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Class 9 Science — How atoms combine, and what a molecule weighs

Use the laws of conservation of mass and of constant proportions, tell a covalent bond from an ionic one, write a formula by crossing over valencies, and add up a molecular or formula unit mass.

Name: ________________________

Before you start

A sodium atom beside a chlorine atom, with each shell named.

1234
  1. 1. Sodium's inner shells, holding 2 then 8 electrons
  2. 2. Sodium's valence shell, holding its single electron
  3. 3. Chlorine's inner shells, holding 2 then 8 electrons
  4. 4. Chlorine's valence shell, holding 7 electrons

Sodium has one electron more than a full inner arrangement; chlorine is one short of eight. Hand the electron across and both are satisfied — that is an ionic bond in one move.

  1. 1.Which of these describes a molecule, as the chapter defines it?

    • a) The simplest whole-number ratio of ions in a crystal
    • b) A single atom of any element
    • c) An electrically neutral group of more than one atom that can exist on its own
    • d) A charged particle formed when an atom gains an electron

  2. 2.Sort each substance by the kind of bond holding it together.

    Groups: Held together by sharing electrons · Held together by transferring electrons

    • Magnesium oxide
    • Sodium chloride
    • Hydrogen molecule
    • Chlorine molecule

  3. 3.Sodium and chlorine combine in the mass ratio 23 to 35.5. If 46 g of sodium reacts completely, what mass of chlorine is needed, in g? Answer with a number.

  4. 4.Taking C as 12 u and H as 1 u, what is the molecular mass of methane, CH4, in u? Answer with a number.

  5. 5.Taking K as 39 u and Cl as 35.5 u, what is the formula unit mass of potassium chloride, KCl, in u? Answer with a number.

  6. 6.A sodium atom, with 11 protons and 11 electrons, loses its single valence electron. What is it now?

    • a) A cation, carrying one positive charge
    • b) A different element, since its proton count changed
    • c) Still a neutral atom, since electrons carry no charge
    • d) An anion, carrying one negative charge

  7. 7.Put the steps for writing a covalent compound's formula in order, starting from Write the symbols of the elements in the compound.

    • Cross the valencies over to the other symbol
    • Write the symbols of the elements in the compound
    • Write each element's valency below its symbol
    • Write them as subscripts, leaving out a subscript of one

  8. 8.A reaction is carried out in a sealed flask on a balance. What does the Law of Conservation of Mass predict about the reading?

    • a) It is the same before and after the reaction
    • b) It falls, because some matter is used up
    • c) It cannot be predicted without knowing the products
    • d) It rises, because new substances have been made

Answer key — Class 9 Science — How atoms combine, and what a molecule weighs

  1. 1. c) An electrically neutral group of more than one atom that can exist on its own
  2. 2. Hydrogen molecule → Held together by sharing electrons; Chlorine molecule → Held together by sharing electrons; Sodium chloride → Held together by transferring electrons; Magnesium oxide → Held together by transferring electrons
  3. 3. 71
  4. 4. 16
  5. 5. 74.5
  6. 6. a) A cation, carrying one positive charge
  7. 7. 1. Write the symbols of the elements in the compound 2. Write each element's valency below its symbol 3. Cross the valencies over to the other symbol 4. Write them as subscripts, leaving out a subscript of one
  8. 8. a) It is the same before and after the reaction