overall charge in the compound, resulting in a neutral species.

Lewis Theory

Lewis theory, or Lewis structures, is a simple way to visualize how bonding occurs between elements. It highlights the electron transfer and how ions form within ionic compounds. Lewis diagrams show valence electrons as dots around the elements’ symbols, and the transfer of electrons is represented by arrows. These structures can predict the resulting ionic compounds and their formulas by illustrating the complete transfer of electrons between atoms to form cations and anions.

The structures emphasize the need for atoms to follow the octet rule and achieve the electron configuration of the nearest noble gas. Lewis theory greatly simplifies the understanding of complex bonding interactions and is a powerful tool in learning and teaching chemistry.