Question

Write both the complete electron-configuration and noble-gas notation for each of the following: a. Ar b. Br c. Al

Short answer

Ar: [Ar], Br: [Ar] 4s² 3d¹⁰ 4p⁵, Al: [Ne] 3s² 3p¹.

Step-by-step solution

- Understanding Electron Configuration Notation

Electron configuration notation describes the distribution of electrons among the orbitals of the atom. Electrons fill from lower to higher energy orbitals in a specific order following the Aufbau principle.

- Using the Periodic Table

Use the periodic table to determine the number of electrons in each element: Ar has 18 electrons, Br has 35 electrons, and Al has 13 electrons.

- Writing Electron Configuration for Ar

The atomic number of Argon (Ar) is 18. Write its electron configuration by filling orbitals up to 18 electrons: 1s², 2s², 2p⁶, 3s², 3p⁶.

- Noble-Gas Notation for Ar

Argon is a noble gas itself. Its configuration can be written as [Ar].

- Writing Electron Configuration for Br

The atomic number of Bromine (Br) is 35. Write its electron configuration: 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p⁵.

- Noble-Gas Notation for Br

Find the closest previous noble gas (Ar) and use it as a shorthand: [Ar] 4s² 3d¹⁰ 4p⁵.

- Writing Electron Configuration for Al

The atomic number of Aluminum (Al) is 13. Write its electron configuration: 1s², 2s², 2p⁶, 3s², 3p¹.

- Noble-Gas Notation for Al

Find the closest previous noble gas (Ne): [Ne] 3s² 3p¹.

Key concepts

Noble-Gas Notation

Noble-gas notation is a shorthand way of writing electron configurations, using the symbol of the nearest noble gas that precedes the element in question.
This simplification helps to quickly convey the electron configuration of an element without listing all the occupied orbitals. For example, for Bromine (Br), the complete electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁵. Using noble-gas notation, you can simplify this to [Ar] 4s² 3d¹⁰ 4p⁵.
The noble gases are a group of elements located in Group 18 (or VIII) of the periodic table, and they are known for having a full outer shell of electrons, which makes them very stable.

Aufbau Principle

The Aufbau principle states that electrons occupy orbitals starting from the lowest energy level before filling higher levels.
This principle is crucial for understanding electron configurations because it guides you on how to arrange the electrons in an atom's orbitals.
The order, following the Aufbau principle, is generally: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s, and so on. For example, the electron configuration for Aluminum (Al) starts with the lowest energy level: 1s², then 2s², followed by 2p⁶, 3s², and finally 3p¹.

• Always start filling the 1s orbital first.
• Move to the next orbital only after the previous one is fully occupied.

Periodic Table

The periodic table is an essential tool for determining an element's electron configuration. Each element's position in the table helps you identify its atomic number, which is the number of electrons in a neutral atom.
For example, Argon (Ar) is located in period 3 and group 18, indicating it has 18 electrons. By using the periodic table, you can find the order in which these electrons occupy the different orbitals.
The table is divided into s-block, p-block, d-block, and f-block, based on the type of atomic orbitals the electrons are filling. This division makes it easier to write electron configurations.

• s-block elements include Groups 1 and 2.
• p-block elements include Groups 13 to 18.
• d-block elements are the transition metals in Groups 3 to 12.

Understanding these groups and periods allows you to correctly apply the Aufbau principle.