Ch.7 chem by Izabela Anic

Valence Electrons

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the electrons in the highest occupied energy level of an element's atom

*the number of valence electrons largely determines the chemical properties of the element

#valence electrons = group # (representative elements)

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Valence electrons are usually the only ones involved in chemical bonds

Electron dot structures: diagrams that show valence electrons in the atoms of elements as dots

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Gilbert Lewis

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in 1916 he developed the octet rule: states that in forming compounds, atoms tend to achieve the electron configuration of a noble gas

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octet: set of 8

noble gas
have 8 electrons in highest occupied energy level

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-atoms of metals tend to lose their valence electrons, leaving complete octet in the next-lowest energy level

atoms of some nonmetals tend to gain electrons or share electrons with another nonmetal atom to achieve a complete octet

transition metal cations

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iron may lose 2 electrons
Fe^2+
or could lose 3: Fe^3+

Silver - Ag: 1s^2 2s^2 2p^6 3s2 3d^10 4s^2 4p^6 4d^10 5s^1
-Would have ti lose 11 electrons to get to Kr
-Would have to gain 7 electrons to get to Xe
-Exceptions to octet rule
-Cations of 3 or greater are uncommon

Pseudo Noble-gas configuration

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Ag^+ (lose 5s^1)
configuration ends in 4s^2 4p^6 4d^10

with 18 electrons relatively favorable

Pseudo noble-gas configurations

ionic compound

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composed of ions, electrically neutral

ionic bonds

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electrostatic forces that hold ions together

chemical formula

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shows the number of atoms of each elements in the smallest representative unit of the compound

NaCl - does not represent a single unit

formula unit

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refers to chemical formula

lowest whole number ration of ions in an ionic compound

NaCl - 1:1
AlBr3 - 1:3

Ionic Bonding

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cations and anions are held together by opposite charges (+and-)

Ionic compounds are called salts

the bond is formed through the transfer of electrons (lose and gain)

electrons are transferred to achieve noble gas configuration

metal+nonmetal

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Properties of Ionic Compunds

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1. crystalline solids are room temperature

2. component ions are arranged in repeating 3D patterns

2.

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ionic compound have high melting points

Coordination Number

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# of ions of opposite charge that surround the ion in the crystal

3.

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can conduct an electric current when melted or dissolved in water

(charges are allowed to move)

ionic compunds

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metal & nonmetal

Metallic Bonds:

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how metal atoms are held together in a solid

-hold on to their valence electrons very weakly

Sea-of-electrons=properties of metals

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-good conductors of electricity (because electrons flow freely)

-ductile-drawn into wires

-malleable-hammered or pressed into shape

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ductility and malleability of metals is explained by mobility of valence electrons

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insulate metal cations from one another easily slide past each other under pressure
(oil in ball bearing)

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Metals are also crystalline structures

-metal atoms are arranged in very compact and orderly patterns

*for spheres of identical size (such as metal atoms), three arrangements are possible

-body centered cubic
-face centered cubic
-hexagonal close packed

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In a body- centered cubic structure, every atom has 8 neighbors (except those on the edge).

The metallic elements sodium, potassium, iron, chromium, and tungsten crystallize in a body centered cubic pattern

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In a face centered cubic arrangement, every atom has 12 neighbors. Among the metals that form a face-centered cubic arrangement are copper, silver, gold, aluminum, and lead.

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In a hexagonal close-packed arrangement, every atom also has 12 neighbors. However because of its shape, the pattern is different from the face-centered cubic arrangement. Metals that have a hexagonal close-packed structure include magnesium, zinc, and aluminum.

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Very few objects are made out of single kind of metal, instead most of the metals we encounter are

ALLOYS:
mixture of 2 or more elements, at least 1 of which is orbital
*their properties are often superior to those of their component atoms

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Brass: copper and zinc
Bronze: copper and Tin
Sterling Silver: 92.5% silver and 7.5% copper: more durable than just silver, still soft enough for tableware and jewelry

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Most important alloy that we use in our life are steels
-corrosion resistant, ductility, hardness, & toughness
Steel: iron, carbon, and a mixture of other metals

Substitutional Alloy

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atoms in the components are about the same size (they replace each other in crystal)

Interstitial Alloy
atomic sized are quite different
(smaller atoms fill in spaces between the larger atoms)
ex:steel with carbon & iron

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Only noble gas elements like He, exist as uncombined atoms they are monatomic, they exist as single atoms

ionic compound

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generally crystalline solids with high melting points ionic bonds form when the combining atoms give up or accepts electrons

other electrons have attractions that cannot be explained by ionic bonding, they do not involve a transfer of electrons

Covalent bond

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atoms share electrons, there is a 'tug of war' for electrons bonding the atoms together

Molecule

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neutral group of atoms joined together by covalent bonds

other examples

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hydrogen, nitrogen, and the halogens

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oxygen gas consists of oxygen molecules each oxygen molecule consists of 2 covalently bonded atoms diatomic molecule with 2 atoms

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molecules can also be made of atoms of different elements

molecular compound: compound composed of molecules

example: H2O
molecules in water are all the same - each water molecule is a tightly bound unit of 2 hydrogen atoms and 1 oxygen

Molecular Formula

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chemical formula of a molecular compound; it shows how many atoms of each element a substance contains

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molecular formula reflects the actual number of atoms in each molecule, not necessarily the lowest whole-number ratios

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O2
a molecular formula
(doesn't have to have different elements

molecular formula does not tell us about structure

Molecular Structure

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arrangement of the atoms within a molecule

Molecular Compounds vs. Ionic Compounds

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molecule formula unit (lowest whole-number ratio of ions)

representative unit

molecule: made up of 2 or more atoms that act as a unit

ionic compounds are continuous arranged positively and negatively charged ions in repeated 3D patterns

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molecular compounds tend to have lower melting and boiling points than ionic compound

*many are gas or liquids at room temperature
*most are composed of atoms of 2 or more nonmetals
vs. ionic compound which has a metal combined with a nonmetal

pg.225

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In covalent bonds, electron sharing usually occurs so that atoms attain: the electron configuration of noble gases

nonmetal and metalloids in Group 4A, 5A, 6A, and 7A

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single covalent bond

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2 atoms held together by sharing one pair of electrons

structural formula

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represents covalent bonds as dashes & shows the arrangement of covalently bonded atoms

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the halogens also form single covalent bonds in their diatomic (molecule with 2 atoms) molecules

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a pair of valence electrons that is not shared between atoms is called an unshared pair
lone pair
non bonding pair

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each F atom has 3 unpaired shared electrons

Water (molecule of compound)

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electron dot structure for ammonia NH3

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Atoms form double or triple covalent bonds if they can attain a noble as structure by sharing 2 pairs or 3 pairs of electrons.

triatomic molecule (3 atoms)

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N2 is an example of an element whose molecules contain triple covalent bonds

Exceptions to Octet Rule

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cannot be satisfied in molecules whose total number of valence electrons is an odd number

a few atoms, especially phosphorus and sulfur sometimes expand the octet rule to 19 or 12 electrons

coordinate covalent bond

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covalent bond in which one atom contributes both bonding electrons

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in a coordinate covalent bond, the shared pair comes from one of bonding atoms

Polyatomic ion

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a tightly bound group of atoms that has a positive or negative charge and it behaves as a unit

Bond dissociation energy

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energy required to break the bond between 2 covalently bonded atom

kilo Joule<--kJ/mol-->mole

high dissociation energy = strong covalent bond

resonance structures

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s=structures that occur when it is possible to draw 2 or more electron dot structures that have the same number of electron pairs for a molecule or ion

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