Ionic and Metallic Bonding - Dr Collings' Science...
Transcript of Ionic and Metallic Bonding - Dr Collings' Science...
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Ionic and Metallic Bonding Section 7.1
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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
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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
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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
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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
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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
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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
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So what happens in group 2A
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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
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Pseudo Noble Gas Electron Configurations Copper can not replicate the electron configuration of a noble gas, but it can get close - how?
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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)
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Examples of cations and anions
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Section 7.2Ionic Bonds and Ionic Compounds
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What is this? How does it form?
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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
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How does this relate to the octet rule?
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It is not always that easyThe ratio of cations to anions is not always 1:1
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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
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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
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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
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They can form some awesome crystals
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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
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Section 7.3Bonding in metals
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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
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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
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Metallic bonds
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Metals vs Ionic Crystals
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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
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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
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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