The question asks to identify the last element belonging to the fifth period of the periodic table. Let's explore the structure of the periodic table to find the answer.

Understanding Periodic Table Periods

The periodic table organizes chemical elements into rows and columns. The horizontal rows are known as periods. Each period corresponds to a specific principal energy level (or shell) for the electrons in an atom. The period number indicates the highest principal quantum number ($n$) occupied by electrons in that element's ground state configuration.

Elements in the Fifth Period

The fifth period ($n=5$) begins with the alkali metal Rubidium ($Rb$) and ends with the noble gas Xenon ($Xe$). This period includes elements that fill the $5s$ orbital, followed by the transition metals that fill the $4d$ orbitals, and finally elements that fill the $5p$ orbitals.

The elements in the fifth period are:

• Rubidium ($Rb$, atomic number $37$)
• Strontium ($Sr$, atomic number $38$)
• Yttrium ($Y$, atomic number $39$)
• Zirconium ($Zr$, atomic number $40$)
• Niobium ($Nb$, atomic number $41$)
• Molybdenum ($Mo$, atomic number $42$)
• Technetium ($Tc$, atomic number $43$)
• Ruthenium ($Ru$, atomic number $44$)
• Rhodium ($Rh$, atomic number $45$)
• Palladium ($Pd$, atomic number $46$)
• Silver ($Ag$, atomic number $47$)
• Cadmium ($Cd$, atomic number $48$)
• Indium ($In$, atomic number $49$)
• Tin ($Sn$, atomic number $50$)
• Antimony ($Sb$, atomic number $51$)
• Tellurium ($Te$, atomic number $52$)
• Iodine ($I$, atomic number $53$)
• Xenon ($Xe$, atomic number $54$)

Identifying the Last Element of the Fifth Period

The fifth period starts with the element having the atomic number $37$ ($Rb$) and concludes with the element having the atomic number $54$ ($Xe$). The period ends when the outermost electron shell being filled (in this case, the $n=5$ shell, specifically the $5p$ sublevel) is completed.

Therefore, the last element in the fifth period is Xenon ($Xe$), which has the atomic number $54$.

Analysis of Options

Let's review the provided options:

• Helium: This element ($He$) is the first element in the first period ($n=1$).
• Yttrium: This element ($Y$) is found in the fifth period ($n=5$), but it is the element with atomic number $39$, positioned after Rubidium and Strontium. It is not the last element.
• Rubidium: This element ($Rb$) has the atomic number $37$ and is the first element in the fifth period ($n=5$).
• Xenon: This element ($Xe$) has the atomic number $54$. It is the final element in the fifth period ($n=5$), completing the filling of the $5p$ sublevel.

Based on this analysis, Xenon is the correct element that concludes the fifth period.