Question

storyline 1 exam
a mystery element has 18 protons. which one of the following statements is true about the element?
the element belongs in row 4 of the periodic table and is classified as an alkali metal.
the element belongs in row 5 of the periodic table and is classified as an alkaline earth metal.
the element belongs in row 2 of the periodic table and is classified as a halogen.
the element belongs in row 3 of the periodic table and is classified as a noble gas.

Explanation:

• The number of protons in an atom is equal to its atomic number. So, the mystery element has an atomic number of 18.
• Looking at the periodic table, the element with atomic number 18 is argon (Ar).
• Argon is in row 3 (period 3) of the periodic table.
• Argon is a noble gas as it has a full valence shell (8 electrons in its outer shell, following the octet rule), which makes it stable and unreactive under normal conditions.

Answer:

The element belongs in row 3 of the periodic table and is classified as a noble gas.