unit 7 section 1 notes
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Unit 7 Section 1 Notes. Compounds and Molecules. What are Compounds?. A compound is made of 2 or more elements joined together. When elements combine to form a compound , the compound has very different properties than the elements that made it. Example of a Compound. - PowerPoint PPT PresentationTRANSCRIPT
Unit 7 Section 1 NotesUnit 7 Section 1 Notes
Compounds and Compounds and MoleculesMolecules
What are Compounds?What are Compounds?
A compound is made of 2 or more elements joined together.
When elements combine to form a compound, the compound has very different properties than the elements that made it.
Example of a CompoundExample of a Compound
Sodium, a metal, combines with chlorine, a gas, to make table salt, a small white crystal.
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Chemical BondsChemical Bonds
The forces that hold atoms or ions together in a compound are called chemical bonds.
Chemical bonds are the difference between compounds and mixtures– A compound is formed through
chemical bonding.
Chemical Bonds vs. MixturesChemical Bonds vs. Mixtures
Mixtures are made of different substances just placed together– Each substance in the mixture keeps its
own properties– Example: Mixing yellow and blue paint
gives green paint, but the properties of the paint have not changed.
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When is a compound formed?When is a compound formed?
A compound is formed when chemical bonds are broken and atoms are rearranged.
Chemical FormulaChemical Formula
A compound ALWAYS has the same chemical formula.
A chemical formula shows the types and numbers of atoms or ions making up the simplest unit of the compound.
Examples of Chemical FormulasExamples of Chemical Formulas
H2O: Water: 2 Hydrogen atoms; 1 Oxygen atom
C12H22O11: Sugar: 12 Carbon atoms; 22 Hydrogen atoms; 11 Oxygen atoms
NaCl: Table Salt: 1 Sodium atom; 1 Chlorine atom
Note: When there is no subscript, it is understood to be “1”.
CompoundsCompounds
Compounds are ALWAYS made of the same elements in the same proportion!!!!!!!
Chemical StructureChemical Structure
Chemical Structure is the way the compound’s atoms are bonded to make the compound.
Just as the structure of buildings can be represented by blueprints, the structure of chemical compounds can be shown by various models. Different models show different aspects of compounds.
Ball-and-Stick ModelBall-and-Stick Model
This model helps you see a compound’s structure by showing how the atoms or ions are arranged in a compound. – Atoms: represented by balls– Bonds: represented by sticks
Ball-and-Stick model for water.
104.45˚
Arrangement of CompoundsArrangement of Compounds
Two words are used to tell how atoms or ions are positioned in a compound:– Bond length: the distance between the
nuclei of two bonded atoms.– Bond angle: the angle formed by two bonds
to the same atom. The Ball-and-Stick model helps you see
bond length and bond angle.
Structural FormulaStructural Formula
Structural formulas can also show the structures of compound
In a structural formula, atoms are represented by chemical symbols.
O
H H
Space-Filling ModelsSpace-Filling Models
Space-filling model shows how much space atoms take up– The different sizes of the atoms can be
seen, but not bond lengths.
Bonds are FlexibleBonds are Flexible
Chemists often use a solid bar to show a bond between two atoms, but bonds are actually flexible, like springs.
Atoms can actually move back and forth a little bit.
Properties of CompoundsProperties of Compounds
What determines many of a compound’s properties?– The atoms present in a compound– The way the atoms are arranged (chemical
structure)
How Does Structure Affect How Does Structure Affect Properties?Properties?
Compounds with network crystals are strong solids– Quartz is made of silicon dioxide, SiO2
Every Si atom is bonded to 4 oxygen atoms; all bond angles are the same at 109.5˚, which holds the atoms together in a strong, rigid structure
The bonds that hold quartz together is very strong and takes a lot of energy to break the bonds, which is why the melting and boiling points are so high
How does Structure Affect How does Structure Affect Properties?Properties?
Some networks are made of bonded ions, which have very strong attractions between the positively and negatively charged ions– The strong attraction requires a lot of energy to
break the bonds, which is why these network structures have high melting and boiling points.
– Examples: table salt, NaCl; magnesium oxide, MgO
How Does Structure Affect How Does Structure Affect Properties?Properties?
Some compounds are made of molecules– A molecule of sugar is made between bonds
of carbon, hydrogen, and oxygen and the attraction between the molecules is much weaker than an ionic bond.
– Compounds made of molecules do not have a strong attraction holding atoms together, so they have a lower melting and boiling point than compounds with ionic bonds.
Strength of Attraction Between Strength of Attraction Between MoleculesMolecules
The weaker the force of attraction between molecules, the easier the substance will be to break apart
Molecules of gases have weak attractions to one another, so they spread out and take up a lot of space.