Valence Electrons VValence electrons - electrons in the highest occupied energy level of an element’s atom. # of valence electrons determines the chemical properties of an element. Group # of representative elements = # of valence electrons. Valence electrons are usually the only electrons used in chemical bonds.
Electron (Lewis) Dot Structures Electron dot structures are diagrams that show valance electrons as dots around the chemical symbol.
Draw the dot structure Magnesium Mg Aluminum Al Carbon C Chlorine Cl
The Octet Rule OOctet comes from the Greek work okto meaning 8. How does it relate to chemistry? Nobel gases have 8 electrons in their outermost energy level (except He). In forming compounds atoms tend to achieve the electron configuration of a noble gas; that is an octet of electrons in the highest energy level, hence the Octet Rule (Proposed by chemist Gilbert Lewis).
How Atoms Achieve an Octet Atoms of the metallic elements tend to lose their valence electrons, leaving a complete octet in the next- lowest energy level. Atoms of nonmetallic elements tend to gain electrons or (to share electrons with another nonmetallic element) to achieve a complete octet.
Formation of Cations An atom’s loss of valence electrons produces a cation, or positively charged ion. For metallic elements the name of the cation is the same as the element. For transition metals, the charge of the cation may vary, that is it does not always lose enough electrons to have a electron config of a noble gas. A Roman numeral is used after the name to indicate the value of the charge (# of electrons lost) Copper(I) or Copper(II)
More on Cations PPseudo noble-gas electron configuration Cadmium - how many valence electrons? 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 4d 10 5s 2 How many electrons would Cd have to lose to to have a noble gas config? How many would it have to gain? Ions with a charge of greater than 3 are extremely unlikely ; thus Cd can’t attain noble gas config., but if it loses two it has 18 valence electrons and is relatively favorable in compounds. 18 valence electrons is know as a pseudo noble gas electron config. GGenerally occurs with elements on the right side of the d block.
Formation of Anions The gain of negatively charged electrons by a neutral atom produces an anion. The name of an anion of a nonmetallic element is not the same as the element name; anion names typically ends with “ide”. Ions produced when halogens (group 7A) gain electrons are called halide ions. How many electrons do the halogens have to gain to attain a noble gas electron config?
Ionic Bonds Anions & cations have opposite charges and are attracted to each other by electrostatic forces. This electrostatic force is called ionic bonds and it hold ions together to create compounds.
Chemical Formula & Formula Units Chemists represent the composition of substances by writing chemical formulas. A chemical formula shows the kind and # of atoms in the smallest representative unit of a substance. I.e. NaCl. Chemical formulas refer to a ratio know as a formula unit. A formula unit is the lowest whole-number ratio of ions in an ionic compound. One Na + to each Cl - or 1 to 1 ratio or NaCl. One Mg 2+ to two Cl -, what would the ratio be? 1:2 or MgCl 2
Formation of Ionic Compounds Compounds composed of cations & anions are called ionic compounds. Usually composed of metal cations and nonmetal anions. Although they are composed of ions, ionic compounds are electrically neutral. Formula must be balanced to show it is electrical neutral. Ionic bonding is responsible for the formation of all types of salts; ionic compounds are often referred to as salts.
Balancing (Ionic) formulas Crisscross method The numerical value of the charge of each ion is crossed over and becomes the subscript for the other ion.
Polyatomic Ions ( see handout ) A polyatomic ion is a tightly bound (bonded) group of atoms that has a positive or negative charge and behaves as a unit, i.e. ammonium ion (NH 4 + ).
Naming Ionic Compounds Binary Ionic Compounds are compounds composed of two elements. To name any binary ionic compound, state the cation name first, followed by the anion name. Terniary Ionic Compounds are composed of more than two elements (contain a polyatomic ion) To name any ternary ionic compound state the cation name first, followed by the anion name.
Practice Naming of Binary Ionic Compounds Easy one first! NaCl Sodium Chloride K2SK2S Potassium Sulfide LiBr Lithium Bromide Cs 2 O Cesium oxide A little harder! CuO Copper(II) oxide FeO Iron(II) oxide SnS 2 Tin(IV) sulfide
Writing Formulas for Binary Ionic Compounds If you know the name of a binary ionic compound, you can write the formula. Write the symbol of the cation and then the anion. Add whatever subscripts are needed to balance the charges. Remember the Crisscross method
Practice Writing Formulas of Binary Ionic Compounds Easy ones first Sodium Chloride NaCl Potassium Chloride KCl Lithium Fluoride LiF A little Harder Calcium bromide CaBr 2 Potassium Nitride K 3 N Tin(IV) sulfide SnS 2 Iron(III) oxide Fe 2 O 3
Practice Writing Formulas for Polyatomic (Ternary) Ionic Compounds A few easy ones Sodium nitrate NaNO 3 Calcium carbonate CaCO 3 Lead(II) sulfate PbSO 4 Lets try a couple of harder ones Calcium Nitrate Ca(NO 3 ) 2 Lithium Carbonate Li 2 (CO 3 )
Properties of Ionic Compounds MMost ionic compounds are crystalline solids at room temperature. IIonic compounds generally have high melting points. Results from a repeating 3-dimensional arrangement of ions that maximizes attractions and minimizes repulsions. Large attractive forces make ionic compounds very stable. II.e. melting point of NaCl is 800 o C. The coordination # of an ion is the number of ions of the opposite charge that surround it in a crystal solid
Properties of Ionic Compounds (cont) Another property of ionic compounds has to do with conductivity. Ionic compounds can conduct an electric current when melted or dissolved in water.
Metallic Bonds Metals are made up of closely packed cations rather than neutral atoms. The valence electrons of metal atoms can be modeled as a “ sea of electrons”. valence electrons are mobile and can drift freely from one part of the metal to another. Metallic bonds - consist of the attraction of the free floating valence electrons for the positively charged metal ions.
Metallic Bond The metallic bond consists of a series of metals atoms that have all donated their valence electrons to an electron cloud that permeates the structure. This electron cloud is frequently referred to as an electron sea. It might help to visualize the electron sea model as if it were a box of marbles that are surrounded by water. The marbles represent the metal atoms and the water represents the electron sea.
Metallic Properties The free flowing valance electrons allow metals to have certain properties not found in non-metals. Conductive – conduct energy Ductile – can be drawn into wire Malleable – can be shaped (hammered)
Crystalline Structure of Metals LLike ionic compounds metals also are crystalline. Metals that contain just one kind of atoms are among the simplest forms of all crystalline solids. MMetals atoms are arranged in very compact and orderly patterns. BBecause metal atoms are spherical there are several types of crystalline arrangements possible: Body-centered cubic Face-centered cubic Hexagonal close packed
Alloys Alloys are mixtures composed of two or more elements, at least one of which is a metal. Alloys are important because their properties are often superior to those of their component elements. Sterling sliver – Ag and Cu Bronze – Cu and Sn Cast iron – Fe and C Stainless steel – Fe, Cr, C, and Ni Surgical steel – Fe, Cr, Ni, an Mo Nonferrous (non-iron) alloys used in coins