Chapter 18 – Chemical Bonds

Ionic Bonding

Vocabulary  electron dot diagram

(Lewis dot structure, LDS)   ion  anion  cation  chemical bond   ionic bond  chemical formula  crystals

Recognize stable electron configurations

 Keep in mind that we are only dealing with the representative elements.

 When the highest occupied energy level of an atom is filled with electrons, the atom is stable and not likely to react.

 The noble gases have stable electron configurations.

Draw Lewis dot structures for the representative elements

 Valence electrons determine the chemical properties of an element.

 A Lewis dot structure (LDS) is a model of an atom that shows only the valence electrons in the atom.

X element symbol – surrounded by valence electrons

 sodium

Practice drawing LDS

 barium

Practice drawing LDS

 krypton

 sulfur

Practice drawing LDS

 nitrogen

 aluminum

Practice drawing LDS

 helium

1 2 13 14 15 16 17 18

Describe how an ionic bond forms

 Elements “want” to have the same electron configuration as their nearest noble gas neighbor…because that is a stable electron configuration.

 Some elements achieve stable electron configurations by gaining or losing electrons.

 An ion is an atom that has lost or gained electrons and now has a net charge.

 Remember…  a loss of electrons creates a positively charged ion,

a cation.  a gain of electrons creates a negatively charged

ion, an anion.

 Remember…  a loss of electrons creates a positively charged ion,

a cation.  a gain of electrons creates a negatively charged

ion, an anion.

 What is the charge on an ion that has gained 3 electrons?

 Remember…  a loss of electrons creates a positively charged ion,

a cation.  a gain of electrons creates a negatively charged

ion, an anion.

 What is the charge on an ion that has lost 2 electrons?

 A chemical bond is the force that holds atoms/ions together.

 An ionic bond is a chemical bond that forms due to a charge attraction after electrons have been transferred between atoms.

 Remember that opposite charges attract…  when cations and anions get close enough to each

other, an ionic bond forms.

Predict the composition of an ionic compound

 Ionic compounds contain ionic bonds and will have a net charge of zero on the compound.

 The charges on the ions formed by sodium and chlorine cancel each other out in a 1:1 ratio, so only one ion of each type is needed for the chemical formula.

 What is the chemical formula when magnesium combines with chlorine?

 Magnesium has two electrons to lose, but chlorine can only gain one electron.

 What is the chemical formula when magnesium combines with chlorine?

 Magnesium has two electrons to lose, but chlorine can only gain one electron.

 What is the chemical formula when magnesium combines with chlorine?

 What is the chemical formula when magnesium combines with chlorine?

 Therefore, the chemical formula is

 What is the chemical formula when cesium combines with iodine?

 What is the chemical formula when potassium combines with nitrogen?

 What is the chemical formula when aluminum combines with chlorine?

 What is the chemical formula when calcium combines with phosphorus?

Relate the properties of ionic com-pounds to the structure of crystal lattices

 Chemical formulas for ionic compounds only give you the ratio of ions in the compound.

 Ionic compounds form crystals. The shape of the crystal depends on the arrangement of the ions.

 The arrangement of the ions depends on the ratio of ions and their relative size (the size of the charges).

 This crystal structure is what makes ionic compounds brittle.

 The close packing in crystals can be explained by the strong attractions in ionic compounds.

 The strong ionic bonds also explain the high melting points of ionic compounds. It takes more energy to get those strong attractions to release and melt.

 Those strong attractions also explain why the solid form does not conduct electricity. However, once the solid is melted or dissolved in water, ionic compounds are very good conductors of electricity.