Question

Use the Aufbau principle to obtain the ground-state electron configuration of selenium.

Short answer

The ground-state electron configuration of selenium is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴.

Step-by-step solution

Identify the Atomic Number

The atomic number of selenium (Se) is 34, which means it has 34 electrons.

Apply the Aufbau Principle

The Aufbau principle states that electrons fill orbitals starting with the lowest energy levels before moving to higher ones. This means we need to fill according to the order of increasing energy shells and subshells.

Order of Orbital Filling

The order for filling is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. For selenium, we will use this order to distribute its 34 electrons.

Fill Electrons into Orbitals

Following the order: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴. This accounts for all 34 electrons.

Key concepts

Electron Configuration

Electron configuration is a way to represent the distribution of electrons among the various atomic orbitals of an atom. It not only describes how many electrons are in each orbital, but also the specific order in which the orbitals are filled. This plays a crucial role in understanding the chemical properties and behaviors of elements. To write the electron configuration, you use a series of numbers and letters that denote the energy levels and types of orbitals occupied by the electrons. For example, the configuration of oxygen is written as 1s² 2s² 2p⁴. Key points to remember about electron configurations:

• Each number represents an energy level.
• The letter indicates the type of orbital (s, p, d, f).
• The superscript tells you how many electrons are present in that orbital.

Understanding electron configurations helps predict the bonding patterns, magnetism, and even the color of an element.

Atomic Number

The atomic number is one of the most fundamental properties of an element in the periodic table. It denotes the number of protons that reside in the nucleus of an atom and consequently determines the number of electrons that must be present in a neutral atom. For selenium, the atomic number is 34, which means it has 34 protons and, typically, 34 electrons. The atomic number is crucial because:

• It defines the identity of the element. Every element has a unique atomic number.
• It helps determine the position of the element in the periodic table.
• It applies directly to determining electron configuration.

So, when working out the electron configuration for selenium, its atomic number informs us of the total number of electrons to account for. This is a key step in achieving the correct configuration.

Orbital Filling Order

Orbital filling order refers to the sequence in which atomic orbitals are filled with electrons. This order is guided by the Aufbau principle, where 'aufbau' means 'build up' in German. Electrons are added one at a time to the lowest energy orbitals available before filling higher ones. The order in which orbitals are filled is not strictly linear; instead, it follows a specific *energy level* pattern: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on. Important aspects of orbital filling:

• Subshells fill in a manner that minimizes the energy of the atom.
• s orbitals fill before p orbitals of the same main energy level.
• Within a main energy level, the sequence is usually s < p < d < f.

This order ensures that the electron configuration reflects the most stable energy arrangement for an atom in its ground state.

Ground-State Configuration

The ground-state configuration of an atom describes the most stable arrangement of electrons in orbitals under normal conditions. This is the natural state for electrons when they are not excited by energy. In this state, electrons occupy the lowest available energy levels as dictated by the Aufbau principle and other rules like Hund's rule and the Pauli exclusion principle. For selenium, with an atomic number of 34, the ground-state electron configuration is: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴. Features of a ground-state configuration:

• It minimizes energy, representing the most stable condition of the atom.
• It is notated in a specific order based on orbital energy levels.
• This configuration is a baseline from which excited states can be derived.

By understanding the ground-state configuration, one can infer chemical reactivity, potential ion formation, and other atomic behaviors.