Chemistry

CHEM& 140 – Checkpoint 9 – Show It HW Name ________________________________ Section ___

1. A homogeneous mixture of nitrogen gas and hydrogen gas may contain any proportions of nitrogen gas and hydrogen gas, but ammonia as a compound can only exist with a fixed proportion of nitrogen and hydrogen.

a. Which law states that elements combine in fixed proportions? Which scientist discovered this law? (Refer back to Checkpoint 5.)

b. (Similar to Example 5.1 in Section 5.2) Two samples of ammonia, collected from two different sources are decomposed into their constituent elements. One sample produces 7.0 g of nitrogen and 1.5 g of hydrogen. The other sample produces 28.0 g of nitrogen and 6.0 g of hydrogen. Show that these results are consistent with the law you stated in the previous part by calculating the nitrogen:hydrogen mass ratio.

c. Report the atomic masses of nitrogen and hydrogen. Include units.

(Look up atomic masses on Periodic Table. Recall that no single atom of N, or H, will actually have this mass, but instead it is the weighted average mass of atoms of the element based on the masses and abundances of the common isotopes of that element. Refer to Checkpoint 6.)

atomic mass of nitrogen: _____________ atomic mass of hydrogen: _____________

d. Taking into account the atomic masses of nitrogen and hydrogen, which ratio of N:H atoms matches the mass ratio that you calculated in the previous part?

Correct atom ratio (circle one):

1 N : 1 H 1 N : 2 H 1 N : 3 H 2 N : 1 H 3 N : 1 H

2. What is a chemical formula? What is the order of listing nonmetal elements in a chemical formula? Give an example.

3.

a. What are polyatomic ions?

b. When more than one polyatomic ion is present, we use parentheses in the formula. How do you calculate the number of atoms of an element within parentheses? Provide an example.

4. What is the difference between a molecular element and an atomic element (also referred to as a monatomic element)? List the elements that occur as diatomic molecules.

5. What is the difference between an ionic compound and a molecular compound? What is the difference between ionic bonding and covalent bonding? (Make sure to discuss what type of elements are involved in each, and discuss what is happening with the valence electrons in formation of bonds according to the Octet Rule.)

6. Ionic compounds do NOT exist as molecules.

a. What is the basic unit (simplest repeating unit) of ionic compounds? _____________________

b. On the macroscale (what you can see by eye) ionic compounds come in the form of crystals. On the atomic level, a crystal of an ionic compound is formed by ions alternating in a three-dimensional _________________.

c. Draw atomic-level diagrams (use spheres to represent atoms or ions) that show the difference between molecules and ionic compounds.

7. For each type of substance on the left, provide an example (as a chemical formula) and identify the basic units that of which it is composed (options for basic units: molecules, formula units, single atoms).

monatomic element

molecular compound

ionic compound

molecular element

8. For the element calcium, Ca:

a. Write out the electron configuration.

b. Draw a Bohr model diagram.

c. Label the valence electrons in both (configuration and diagram).

d. Does an atom of Ca gain or lose electrons when it becomes an ion? Label these electrons in your Bohr model diagram and your electron configuration.

9. (Similar to Example10.1 on page 327 in textbook.) Draw Lewis dot symbols of Mg, Si, S, and Ar.

10. (Similar to Example 10.2 & 10.3 on pages 327-328 in textbook.) Draw the Lewis structure of the compound that forms between sodium and sulfur. State the chemical formula of the compound that forms.