Compounds

• What does a chemical formula represent?

  Elements and their proportions in a compound
• How does the chemical formula H₂O differ from H₂O₂?

  H₂O has two H and one O; H₂O₂ has two O
• Why are chemical symbols important in writing formulae?

  They simplify writing and understanding chemical formulae
• What is the chemical symbol for Hydrogen?

  H
• What is the valency of oxygen (O)?

  -2
• What is the chemical symbol for Oxygen?

  O
• What is valency in chemistry?

  Element's ability to combine with others
• What is the first step in the criss-cross method?

  Write the symbols of the elements
• What is the purpose of simplifying subscripts in a chemical formula?

  To express the formula in simplest terms
• What is the procedure to find the valency of elements?

  1. Look at the group number on the periodic table (without transition metals).
  2. Group 1 elements have valency +1.
  3. Group 2 elements have valency +2, and so on.
• What does "criss-cross" refer to in the criss-cross method?

  Transferring valency numbers as subscripts
• What is the process of simplifying a chemical formula?

  • Identify the subscripts in the formula
  • Determine the greatest common factor
  • Divide each subscript by the greatest common factor
  • Write the simplified formula
• Why is valency important in writing chemical formulae?

  It ensures charges balance in compounds
• What does the simplified formula MgO indicate about the ratio of magnesium to oxygen?

  It indicates a 1:1 ratio
• How does the criss-cross method apply to magnesium oxide (Mg and O)?

  • Mg has a valency of +2
  • O has a valency of -2
  • Criss-crossing gives Mg₋₂O₊₂
  • Simplifies to MgO
• What is the criss-cross method used for?

  To balance charges in chemical formulae
• What is the chemical formula for water using symbols?

  H₂O
• How would you simplify the formula Mg₂O₄?

  By dividing subscripts by 2
• What do you write beside the symbols in the criss-cross method?

  Their valencies
• What are the steps in the criss-cross method?

  1. Write the symbols of the elements.
  2. Write their valencies above/beside the symbols.
  3. Criss-cross the numbers as subscripts to each element.
  4. Simplify if necessary.
• What do the small numbers in a chemical formula signify?

  How many atoms of each element are present
• How does valency determine compound formation?

  It indicates how many atoms combine
• What is the greatest common factor used for in simplifying chemical formulas?

  It is used to divide the subscripts
• What does the chemical formula H₂O indicate?

  Two hydrogen atoms and one oxygen atom
• What are chemical symbols used for?

  To represent elements in chemical formulae
• How are most chemical symbols derived?

  From the first one or two letters of the element's name
• What should you do after criss-crossing the numbers in the criss-cross method?

  Simplify if necessary
• What is the valency of sodium (Na)?

  +1