How many elements are there in the fourth period of the periodic table? Explain why it is not possible for there to. be another element in this period.
The Correct Answer and Explanation is:
There are 18 elements in the fourth period of the periodic table.
Explanation:
The periodic table is structured based on the arrangement of electrons in atomic orbitals, following the principles of quantum mechanics. The fourth period begins with potassium (K), atomic number 19, and ends with krypton (Kr), atomic number 36. The reason there are exactly 18 elements in this period is due to how electrons fill the available orbitals.
In the fourth period, the filling of the electron shells follows this order:
- The 4s orbital is filled first with 2 electrons
- The 3d orbitals accommodate up to 10 electrons
- The 4p orbitals hold a maximum of 6 electrons
Adding these together gives:
- 2 electrons for the 4s orbital
- 10 electrons for the 3d orbitals
- 6 electrons for the 4p orbitals
Total: 2 + 10 + 6 = 18 elements
The specific elements in the fourth period are:
Potassium (K), Calcium (Ca), Scandium (Sc), Titanium (Ti), Vanadium (V), Chromium (Cr), Manganese (Mn), Iron (Fe), Cobalt (Co), Nickel (Ni), Copper (Cu), Zinc (Zn), Gallium (Ga), Germanium (Ge), Arsenic (As), Selenium (Se), Bromine (Br), and Krypton (Kr)
It is not possible to have more than 18 elements in the fourth period because after krypton, the next electrons begin filling the 5s orbital, which belongs to the fifth period. The number of elements per period depends entirely on how many electrons can occupy the available energy levels and sublevels based on the Pauli Exclusion Principle and the Aufbau Principle, which dictate electron configurations.
Thus, the structure of electron shells naturally limits the fourth period to exactly 18 elements.
