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LESSON 2.1

LESSON 2.1. CHEMICAL SHORTHAND. CHEMICAL FORMULAS. A chemical formula of a compound tells how much of each element is present. Examples : H ₂O shows two atoms of hydrogen and one atom of oxygen. OXIDATION NUMBERS.

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LESSON 2.1

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  1. LESSON 2.1 CHEMICAL SHORTHAND

  2. CHEMICAL FORMULAS • A chemical formula of a compound tells how much of each element is present. • Examples: H₂O shows two atoms of hydrogen and one atom of oxygen

  3. 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.

  4. 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

  5. BINARY COMPOUNDS • A compound made up of two elements is a binary compound. • Binary compounds are formed by two elements combining to become stable. • When compounds are formed, chemical formulas are used to represent reactions.

  6. A chemical formulas consists of • symbols with subscripts. • Symbols represent the elements present. • Subscripts represent the number of atoms present of a particular compound. H₂SO₄ This is Sulfuric Acid. This means that in this compound there are: 2 hydrogen atoms 1 sulfur atom 4 oxygen atoms

  7. LET’S PRACTICE Give the name and number of atoms present in the following compounds. • H₂O Hydrogen oxide (Water) • HNO₃ Nitric acid • NaCl Sodium chloride • K₂SO₄ Potassium sulfate • Al₂O₃ Aluminum oxide

  8. 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

  9. 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)F₂ • Rewrite the formula with subscripts. • Ca⁺² + F⁻ CaF₂

  10. 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

  11. LET’S PRACTICE • Lithium + oxygen Li + O Li⁺+ O ⁻² Li⁺ + O ⁻²Li₂O(1) Li + O Li₂O

  12. MORE PRACTICE • Magnesium + nitrogen • Lithium + sulfur • Calcium + phosphorus • Potassium + chlorine • Barium + oxide

  13. 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: Na₂S sodium sulfide • MgO magnesium oxide

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

  15. 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: K₂SO₄ is potassium sulfate

  16. 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: K₂CO₃ -one carbonate ion • Si(CO₃)₂ -two carbonate ions

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