Essential Concepts: Formation of Compounds Chapter 4

Essential Concepts: Formation of compounds -- Chapter 4

Obj 1 Distinguish the properties of compounds from those of the elements of which they are composed.

Atoms combine to form compounds only if doing so increases their stability. Atoms share or transfer electrons in order to end up with their outer energy levels filled with valence electrons, like the stable noble gases. Thus compounds are usually more stable and less reactive than their component elements, and usually have very different properties from their parent elements.

Sodium chloride is an example. Sodium (Na) and chlorine (Cl) are pure elements that combine to form sodium chloride (NaCl), a compound. Sodium is highly reactive and reacts vigorously with water. Chlorine is a green, poisonous gas. Both are unstable and dangerous. Yet when sodium transfers its lone valence electron to chlorine, the resulting ions are stable and will remain unchanged for hundreds of years. The resulting compound, sodium chloride (table salt) is unreactive and extremely safe, and, in fact, is essential for good health. (By the way, when an atom becomes a negative ion, we change the ending of its name to -ide. A neutral chlorine atom becomes a negatively charged chloride ion.)

Obj 2 Contrast the properties of ionic compounds with molecular compounds

Ionic Compounds Molecular Compounds Bonds Ionic (attraction of oppositely charged Covalent (results from the ions, that result from the transfer of sharing of electrons to achieve electrons to achieve noble gas stability) noble gas stability) Components Metal with nonmetal or polyatomic Nonmetals only ions (those made of several atoms) Solubility Highly soluble in water Some soluble, many not Melting point High (tend to be solid crystals) Relatively low (many are liquid or gas at room temperature) Conductivity Strong conductors of electricity Do not conduct electricity

Obj 3 Use the periodic table, and knowledge of valence electron trends, to predict the number of electrons an atom needs to gain or lose to achieve greater stability.

. Metals are to the left of the stairstep of the periodic table and tend to lose electrons to become positive ions with noble gas stability.

. Nonmetals are to the right of the stairstep and tend to either gain electrons (from metals) to form negative ions, or they share electrons with other nonmetals to achieve noble gas stability.

Obj 4 Model the formation of compounds using Lewis Dot Structures Drawing Lewis Electron Dot Structures

1. Use the element's symbol to represent everything but the valance electrons.

2. Draw dots representing valance electrons on the four sides of the symbol (right, bottom, left, and top). Draw one per side for the first 4, then double up until all valance electrons are accounted for.

Demonstrating the formation of ionic bonds with Lewis Electron Dot Structures

1. Draw Lewis dot structures for the component atoms. Na Cl

2. Indicate the transfer of electrons using arrows Na Cl

3. Show the resulting ions with their appropriate charges. Na Cl

Complete demonstration: Na + Cl  Na Cl (NaCl)

Demonstrating the formation of covalent (molecular) bonds with Lewis Electron Dot Structures

1. Draw Lewis dot structures for the component atoms. H O H

2. Circle the pairs of electrons that are shared H O H

3. Replace each shared pair of circled electrons with a line H O H

Complete demonstration: H + O + H  H O H (H2O)

Obj 5 Recognize the name, formula, and charge of common polyatomic ions.

A polyatomic ion is a tightly (covalently) bound group of atoms that behave as a unit and have a charge. The polyatomic ions you need to know (there are many others) are:

+ - ammonium NH4 permanganate MnO4

- - 2- acetate CH3COO or C2H3O2 carbonate CO3

- 2- chlorate ClO3 chromate CrO4

- 2- hydroxide OH sulfate SO4

- 3- nitrate NO3 phosphate PO4