Bonding

Valence Electrons Neon Argon Radon The electrons on the outside edge of the atom This is where the action is- where bonding takes place Atoms have no more than 8 valence electrons Neon 1s22s22p6 Argon 1s22s22p6 3s23p6 Radon [Xe]6s24f 145d106p6

Maximum Number of Electrons Max number of Valence Electrons Energy Level (Shell) Maximum Number of Electrons Max number of Valence Electrons 1 2 8 3 18 4 32 5 50 6 72 7 98

Electron-dot diagrams can be used to give the number of valence electrons The number of valence electrons is equal to the element’ group number or # -10 Group 1 Group 2 Group 13 Group 14 Group 15 Group 17                                                                   

iodine phosphorus gallium argon Write the electron-dot symbols for the following elements: iodine phosphorus gallium argon

The Octet Rule: Atoms will combine to form compounds in order to reach eight electrons in their outer energy level. Atoms with less than 4 electrons tend to lose electrons. Atoms with more than 4 electrons tend to gain electrons. Some atoms share electrons

Types of Chemical Bonds Ionic bond - a transfer of one or more electrons from one atom to another Forms attractions due to the opposite charges of the atoms Covalent bond - attractive force due to the sharing of electrons between atoms Some bonds have characteristics of both types and not easily identified as one or the other

Why do compounds form? Atoms are trying to get 8 valence electrons How do compounds form? By ionic or covalent bonding How can you tell if a compound is ionic or covalent? By the types of elements in the compound

Ionic compounds form between metals and nonmetals

Covalent compounds form between 2 nonmetals

Note Question 4: Indicate whether a bond between the following would be 1) Ionic 2) covalent ____ A. sodium and oxygen ____ B. nitrogen and oxygen ____ C. phosphorus and chlorine ____ D. calcium and sulfur ____ E. chlorine and bromine

Ions Atoms with extra electrons or missing electrons Extra electrons give an ion a negative charge Missing electrons give an ion a positive charge If they have to choose, atoms would rather be stable than neutral.

How Does This Happen? Some atoms have a few too many electrons Some atoms only need a few electrons

What do you do if you are a sodium (Na) atom? Go look for an atom that wants the extra electron!

Now you are a sodium ion With 10 electrons Just like neon, with 8 valence electrons But you’re not neon You’re not a plain sodium atom But you’re happy!

You are now quite attractive... Electrically.

Ionic Bonding Attraction that holds oppositely charged ions together. Formed by the transferring of electrons

Ions from Metal Ions In ionic compounds, metals in 1, 2, 13 and 3-12 lose electrons to nonmetals Metals lose electrons to achieve the octet arrangement in the next lowest energy level The names of metal ions are the same as the elements Metal ionic charge: +1, +2, +3, or +4

Ions from Nonmetals In ionic compounds, nonmetals in 15, 16, and 17 gain electrons from metals Nonmetal add electrons to achieve the octet arrangement Nonmetal ionic charge: -3, -2, or -1 The names of nonmetal ions end in -ide

Formation of Sodium Ion Sodium atom Sodium ion Na   Na + + e 2-8-1 2-8 ( = Ne) 11 p+ 11 p+ 11 e- 10 e- 0 1+

Formation of Magnesium Ion Magnesium atom Magnesium ion Mg:  Mg2+ + 2e 2-8-2 2-8 (=Ne) 12 p+ 12 p+ 12 e- 10 e- 0 2+

Fluoride Ion unpaired electron octet     1 - F  + e F     2-7 2-8 (= Ne) 9 p+ 9 p+ 9 e- 10 e- 0 1 - ionic charge

Charges on Ions in Group 1-17 +1 +2 +3 -3 -2 -1

Ions A. Number of valence electrons in aluminum 1) 1 e- 2) 2 e- 3) 3 e- B. Change in electrons for octet 1) lose 3e- 2) gain 3 e- 3) gain 5 e- C. Ionic charge of aluminum 1) 3- 2) 5- 3) 3+

Learning Check B 3 Give the ionic charge for each of the following: A. 12 p+ and 10 e- 1) 0 2) 2+ 3) 2- B. 50p+ and 46 e- 1) 2+ 2) 4+ 3) 4- C. 15 p+ and 18e- 2) 3+ 2) 3- 3) 5-

What If No One Will Give Up An Electron? Atoms with less than 8 valence electrons can move close to each other and share their electrons The electrons spend their time around both atoms. And they lived happily ever after!

Ionic Bonding Ionic bond – attraction between a cation and an anion Attraction of Na+ with Cl- forms NaCl ion pairs that aggregate into a crystal lattice

Features of Ionic Bonding • Ion formation takes place by electron transfer • The ions are held together by the electrostatic force of the opposite charges • Reactions between metals and nonmetals (representative elements tend to be ionic)

Ionic Compound Properties brittle high melting points conduct electricity in molten state or when dissolved in water

Covalent Bonding Let’s look at the formation of H2: H + H H2 Each hydrogen has one electron in its valance shell Both hydrogen atoms have an equal tendency to gain or lose electrons Electron transfer from one H to another usually will not occur under normal conditions (No one will let go!)

Instead, each atom attains a noble gas configuration by sharing electrons

Covalent bonds form between atoms with similar tendencies to gain or lose electrons Compounds containing covalent bonds are called covalent compounds or molecules The diatomic elements have pure covalent bonds (totally equal sharing)

The Polar Covalent Bond Ionic bonding involves the transfer of electrons Covalent bonding involves the sharing of electrons Polar covalent bonding - bonds made up of unequally shared electron pairs

Polar Covalent Bonding The electrons spend more time with fluorine This sets up a polar covalent bond A pure covalent bond can only occur when both atoms are identical

Electronegativity Electronegativity - a measure of the ability of an atom to attract electrons in a chemical bond Elements with high electronegativity have a greater ability to attract electrons than do elements with low electronegativity The difference in electronegativity determines the extent of bond polarity

Electronegativity and Bond Polarity The most electronegative elements are found in the upper right corner of the periodic table The least electronegative elements are found in the lower left corner of the periodic table

Know the trend! Which is more electronegative Boron or gallium? Calcium or zinc?

Table of Electronegativities (on your worksheet)

Electronegativity and Bond Polarity The greater the difference in electronegativity between two atoms, the greater the polarity of their bond Which would be more polar, a H-F bond or H-Cl bond? H-F … 4.0 - 2.1 = 1.9 H-Cl … 3.0 - 2.1 = 0.9 The HF bond is more polar than the HCl bond

Look at your handout There is a chart in the lower right hand corner of the back This gives the break-off points for deciding what type of bond you have between two atoms If the difference is 0, Pure covalent If the difference is ≤0.4, Nonpolar covalent If the difference is 0.5-1.7, Polar covalent If the difference is >1.7, Ionic

Using the table of electronegativites, classify the following bonds as ionic, polar covalent or nonpolar covalent. BCl3 CS2

There is no sharp distinction between bonding types. The positive end (or pole) in a polar bond is represented δ+ and the negative pole δ-.

Dipole Moments Consider HF: – The difference in electronegativity leads to a polar bond. – There is more electron density on F than on H. – Since there are two different “ends” of the molecule, we call HF a dipole.

We can show this in two ways:

Some covalent molecules will have polar bonds, but the molecules themselves will be nonpolar The polarity of the bonds cancel each other out

Water Water is a very polar molecule This accounts for some of water’s unusual properties.

Water dissolves ionic and polar substances

Water molecules hold on tightly to each other This accounts for properties like surface tension

Water has a high heat capacity Water is a liquid at room temperature, even though other compounds in its size range are gases