4.2 How Elements Form Compounds

Objectives Model two types of compound formation: ionic and covalent at the atomic level. Demonstrate how and why atoms achieve chemical stability by bonding. Compare, using examples the effect of covalent and ionic bonding on the physical properties of compounds.

How Elements Form Compounds In the 19th Century chemists tried to picture how atoms combine. Early models included atoms with “hooks” that allowed them to attached to one another.

When Atoms Collide Atoms collide when they react These collisions determine what kind of compounds are formed. When atoms collide it is the electron clouds that interact with one another. To be more precise it is the valence electrons that interact. Remember: valence electrons are responsible for the chemical properties of elements

Valence Electrons and Bonds To understand how valence electrons affect the bonds of compounds let look at elements with unusual properties….the noble gases

Noble Chemical Stability Noble Gases are located in Group 18 They are inert or unreactive This lack of reactivity makes them useful as incandescent light bulbs (argon or krypton) and neon lights (orange-neon, blue-argon, yellow-helium) Occur naturally in the environment

Noble Gases Not a single compound of these elements has ever been found naturally in the environment. In 1960 chemists were able to react fluorine with krypton and xenon under high temperature and pressure. Since then a few additional compounds of xenon and krypton have been synthesized, but none with helium, neon or argon.

The Octet Rule Lack of reactivity of the noble gases must mean that they are stable. Elements in the same group have similar valence arrangements. Noble gases have 8 valence electrons, except for helium that has 2. Because the electron arrangement determines chemical properties  unreactive nature of noble gases

Octet Rule Atoms combine because they become more stable by doing so. Modern model of bonding is based on the fact that the stability of noble gases is a result of their valence electron arrangements. Octet Rule- atoms can become stable by having eight electrons in their outer energy level (or two electrons in the case of some of the smaller atoms).

Noble Gas Configuration Atoms obtain stability by achieving the same configuration of valence electron as found in noble gases; most stable configuration.

Valence Electron Review How many valence electrons do you find in Group 1, 2, 13, 15, 16 and17?

Review What is the octet rule?

Ways to Achieve a Stable Outer Energy Level 4.2 Continued Ways to Achieve a Stable Outer Energy Level

Forming Compounds When atoms collide with enough force their outer electrons may rearrange to achieve a stable octet of valence electrons. When it occurs they achieve a noble gas configuration and the atoms form compounds

How do the electrons rearrange? Electrons are transferred from one atom to another Electrons are shared between atoms

Electrons Can Be Transferred Example Sodium (Na) and Chlorine (Cl) Na has one valence electron Cl has seven valence electrons Na transfers its 1 to Cl 7  Na now has 8 electrons in its outer shell and Cl now has 8 in its outer shell Na now has 1 less electron  has +1 charge Cl now has 1 more electron has a -1 charge

Ions Atom or group of atoms that has a charge b/c they have either lost or gained electrons Ions form when valence electrons rearrange by transfer Compounds composed on ions are ionic compounds

Table 4.2 p. 134

Ions Attract Each Other Opposites attract Positive sodium is attracted to the negative chloride ion. Positive ions are called cations Names stays the same Negative ions are called anions Add –ide to the end Sulfur  Sulfide ion

Ionic bond Strong attractive force between ions of opposite charge Hold ions together in an ionic compound Crystal is a regular, repeating arrangement of atoms, ions or molecules

The Results of Ionic Attraction b/c of the strong attractive forces and the degree of organization  solid at room temp Raising temp causes particles to move faster  particles overcome attractive forces and the crystal organization breaks down

Representing Compounds with Formulas Sodium Chloride Formula: NaCl Write cation first followed by the anion Always in the smallest whole number ratio

Review What is a negatively charged ion called? What is a positively charged ion called? What is the noble gas configuration?

Electrons Can Be Shared What happens when hydrogen and oxygen collide? Oxygen has 6 valence electrons Hydrogen has 1 valence electron Can hydrogen lose its one valence electrons?

Colliding atoms Transfer electrons only when one atom has a stronger attraction for valence electrons In the case of sodium and chlorine, chlorine attracts sodium’s valence electrons strongly and sodium is holding its electrons weakly In the case of hydrogen and oxygen neither atom attracts strongly enough to take electrons from the other.

Hydrogen and Oxygen They will share their electrons When atoms collide with enough energy to react, but neither atom attracts electrons strongly enough to take electrons from the other, the atoms combine by sharing valence electrons.

Let’s Look at Water

Formation of Compounds Like the formation of sodium chloride all the components present before the reaction are still present after the reactions The valence electrons no longer reside in the same position. They have rearranged

Electrons Sharing Produces Molecules Covalent bond- the attraction of two atoms for a shared pair of electrons Electrons are shared Neither atom has an ionic charge Covalent compound- compound whose atoms are held together by a covalent bond, also called molecular compounds Water is an example Molecules-uncharged group of two or more atoms held together by covalent bonds (water molecules)

More Than Two Electrons Can Be Shared Let’s look at Carbon Dioxide Carbon has 4 valence electrons Oxygen has 6 valence electrons

Covalent and Ionic bonds Two nonmetallic elements usually form molecular or covalent compounds by sharing electrons Nonmetallic and a metallic element usually form ionic compounds

Review What is a covalent bond? What is the definition for a molecular compound? Give me an example of a molecular compound?

Review What is a covalent bond? What is the definition of a molecular compounds? What are two examples of a molecular compound?

How do ionic and covalent compounds compare?

Bonding When elements combine they either form ions or molecules…there are no other possibilities. When particles change dramatically, when they change from sodium to sodium ions or from hydrogen and oxygen to water molecules. These changes explain why compound have different properties from the elements they are composed of.

Explaining the Properties of Ionic Compounds Physical properties are a result of the well-organized, tightly bound ions. Ions forms strong, three-dimensional crystals

Properties of Ionic Compounds Generally ionic compounds are crystalline solids at room temperature. Generally hard, rough and brittle High melting points and boiling points This is because of the strong attraction between ions Usually dissolve in water

Properties of Ionic Compounds Solutions will conduct electricity Liquid state (melted state) will conduct electricity They are electrolytes- any compound that conducts electricity when melted or dissolved in water In order to conduct electricity the ions must be free to move. Must take on or give up electrons Solid state does not conduct electricity. Why?

Explaining the Properties of Covalent Compounds Composed of molecules Held together by covalent bonds-make the molecule a stable unit Molecules have no ionic charge Attractive forces between molecules are usually weak Interparticle forces- forces between particles that make up a substance

Properties of Covalent Compounds Many are liquids or gases at room temperature Low melting points and boiling points Do not conduct electricity Some do not dissolve in water Vegetable oil and gasoline In general they are less soluble in water than ionic compounds

Ionic Vs. Covalent It is the strength between interparticle forces in covalent compounds compared to the strong attractive forces of ions in ionic compounds is what explains many of the differences in their physical properties Because covalent compounds do not have ions you would expect that they don’t conduct electricity Ionic compounds are more soluble in water because ions are attracted by the water molecules, while molecular compounds are not

Ionic Vs. Covalent Compounds High melting point High boiling point Solid at room temperature Soluble in water Excellent conductor of electricity Many crystalline Low melting point Low boiling point Liquid or gas at room temperature Less soluble in water Poor conductor of electricity