Question

How many elements are there in the fourth period of the periodic table? Based on quantum theory, explain why it is not possible for there to be another element in this period.

Short answer

There are 18 elements in the fourth period; quantum theory limits this by shell filling.

Step-by-step solution

Understanding the Fourth Period

The fourth period in the periodic table starts from potassium (K) and ends at krypton (Kr). We need to count the elements starting from atomic number 19 to atomic number 36.

Counting the Elements

To calculate how many elements are in the fourth period, we list them: 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). This gives us a total of 18 elements.

Explaining the Quantum Theory Limit

According to quantum theory, the number of elements in a period is dictated by the filling of electron shells. The fourth period involves filling the 4s, 3d, and 4p subshells. The 4s subshell holds 2 electrons, 3d subshell holds 10 electrons, and 4p subshell holds 6 electrons, making a total of 18 electrons. Thus, it is not possible to have more elements because any additional electrons would start filling the 4d subshell, which corresponds to the next period.

Key concepts

Quantum Theory

Quantum theory provides the framework behind the structure of atoms. It helps us understand the behavior of electrons within an atom. According to this theory, electrons move in regions around the nucleus known as orbitals, which are grouped in shells and subshells. Each orbital can hold a specific number of electrons determined by quantum numbers.
The quantum numbers include the principal quantum number, which indicates the shell (energy level) an electron occupies, and the azimuthal quantum number, which denotes the shape of the orbital. These quantum numbers allow electrons to be organized in a way that every electron within an atom has a unique set of quantum numbers. This organization helps explain why each period in the periodic table has a limited number of elements, as only certain electron configurations are possible within each period.

Electron Shells

Electron shells are layers around an atom's nucleus where electrons are likely to be found. They are called shells because they encompass the atom at varying distances, like layers of an onion. Each shell can contain a predetermined number of electrons.
The first shell (closest to the nucleus) can hold up to 2 electrons, the second shell can hold up to 8, and it continues to increase with each shell. In the fourth period of the periodic table, electrons fill the 4s, 3d, and 4p subshells. These subshells within the shell have specific limits to how many electrons they can hold:

• The 4s subshell can hold 2 electrons.
• The 3d subshell can hold 10 electrons.
• The 4p subshell can hold 6 electrons.

This totals to 18, which is the reason there are 18 elements in the fourth period. Beyond these 18 electrons, additional electrons begin to fill the 4d subshell, marking the start of the fifth period.

Atomic Number

The atomic number of an element is a fundamental property that determines its position on the periodic table. It represents the number of protons found in the nucleus of an atom. Since atoms are electrically neutral, the atomic number also tells us the number of electrons surrounding the nucleus.
For example, in the fourth period of the periodic table, potassium starts at atomic number 19, and it continues up to krypton, which is atomic number 36. Therefore, the elements in the fourth period have atomic numbers ranging from 19 to 36, including consecutive elements within these numbers. The atomic number is not just a counting number; it also indicates how electrons are distributed among different shells, influencing the chemical properties of the element.

Periodic Table

The periodic table is a systematic arrangement of elements, organized by increasing atomic number. Elements are placed in rows called periods and columns known as groups. Each period corresponds to the filling of a principal energy level, beginning each new shell of electrons.
The fourth period spans elements with atomic numbers from 19 (potassium) to 36 (krypton). As you go across the period, new electrons fill in beginning from the 4s subshell, proceeding through the 3d, and ending in the 4p subshell. This ordering not only determines how many elements are in the period but also their chemical behavior, as elements in the same group tend to have similar properties.
By following these trends, the periodic table helps us predict the properties of elements and their compounds, emphasizing the significance of their atomic numbers and their electron configurations.