ionic and metallic bonding - dr collings' science...

Ionic and Metallic Bonding Section 7.1

What is a Valence Electron? ● Valence electrons are the electrons in the highest occupied energy level of an

element’s atoms ● The valence electrons largely determine the chemical properties of an

element

How do you determine the number of valence electrons? ● The number of valence electrons in an atom of an element is related to the element's

group number in the periodic table ● For a representative element, the number of electrons is the same as the group number

○ Group 1A elements have one valence electrons○ Group 4A elements have four valence electrons ○ Helium is the only exception: it is in group 8A but Helium only has 2 valence electrons

How can this information be displayed easily? ● Electron dot diagrams display the valence electrons as dots● This is because only valence electrons are used in chemical bonds● All of the elements in a group have the same number of electron dots in their

structures

What is the Octet rule? ● Atoms of each noble gas, except helium, have eight

valence electrons . ● Gilbert Lewis used this fact to explain what atoms form

certain types of ions and molecules ● Electron configuration of noble gasses: ns2np6● This set of eight is referred to as an octet ● Atoms tend to form compounds in a way that allows them

to have eight electrons in their highest occupied energy level

● Atoms of metals tend to lose their valence electrons, leaving an octet in the next lowest energy level.

● Nonmetals can gain electrons to form an octet

Formation of cations ● A cation forms when an atom loses one or more valence electrons● For metals the cations name is the same as element’s name, but is written

with a + sign - Na+

● Although names are the same, properties are very different● Sodium metal is highly explosive, whereas Sodium cations are quite non

reactive

https://www.youtube.com/watch?v=-3zipNGwqjE

Formation of cations continued…..

● Most cations are caused by the loss of 1 - 3 valence electrons

● These valence electrons are easily removed

● End result is electron configuration the same as noble gas

So what happens in group 2A

And Transition metals? ● Transition metals can he highly variable ● Some do not form noble gas electron configurations - exceptions to the octet rule● How many electrons would copper have to lose to be like Aron? How many

electron would copper have to gain to be like Krypton? ● Ions with charges of 3 or more are not common

Pseudo Noble Gas Electron Configurations Copper can not replicate the electron configuration of a noble gas, but it can get close - how?

What about Anions? ● Anions form when there are more electrons than protons in an atom● The name of an anion is NOT the same as the element name● The name of anions end in -ide ● Chlorine forms chloride ions, an example of a halide ion

○ Halide ions form when a halogen atom gains a valence electron

● Oxygen forms oxide ions

Chlorine Chloride (same electron structure as argon)

Examples of cations and anions

Section 7.2Ionic Bonds and Ionic Compounds

What is this? How does it form?

What is special about Salt? ● Salt is an ionic compound ● All ionic compounds are made of

cations and anions, but are electrically neutral

○ Positive charge = equals negative charge

● Cations and anions are held together by electrostatic force - referred to as an ionic bond

How does this relate to the octet rule?

It is not always that easyThe ratio of cations to anions is not always 1:1

How does this relate to chemical formulas? ● A chemical formula shows the number of atoms

of each element in the smallest representative unit of a substance

○ NaCl : Chemical Formula for Sodium Chloride○ MgCl2 : Chemical Formula for Magnesium Chloride

● Ionic compounds do not exist as single units, but instead are arranged in repeating patterns, so the chemical formula refers to a ratio known as a formula unit

○ You do not get simply one sodium bonded to one chloride ion, instead you get a lattice

● A formula unit is the lowest whole number ratio of ions in an ionic compound

○ Ionic charges are not shown in the formula unit

Practice problems - section 7.2 Use electron dot structures to predict the formulas of the ionic compounds produced from the following elements:

1. Potassium and Oxygen2. Magnesium and Nitrogen3. Potassium and Iodine4. Lithium and Fluorine5. Aluminum and Oxygen6. Aluminum and Sulfur

What properties do Ionic compounds share? ● Most ionic compounds are crystalline at

room temperature ● Ions are arranged in repeating 3-D

patterns ○ In Sodium Chloride, each chloride ion is

surrounded by six sodium ions○ Each ion is attracted strongly to its neighbor

● The large attractive forces result in a very stable crystal structure

○ This means a lot of energy is required to convert it into a liquid form

○ Melting point of NaCl is 800 C

● High melting points are common for ionic compounds

They can form some awesome crystals

More properties ● Ionic compounds can conduct an electric current when melted or dissolved in

water ● The crystal structure breaks down and ions are free to move● Cations will move to negative electrode, anions will move to positive electrode,

allowing current to move

Section 7.3Bonding in metals

What are some of the key properties of metals? ● Metals are in general:

○ Tough○ Malleable○ Ductile○ Corrosion-resistant○ Melt at high temperatures

● All of these are due to the way that metal ions form bonds with one another

Why do metals have these properties?● It is all due to how metal ions are bonded together● Metals are NOT made up of neutral atoms● Instead, metals consist of closely packed cations and loosely held valence

electrons ● The valence electrons are free to move from one part of the metal to another ● A metallic bond is the force of attraction between the free floating valence

electrons and positively charged metal ions ● These free floating electrons explain why metals are

○ Ductile (can be drawn into wires)○ Malleable (can be hammered into sheets)○ Good conductors of electric current

Metallic bonds

Metals vs Ionic Crystals

Are metals crystalline? ● Yes! ● Atoms in metals are

arranged in compact and orderly patterns

● Chromium - each atom has 8 neighbors

● Gold - Each atom has 12 neighbors

● Zinc - Again each atom has 12 neighbors, but in a different arrangement

What is an alloy? ● Most metals that we use on a daily basis

are alloys ● Alloys are mixture of two or more

elements, at least one of which is a metal○ Brass - Copper and Zinc

● Properties are often superior to their component elements

● Steels are the most important alloys used today

○ Most are made of Iron, carbon and small amounts of other metals

● Important properties that can be changed are corrosion resistance, ductility, hardness and toughness

How are alloys made? ● If a similar size, the new

element simply replaces the element in the lattice - a substitutional alloy

● If the atomic sizes are different, the smaller atoms can fit in the spaces between the larger atoms

● This is common with steel - carbon occupies space between Iron atoms