Compounds Atoms are more stable if their outermost (valence) shell is full

most element atoms tend to lose or gain electrons elements are usually found in nature combined with another element

Compound - a pure substance formed when two or more different elements combine

Compounds are always formed from a specific combination of elements in a fixed ratio. Ex.: H2O, CO2

Compounds cannot be broken down into simpler compounds or elements by physical means heating, cooling, vaporizing, compressing do not change the molecules in compounds

Atoms, Elements, and Compounds (2.1 cont.)

Chemical BondsCovalent bondForce of attraction (chemical bond) that

forms when electrons are shared;forms between elements that want to gain electrons to fill their outer shells

MoleculeThe smallest piece of a compound in which the atoms are held together by covalent bonds

Covalent bonds can be broken by chemical reactions, not by physical processes (boiling, freezing, etc.).

For example, water molecules can be broken to form hydrogen and oxygen gases by an electrical current; the current provides energy for the reaction to happen.

Electricity

2H2O 2H2 + O2

Covalently Bonded Molecules

(diagrams and structural formulas)

Notice that covalent bonds can be:

•single (1 pair e- shared)

•double (2 pairs e- shared)

•triple (3 pairs e- shared).

Mnemonic for Covalent Bonds

H O N C

Some elements are more stable when they form molecules: H2, O2, N2, etc. Check the molecules and decide if they are elements, compounds, or a mixture.

Element or compound?

CO2

Element or compound?

C (Carbon)

Element or compound?

CO2H2O

Element or compound?

O2

H2O

Element or compound?

Element or compound?

O2H2

Element or compound?

H2O

Ions Atoms can form ions by gaining or losing electrons.

Metals tend to lose one or more electrons to form positive ions called cations.

– Cations are generally named by using the name of the parent atom.

Nonmetals tend to gain one or more electrons to form negative ions called anions.

Ions

• Anions are named by using the root of the atom name followed by the suffix –ide.

Ionic Bond Electrical attraction (weaker than covalent) between two oppositely

charged atoms (ions) or groups of atoms – electrons are donated/accepted

Forms between an atom that wants to lose and one that wants to gain electrons to have a full valence shell

Ionic compound name: Sodium chloride

Formulas of Ionic compounds

• Write the cation element symbol followed by the anion element symbol; use as many of each ion as needed to make the charges add up to zero.

Ionization / Dissociation - ionic Compounds in water ionic bond is broken (weak) because water molecules interact with the ions and

separate them.

Compound:Magnesium chloride

NaClSodium Chloride

Dissolved in H2O

Dissociation / ionization in water

Sodium and chloride ions in solid compound:Why do the ions stick together?

What can we infer about water molecules?

IonsIon Charges and the Periodic Table

• The ion that a particular atom will form can be predicted from the periodic table.

• Metals tend to lose electrons (oxidation) cations• Nonmetals tend to gain electrons (reduction) anions

Ion Charges and the Periodic Table

* Mark the charges for each column on your table. *

Can you tell if the pairs will form an ionic or covalent compound?

Calcium (Ca) and Chlorine (Cl) Ionic (CaCl2, calcium chloride) Oxygen (O) and Phosphorus (P) Covalent (PO4, phosphate) Sodium (Na) and Bromine (Br) Ionic (NaBr, sodium bromide) Magnesium (Mg) and Oxygen (O) Ionic (MgO, magnesium oxide) Nitrogen (N) and Oxygen (O) Covalent (NO3, nitrate)

What’s the pattern?

Electron “givers” and “takers” make ionic compounds.

Two “takers” make a covalent compound. Two “givers” don’t react.

Ion Functions in Organisms

All body fluids contain ions - Na+, Cl-, K+, Mg2+ , Ca2+

Ex. of functions: nerve impulses, muscle contraction, homeostatic balance (water/salt balance)

Chemical reactions happen in saline environment – would not work otherwise

*SUMMARY*

(chloride)