Question

Why does the fourth period in the periodic table contain 18 elements?

Short answer

The fourth period has 18 elements due to the filling of 4s, 3d, and 4p orbitals.

Step-by-step solution

Understanding the Electron Configuration

The fourth period starts with potassium (K) and ends with krypton (Kr). The key to understanding why there are 18 elements in this period lies in electron configuration. Elements in the periodic table are arranged in order of increasing atomic number, and their electron configuration determines the distribution within periods.

Identifying Electron Orbitals

The fourth period involves filling the 4s, 3d, and 4p orbitals. Potassium (K) starts filling the 4s orbital. Potassium and calcium complete the filling of the 4s orbital.

Filling the 3d Orbital

After the 4s orbital is filled, the next set of orbitals to fill is the 3d. These orbitals begin filling starting with scandium (Sc) and continue through zinc (Zn), which takes up the next ten elements in the period.

Completing Orbital Filling with 4p

Following the completion of the 3d orbital filling, the next elements start filling the 4p orbital. Starting with gallium (Ga) and ending with krypton (Kr), six more elements are added.

Counting Total Elements

The sum of electrons that fill these orbitals during the fourth period is 2 (4s) + 10 (3d) + 6 (4p) = 18 electrons, corresponding to 18 elements. This configuration completes the fourth period on the periodic table.

Key concepts

Periodic Table

The periodic table is an essential tool in chemistry and helps in understanding the properties of all known elements. It is arranged in a way that elements with similar properties are grouped together. The arrangement is primarily based on the atomic number, which is the number of protons in an atom's nucleus.
The periodic table is organized in rows called periods and columns called groups. Each period corresponds to the filling of a particular set of electron orbitals, which in turn affects the chemical properties of the elements within it. As we move across a period from left to right, the atomic number of the elements increases by one, indicating the addition of one proton to the nucleus and one electron to the outer electron shell. This structure allows us to predict and understand an element's reactivity, state of matter, and other properties.

Electron Orbitals

Electron orbitals are regions around the nucleus where electrons are likely to be found. They're fundamental in determining how atoms bond and interact with each other.
There are various types of orbitals, identified as s, p, d, and f, each having a different shape and number of possible electrons. For instance:

• s orbitals are spherical and can hold up to 2 electrons.
• p orbitals are dumbbell-shaped and can hold up to 6 electrons.
• d orbitals can hold up to 10 electrons and have a more complex shape.
• f orbitals, although not discussed here much, can hold up to 14 electrons.

As electrons fill orbitals, they do so in a specific order known as the Aufbau principle, which is crucial for understanding the electron configuration of atoms.

Atomic Number

The atomic number is a unique identifier for each chemical element in the periodic table. It represents the number of protons found in the nucleus of an atom and determines the element's identity.
For example, hydrogen has an atomic number of 1, meaning it has one proton in its nucleus, while carbon has an atomic number of 6. As we move across the periodic table from left to right, the atomic number increases sequentially, reflecting the addition of protons in the nuclei of elements. Thus, the atomic number plays a pivotal role in defining the characteristics of each element, including their electron configuration, as it affects the distribution of electrons in orbitals and the chemical behavior of the element.

Electron Distribution

Electron distribution refers to the arrangement of electrons around the nucleus of an atom. It is governed by electron configuration, which follows specific principles and rules like the Aufbau principle, Pauli-exclusion principle, and Hund's rule.
Using the periodic table, we can predict electron distribution across different orbitals for various elements. For instance, in the fourth period of the periodic table, the electron distribution involves filling the 4s, 3d, and 4p orbitals. This period starts with filling the 4s orbital, followed by filling of the 3d orbitals, and concludes with filling the 4p orbitals. The total electron configuration for the fourth period is 2 electrons in 4s, 10 electrons in 3d, and 6 electrons in 4p, summing up to 18 electrons. This distribution explains how electrons are arranged in energy levels within elements in a given period.