In the modern periodic table, gold (Au) is located in which horizontal period?

Introduction / Context:
This question checks familiarity with the layout of the modern periodic table, specifically the position of the element gold. Periods are the horizontal rows in the periodic table, numbered from 1 at the top to 7 at the bottom for known elements. Gold, with symbol Au and atomic number 79, is a well known precious metal, and knowing its period helps with understanding its electron configuration and chemical relatives.

Given Data / Assumptions:
- The element in question is gold, symbol Au, atomic number 79.
- The options for its period number are 11, 5, 6, and 7.
- We assume the standard long form periodic table with 7 periods.

Concept / Approach:
Period number corresponds to the highest principal quantum number (n) that contains electrons in the ground state electron configuration of an element. Elements in period 6 have electrons filling the 6s, 4f, and 5d subshells, among others. Gold has an electron configuration that places it in period 6. There is no period 11 in the standard table, and gold is too heavy to be in periods 5 or 4, but not as heavy as the elements in period 7, which include the actinides and superheavy elements.

Step-by-Step Solution:
Step 1: Recall that gold has atomic number 79 and symbol Au.
Step 2: The approximate ground state electron configuration of gold can be written in a simplified way as [Xe] 4f14 5d10 6s1, indicating that its outermost electrons involve the n equals 6 shell.
Step 3: The highest principal quantum number present in this configuration is 6, which means that gold belongs to period 6.
Step 4: The standard periodic table has only 7 periods. There is no period 11, so option 11 is impossible.
Step 5: Period 5 contains elements with electrons filling up to n equals 5, and period 7 contains even heavier elements than gold, including some radioactive and synthetic elements. Gold does not belong to those periods.
Step 6: Therefore, among the choices given, the correct period for gold is period 6.

Verification / Alternative check:
If you examine a standard periodic table, you will find gold (Au) in the sixth horizontal row, positioned among the transition metals, below silver (Ag) and copper (Cu) in group 11. This visual confirmation from any accurate periodic table matches the electron configuration based reasoning and clearly places gold in period 6, not in periods 5 or 7.

Why Other Options Are Wrong:
- 11: The long form periodic table has only 7 periods, so period 11 does not exist in the standard classification.

Why Other Options Are Wrong (continued):
- 5: Period 5 contains lighter transition metals such as silver (Ag) and cadmium (Cd). Gold is heavier and is placed in the next period down.
- 7: Period 7 includes very heavy and mostly radioactive elements, such as francium, radium, and the actinide series. Gold, with atomic number 79, appears one period above them in period 6.

Common Pitfalls:
Students may confuse group numbers (vertical columns) with period numbers (horizontal rows). For example, gold is in group 11, but that does not mean it is in period 11. Another confusion arises from not having the periodic table memorised and guessing the period based on atomic number alone. Remember that gold is just below silver and copper in the same group, and copper is in period 4, silver in period 5, and gold in period 6, which forms a neat trend down the group.

Final Answer:
6 is the period number in which gold (Au) occurs in the modern periodic table.