C2.2.2 Electronic structures

Learning Outcomes and Specification Context

The study of electronic structures (C2.2.2) within the OCR Gateway GCSE Chemistry for Combined Sciences curriculum focuses on two primary objectives. First, learners should be able to describe the electronic structure of the first $20$ elements. Second, they must explain the relationship between an element's position in the Periodic Table and its specific electronic structure. These outcomes correspond to specification references $C2.2b$ and $C2.2c$.

An illustrative example of complex atomic structure is found in Livermorium ($Lv$), a metallic element discovered at the start of the $21^{st}$ century. While only a few atoms of Livermorium have been synthesized, each atom possesses $116$ electrons. Understanding how these electrons are arranged is fundamental to the study of chemistry.

Organization of the Periodic Table

The Periodic Table is organized into a specific grid where rows and columns are given distinct names to help identify elements and their properties. A horizontal row is defined as a "period," and a vertical column is defined as a "group." These groups and periods are numbered for identification purposes.

When observing the Periodic Table from left to right across any given period, the atomic number increases by exactly $1$ for each subsequent element. Consequently, because the atomic number represents the number of protons and typically equals the number of electrons in a neutral atom, the total number of electrons in each atom also increases by $1$ for every step to the right. A key principle of chemistry is that elements residing in the same group exhibit similar chemical properties, a phenomenon directly attributed to the specific arrangement of their electrons.

Fundamental Principles of Electronic Shells

In an atom, electrons are not scattered randomly but are arranged around the nucleus in distinct layers known as shells or energy levels. The specific "electronic structure" of an element is a description of how these electrons are distributed among the various shells. The outermost shell that contains at least one electron is formally referred to as the "outer shell."

An analogy for this arrangement can be drawn from books on a shelf: just as books are placed on different shelves at varying heights from the floor, electrons are situated in shells at different energy levels around the nucleus. Each shell has a maximum capacity for the number of electrons it can hold, defined as follows:

• The first shell can hold a maximum of $2$ electrons.
• The second shell can hold a maximum of $8$ electrons.
• The third shell can hold a maximum of $8$ electrons.
• The fourth shell can hold a maximum of $18$ electrons.

Determining the Electronic Structure of an Atom

To determine the electronic structure of any given atom, a systematic two-step process is applied. The first step involves filling each of the shells in sequential order, starting with the first shell closest to the nucleus (the lowest energy level). Once a shell reaches its maximum capacity as defined in the shell capacity rules, subsequent electrons are placed into the next shell.

The second step is to document the structure using numerical notation. The electronic structure is written as a sequence of numbers where each number represents the count of electrons in a specific shell. These numbers are separated by a dot ($.$), which serves to indicate the transition from one shell to the next.

Worked Example: The Lithium Atom

Lithium provides a clear example of applying the rules of electronic arrangement. Lithium has an atomic number of $3$, which indicates that it possesses a total of $3$ electrons.

Applying Step 1: The first shell is filled first. Since the first shell has a maximum capacity of $2$, it takes $2$ of Lithium's $3$ electrons. The remaining $1$ electron must be placed into the second shell. Subatomic components of this atom include protons and neutrons in the nucleus, with electrons orbiting in these defined shells.

Applying Step 2: The numerical notation for this arrangement is written as $2.1$. This signifies that there are $2$ electrons in the first shell and $1$ electron in the second (outer) shell.

Questions & Discussion

During the study of these arrangements, the following observation task is presented: "Write down the number of elements in Periods 1, 2, 3, and 4." This requires an analysis of the Periodic Table to count the elements present in each horizontal row, which correlates with the filling of the respective electronic shells.