Question

1. explain the octet rule. why are hydrogen and helium exceptions to the octet rule?
2. use figure 20 to answer each of the following questions. explain your reasoning for each answer.

a. if a is an ion and b is an atom of the same element, is the ion a positive or negative ion?
b. if a and b represent the atomic radii of two elements in the same period, what is their order?
c. if a and b represent the ionic radii of two elements in the same group, what is their order?

1. how many valence electrons do elements in group 1 have? in group 18?
2. figure 21 shows two ways to define an atomic radius. describe each method. when is each method used?
3. chlorine the electron - configuration of a chlorine atom is ne3s²3p⁵. when it gains an electron and becomes an ion, its electron configuration changes to ne3s²3p⁶, or ar, the electron configuration for argon. has the chlorine atom changed to an argon atom? explain.

Explanation:

1. The octet rule states that atoms tend to gain, lose, or share electrons to have eight electrons in their outer - most shell for stability. Hydrogen and helium are exceptions because hydrogen has only one electron shell which can hold a maximum of 2 electrons, and helium has a full outer shell with 2 electrons.

66a. If A is an ion and B is an atom of the same element and A is larger, it is a negative ion. This is because negative ions are formed by gaining electrons, which increases electron - electron repulsion and thus the size.
66b. In the same period, atomic radius decreases from left to right. If A is larger than B, A is to the left of B in the periodic table.
66c. In the same group, ionic radius increases down the group. If A is larger than B, A is lower in the group than B.

1. Elements in group 1 have 1 valence electron. Elements in group 18 have 8 valence electrons (except helium which has 2).
2. One method in Figure 21 is the metallic radius (a), measured as half the distance between the nuclei of two adjacent metal atoms in a metal crystal. It is used for metals. The other is the covalent radius (b), measured as half the distance between the nuclei of two covalently - bonded atoms of the same element. It is used for non - metals that form covalent bonds.
3. The chlorine atom has not changed to an argon atom. Although the electron configuration of the chloride ion is the same as that of argon, the identity of an atom is determined by the number of protons. Chlorine has 17 protons and argon has 18 protons.

Answer:

1. The octet rule is about 8 - electron outer - shell stability. Hydrogen and helium are exceptions due to their 1 - electron - shell nature and 2 - electron capacity.

66a. Negative ion. Reason: Larger size due to electron gain.
66b. A is to the left of B in the period. Reason: Atomic radius decreases left to right in a period.
66c. A is lower in the group than B. Reason: Ionic radius increases down a group.

1. Group 1: 1 valence electron; Group 18: 8 (except He: 2).
2. Metallic radius for metals (half distance between adjacent metal nuclei), covalent radius for non - metals forming covalent bonds (half distance between covalently - bonded atoms).
3. No. Chlorine has 17 protons, argon has 18 protons; electron configuration doesn't change atomic identity.