Chapter 4 Formation of Compounds Section 4.2 How Elements Form Compounds

Chapter 4Formation of Compounds

Section 4.2How Elements Form Compounds

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

When elements react, atoms of the elements must collide

Reactions between atoms only involve their

electron clouds- v.e. of colliding atoms interact

Compound formation

Group 18 elements Unreactive elements, extreme stability Noble gases are used in light displays, neon

lights Lack of reactivity of noble gases indicates

that atoms of these elements must be stable Each noble gas has 8 valence electrons,

except helium has 2

Noble Gases

Achieving the same configuration of v.e.’s (a stable octet) as one of the noble gases

NOBLE GAS CONFIGURATION

Atoms can become stable by having eight electrons in their outer energy level (or 2 for H and He)

If atoms collide with enough energy, their outer electrons may rearrange to achieve a stable octet of v.e.’s → the atoms will form a compound

OCTET RULE

2 possibilities: Transfer (ionic) - metal and nonmetal Sharing (covalent) - 2 nonmetals

Valence electron rearrangement

• Can use Lewis dot structures to represent atoms and the rearrangement of v.e.’s.

• Colliding atoms transfer electrons only when

one atom has stronger attraction for valence electrons than the other atom

• Electrons will move from one atom (metal) to

the other (nonmetal)

Electron Transfer (Ionic Compound)

• Example: Transfer gives both Chlorine and Sodium stable octet

. . . .Na • + • Cl : → [Na] + + [ ; Cl : ] - • • • •

• Try these: Ca and Cl, Li and S, K and P • Atoms are no longer neutral- become ions


ION: Atom or group of combined atoms that has a charge because of the loss or gain of

electrons.

Ions always form when v.e.’s rearrange by electron transfer between atoms


IONIC COMPOUND: A compound that is composed of ions

Once the ions have formed they are strongly attracted to each other.

IONIC BOND: Strong attractive force

between ions of opposite charge


Ions in salt do not arrange themselves into isolated sodium ion/chloride ion pairs. They arrange themselves into a well-organized definite cube structure

CRYSTAL: Regular repeating arrangement of atoms, ions or molecules


Ionic attractions are strong attractive forces so it is hard to break ionic compounds and they

have high melting point temperatures


Generally, ionic compounds:1) Crystalline solids at room temperature (Why?)2) Hard, rough and brittle (Why?)3) High temperatures in order to melt (Why? What

does this look like?)4) They tend to conduct electricity when they dissolve

in water or are melted (Why? What does this look like?)

http://education-portal.com/academy/lesson/ionic-compounds-formation-lattice-energy-and-properties.html

ELECTROLYTE: Compound that conducts electricity

when melted or dissolved in water.

Properties of Ionic Compounds

Predict: 1. Would NaCl dissolved in water conduct

electricity? Would this be an electrolyte? Why or why not?

2. Would NaCl dissolved in ethanol conduct electricity?

Would this be an electrolyte? Why or why not?

3. Would distilled water conduct electricity? Why or Why not?

Formulas of compounds tell1)What elements make up the compound 2)How many atoms of each element are present in

the compound Ex: Water – H2O has 2 hydrogen atoms and 1

oxygen atomSodium Chloride- NaCl has 1 sodium and 1

chlorine atom

Chemical Formulas

When atoms collide with enough energy to react, but neither attracts electrons strongly

enough to take electrons from the other atom, the atoms combine by sharing v.e.’s

Electron Sharing/ Covalent Compounds (molecules)

Ex: Sharing gives both H and O stability

Hydrogen has 2 valence electrons and oxygen has an octet when they share electrons.

• • • • H+ • O : → H : O : (O has 7 e-s) • • • • • •H : O : + H → H :O : • •


COVALENT BOND: Attraction of two atoms for a shared pair of electrons

In a covalent bond, atoms (2 nonmetals) share electrons and neither atom has an ionic charge

COVALENT COMPOUND: Compound whose

atoms are held together by covalent bonds


MOLECULE: Uncharged group of two or more atoms held together by covalent bonds

Covalent bonds= molecular compounds


• More than 2 electrons can be shared in some cases of electron sharing

• Example: Carbon Dioxide . . . . .

: O • + • C • + • O : → • • • : O : :C : :O : •• • •

Sharing More than 2 e-‘s

Attractive force between molecules are usually weak

INTERPARTICLE FORCES: Forces between

particles that make up a substance

Properties of Covalent bonds

Many covalent compounds 1) Are liquids or gases at RT or are solids that

melt at low temperatures2) Do not conduct electricity3) do not usually dissolve in water4)In general, are usually less soluble in water

than ionic compounds.http://ed.ted.com/lessons/how-atoms-bond-george-zaidan-and-charles-morton#review

http://education-portal.com/academy/lesson/covalent-compounds-properties-naming-

formation.html

Properties of Covalent bonds

http://education-portal.com/academy/lesson/lewis-structures-single-double-triple-bonds.html

Steps to determine the number of bonds:1) Have -Total number of V.E.2) Need - Total number of V.E. (Happy)3) Subtract Need – Have4) Divide by 25) Draw with that number of bonds

Lewis Dot Structures(Structural Formulas)

http://education-portal.com/academy/lesson/lewis-dot-structures-polyatomic-ions-and-resonance-structures.html

Polyatomic Structures

Documents

Chapter 4 Formation of Compounds Section 4.2 How Elements Form Compounds