Chemical Bonding Chapter 17 Adapted from “Conceptual Physical Science” Hewitt

Chemical Bonding General theme How can the periodic table can be used to determine chemical bonding?

Electron Affinity Electron Affinity The ability of an atom to attract additional electrons   How many electrons an atom attracts is dependent on its valence shell.

Electron Affinity continued Octet Rule Atoms desire to have a full valence shell. Generally, this means they want 8 electrons in their outer shell. Atoms will gain or lose electrons in order to achieve this stability.

Electron Affinity continued Periodic Table Elements in the upper right corner have the greatest electron affinity. Elements in the lower left corner have the smallest electron affinity.

Electron Affinity continued Atoms are electrically neutral… …until they gain or lose electrons.

Ions How can an atom become an ion? An atom becomes an ion by gaining or losing electrons Each electron has the same quantity of negative charge and the same mass

Ion Ion A charged atom Atoms are electrically neutral, but if an atom becomes charged by gaining or losing an electron it becomes an ion.

Cation Cation Positive ion (+) Loses electrons Metals commonly do this.

Anion Anion Negative ion (-) Gains electrons Non-metals commonly do this.

Cations and anions Cation Any atom or group of atoms that has a positive charge  Anion Any atom or group of atoms that has a negative charge   When bonding, atoms want 8 valence electrons

Metals Metals Valence electrons of most metals are weakly held to the nucleus LOW Metals have a very _______ electron affinity.

Metallic Bond Metallic Bond A chemical bond in which metal atoms are held together by their attraction to a common pool of electrons   Metal ions are held together by freely flowing electrons. This electron mobility accounts for a metal’s high heat and electrical conductivity

Alloy Alloy A mixture of 2 or more metallic elements Example: White Gold Au and Pd Sterling silver 92.5% Ag 7.5% Cu Steel 97% Fe 2% Mn 1% C   Aluminum cans are alloys of Al, Zn and Mn

Ionic bond Ionic Bond Metal atoms lose electrons to become cations (+) Non-metal atoms gain electrons to become anions (-)   Cations and anions are pulled together by an electrical force

Ionic Bond Ionic Bond An electrical force of attraction between two oppositely charged ions   Remember atoms want 8 electrons (in general) in outer shell.

Ionic Bond Sodium wants to get rid of an electron… Na +1 Na Chlorine wants to get an electron… -1 Cl Positive Na is attracted to negative Cl…

Ionic Compounds Ionic Compound A chemical compound made with ionic bonds NaCl, CaF2, KI, LiCl, MgCl2 Ionic compounds typically consist of elements found on opposite sides of the periodic table Fig 17.1, p. 405 (metal ion and non-metal ion)

Ions Na+, Li+, K+ each lost 1 electron Mg2+, Ca2+, Ba2+ each lost 2 electrons O2-, S2- each gained 2 electrons F-, Cl-, Br- each gained 1 electron   Numeral 1 is optional in super script notation

Ionic compounds For all ionic compounds, positive and negative charges must balance   Determine the compound created between Fluorine and Calcium

Bring in another Fluorine! Ionic compounds Ca2+ F- Calcium needs to get rid of two electrons… Fluorine can only accept one electron… WHAT DO WE DO? Bring in another Fluorine! F-

Ionic compounds F- Ca2+ F- Calcium will bond with two Fluorine atoms…

Ionic compounds Aluminum Oxide +3 -2 Al O Al2O3

Electron Dot Structures Valence electrons are usually the only electrons used in chemical bonds, therefore; only valence electrons are shown in electron dot structures.  

Octet Rule Octet Rule In forming compounds, atoms tend to achieve the electron configuration of a noble gas. All noble gases, except He, have 8 valence electrons

Covalent Compounds Covalent bonds primarily involve non-metals. (these are elements with high electron affinities) Noble gases are exception - their outer electron shells are full, therefore, they have no electron affinity and do not form covalent bonds

Covalent Bonds Covalent bond A chemical bond where atoms are held together by their mutual attraction for the 2 electrons they share   Co- means sharing

Covalent Compounds In a covalent bond —there are no ions involved but the force holding the two atoms together is still electrical. Covalent compound- any group of atoms held together by one or more covalent bonds Ex: H2, O2, N2, CO2, H2O

Covalent Compounds Example: how / why bonds form between Hydrogen and Fluorine.

Covalent Compounds Oxygen has affinity for 2 extra electrons Oxygen finds 2 extra electrons when it bonds with 2 H atoms to form water. Each H atom gets access to an additional electron by bonding to O. Win-Win situation for O and H 

Electronegativity Electronegativity The ability of an atom to attract a bonding pair of electrons to itself when bonded to another atom.

This chemical compound is slightly polar Electronegativity - + This chemical compound is slightly polar

Polar Polar Compounds which have a different in charge on each end. Similar to North and South poles of a magnet.

Polar When the electronegativity of the 2 atoms differ, the bond is polar (ex: H-F) The greater the electronegativity, the greater the polarity See figure 17.23 page 414

Polarity To determine bond polarity, simply find the difference in the electronegativity of the bonded elements.

Polarity H-F H = 2.2 F = 3.98 Polarity 3.98-2.2 = 1.78 toward Fluorine

Dipole Dipole Difference of charge which creates two poles   F has a greater electro negativity Slightly (+) H – F Slightly (-) H :F

To Polar or not to polar? A Covalent bond is either polar or non-polar When the 2 atoms have the same electro negativity, no dipole is formed and the bond is non-polar (ex: H2, N2)

Covalent Compounds The number of covalent bonds an atom is able to form is equal to the number of additional electrons it can attract. There may be more than 1 covalent bond between 2 atoms

Double? Triple? Covalent bonds O2 two O atoms connected by 2 covalent bonds called a double covalent bond (4 electrons are shared, 2 from each atom)   O O N2 triple covalent bond, 6 electrons are shared, 3 from each atom N N

Single covalent bond dots Pairs of electrons not shared between atoms are called unshared pairs

Molecules Molecule A group of atoms held tightly together by covalent bonds   A diamond is a single large molecule made of many carbon atoms connected by numerous covalent bonds

Compare and Contrast Metallic bond network of positively charged metal ions are held together within a fluid of freely moving electrons Ionic bond each ion is attracted to all the oppositely charged ions surrounding it   Covalent bond involves only the two atoms sharing electrons (like a marriage)