Periodic Trends and Bond Types

Take notes on the following:

• The trend as you go across and as you go down the periodic table, and why it occurs for:– Atomic size– Ionization Energy– Electronegativity

• Covalent and Polar Covalent bonds• Bonding in water molecules

Resources to use:

• Use your book,• This powerpoint,• The internet,• And each other for information.

• When you are done, begin the book homework

Atomic Size

Shielding Electrons• Shielding electrons are the electrons in the energy

levels between the nucleus and the valence electrons.

• They are called "shielding" electrons because they "shield" the valence electrons from the force of attraction exerted by the positive charge in the nucleus

• The shielding effect explains why valence shell electrons are more easily removed from the atom. The effect also explains atomic size. The more shielding, the further the valence shell can spread out and the bigger atoms will be.

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Electronegativity/Electron Affinity

• Ability of an atom to attract shared electrons to themselves

• The polarity of a bond depends on the difference in electronegativity between the two atoms

Electronegativity Trends

Types of Bonding

• Metallic • Ionic• Covalent– Polar covalent bonds (NOT symmetrical)• Electrons are not shared equally• If an element has a stronger ‘pull’ on electrons it obtains a

partial negative charge δ- and the other element will

have a partial positive charge δ+• This is called a dipole, ‘two poles’ one positive and one

negative

Orbitals in Polar and Nonpolar

What you think would happen.. What actually happens..

H Cl....

... .

Nonpolar Covalent Bonds

• If electrons are shared equally and there is no partial charge, then it is a nonpolar bond

• This happens when identical atoms are bonded together or if the difference in electronegativities is very low

• It may also happen if a molecule is completely SYMMETRICAL– Like with a Hydrogen molecule!

Nonpolar Molecules

• A molecule can have 2 polar bonds but if they are exact opposites of each other they can cancel out

Symmetrical, nonpolar

Types of BondsGenerally:

• Difference of 1.7+ = Ionic Bond• Difference 0.6 - 1.6 = Polar Covalent• Difference of 0.0 - 0.5 = Nonpolar Covalent

Bond Electronegativity Values Bond typeCl-Cl (3.0) (3.0)H-F (2.1) (4.0)H-S (2.1) (2.5)

Electronegativity Trends

Rules for Lewis Structures (for an element)

Lewis Structures are only used for covalent bonds. Steps:• Draw element symbol

– Think of there being 4 sides to the element’s symbol• Draw the number of valence electrons as dots

– Draw the dots starting at the right and move clockwise.– Fill on dots per side until you have used up all the valence electrons

Practice:• Lithium, Berrylium, Boron, Carbon, Nitrogen, OxygenTry on your own: • FluorineTrick:• A “line” represents two electrons.

Rules for Lewis Structures (for a molecule)

1. Count the total number of valence electrons for each atom found in the molecule

2. The element listed first is the one in the middle (exception: H)

3. Connect each element with a single line (a line represents two electrons)

4. Then add the rest of the electrons to the elements that need them by twos working form the outside in, until the octet rule is fulfilled for all elements (due rule for H!) and you have run out of electrons to add.

Practice Lewis Structures

• HCl• H2S

• CH4

• PH4

Additional Rules for Lewis

• **If the octet rule is fulfilled for one element but not another one that is connected to it, share another pair of electrons (using a line, this is a Double Bond)

• There is also such thing as a triple bond…• For ions, if they have a negative charge, add in

that many electrons and if it is positive take that many electrons away

Water• What type of bond occurs between O-H in a

water molecule?• Draw a water molecule, indicating the partial

charges on the correct atoms• Hydrogen bonding occurs in water molecules

Surface Tension

Common tent materials are somewhat rainproof in that the surface tension of water will bridge the pores in the finely woven material. But if you touch the tent material with your finger, you break the surface tension and the rain will drip through.

help the cleaning of clothes by lowering the surface tension of the water so that it more readily soaks into pores and soiled areas.