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

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

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

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

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

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

  4. 4.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.

  5. 5.Why is sodium chloride described by a formula unit mass rather than a molecular mass?

    • a) Its ions form a crystal rather than separate molecules
    • b) Its formula has more than one kind of atom in it
    • c) Its atoms share electrons instead of transferring them
    • d) It is too heavy for a molecular mass to be worked out

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

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

  7. 7.Hydrogen and oxygen combine to make water in the mass ratio 1 to 8. What mass of oxygen combines with 3 g of hydrogen, in g? Answer with a number.

  8. 8.Taking H as 1 u, N as 14 u and O as 16 u, what is the molecular mass of nitric acid, HNO3, in u? Answer with a number.

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

  1. 1. 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
  2. 2. d) It is the same before and after the reaction
  3. 3. c) A cation, carrying one positive charge
  4. 4. 74.5
  5. 5. a) Its ions form a crystal rather than separate molecules
  6. 6. c) An electrically neutral group of more than one atom that can exist on its own
  7. 7. 24
  8. 8. 63