LESSON 2.1 CHEMICAL SHORTHAND

CHEMICAL FORMULAS  A chemical formula of a compound tells how much of each element is present.  Examples: H ₂ O shows two atoms of hydrogen and one atom of oxygen

OXIDATION NUMBERS  The oxidation number is the number of electrons an atom gains, loses, or shares, when bonding with another atom. (This can also be called the valence number.)  Example: an atom of Na has a charge of ⁺ 1, so its oxidation number is also ⁺ 1.

 If an element has more than one oxidation number, you use a Roman Numeral in the name of the compound to indicate the oxidation number.  Example:  Copper(I) Cu ⁺  Copper(II) Cu ⁺ ²

BINARY COMPOUNDS AA compound made up of two elements is a binary compound. BB inary compounds are formed by two elements combining to become stable. WW hen compounds are formed, chemical formulas are used to represent reactions.

A chemical formulas consists of symbols with subscripts.  Symbols represent the elements present.  Subscripts represent the number of atoms present of a particular compound. H ₂ SO ₄ This is Sulfuric Acid. This means that in this compound there are: 2 hydrogen atoms 1 sulfur atom 4 oxygen atoms

LET’S PRACTICE Give the name and number of atoms present in the following compounds. 1.H ₂ O Hydrogen oxide (Water) 2.HNO ₃ Nitric acid 3.NaCl Sodium chloride 4.K ₂ SO ₄ Potassium sulfate 5.Al ₂ O ₃ Aluminum oxide

STEPS TO WRITE A BINARY FORMULA  Write the symbol for the element with the positive oxidation number.  Write the symbol for the element with the negative oxidation number.  Ca + F

 Put in oxidation numbers. Ca ⁺ ² + F ⁻  Criss –cross the valence number to lower right corner of each element.  Drop the signs.*Do not write a “1”. Ca ⁺ ² + F ⁻ Ca ( ₁ ) F ₂  Rewrite the formula with subscripts.  Ca ⁺ ² + F ⁻ CaF ₂

When you have oxidation numbers that are the same but have opposite signs the numbers will cancel each other out. No subscripts need to be written. EX: Na ⁺ + Cl ⁻ Na ⁺ + Cl ⁻ NaCl Ba ⁺ ² + S ⁻ ² Ba ⁺ ² + S ⁻ ² BaS

LET’S PRACTICE  Lithium + oxygen Li + O Li ⁺ + O ⁻ ² Li ⁺ + O ⁻ ² Li ₂ O (1) Li + O Li ₂ O

MORE PRACTICE MM agnesium + nitrogen LL ithium + sulfur CC alcium + phosphorus PP otassium + chlorine BB arium + oxide

NAMING BINARY COMPOUNDS  Write the name of the first element.  Write the root of the name of the second element.  Add the suffix –ide to the root.  Example: Na ₂ S sodium sulfide  MgO magnesium oxide

NAMING COMPOUNDS : 1. Magnesium + Nitrogen 2. Lithium + Sulfur 3. Calcium + Phosphorus 4. Potassium + Chlorine 5. Barium + Oxygen

POLYATOMIC IONS  A polyatomic ion is a group of positively or negatively charged covalently bonded atoms.  Naming: say the name of the element and polyatomic ion  Example: K ₂ SO ₄ is potassium sulfate

WRITING FORMULAS FOR POLYATOMIC IONS  Follow the rules for binary compounds  Add parentheses around the polyatomic ion formula if there is more than one ion needed  Example: K ₂ CO ₃ -one carbonate ion  Si(CO ₃ ) ₂ -two carbonate ions