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Year 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.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) It is too heavy for a molecular mass to be worked out
    • c) Its atoms share electrons instead of transferring them
    • d) Its formula has more than one kind of atom in it

  2. 2.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.

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

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

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

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

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

  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.Put the steps for writing a covalent compound's formula in order, starting from 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
    • Cross the valencies over to the other symbol
    • Write the symbols of the elements in the compound

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

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

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

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

  1. 1. a) Its ions form a crystal rather than separate molecules
  2. 2. 71
  3. 3. c) A cation, carrying one positive charge
  4. 4. 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
  5. 5. 74.5
  6. 6. 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
  7. 7. d) An electrically neutral group of more than one atom that can exist on its own
  8. 8. 16