Problem 19 - Chemistry: The Central Science

Question

Write the electron configuration for each of the following ions, and determine which ones possess noble-gas configurations: (a) \(\mathrm{Sr}^{2+}\), (b) \(\mathrm{Ti}^{2+}\), (c) \(\mathrm{Se}^{2-}\), (d) \(\mathrm{Ni}^{2+}\), (e) \(\mathrm{Br}^{-}\), (f) \(\mathrm{Mn}^{3+}\).

Step-by-Step Solution

Answer

The electron configurations of the given ions are: 1. Sr²⁺: \([Kr]\) 2. Ti²⁺: \([Ar]3d^2\) 3. Se²⁻: \([Kr]\) 4. Ni²⁺: \([Ar]3d^8\) 5. Br⁻: \([Kr]\) 6. Mn³⁺: \([Ar]3d^4\) Sr²⁺, Se²⁻, and Br⁻ possess noble-gas configurations.

Step 1: Write down the electron configurations of the neutral atoms

Before we can find the electron configurations of the ions, we first need to know the configurations for the neutral atoms. To find the electron configurations, we refer to the periodic table and find the atomic number for each element. The atomic numbers will give us the number of electrons for that element. 1. Sr (Strontium) - Atomic number: 38 2. Ti (Titanium) - Atomic number: 22 3. Se (Selenium) - Atomic number: 34 4. Ni (Nickel) - Atomic number: 28 5. Br (Bromine) - Atomic number: 35 6. Mn (Manganese) - Atomic number: 25 Now using Aufbau's principle, Hund's rule, and Pauli's exclusion principle, we can deduce the electron configuration of these neutral atoms.

Step 2: Write down the electron configurations of the ions

Now that we know the electron configurations of the neutral atoms, we can modify them to find the electron configurations for the ions. 1. Sr²⁺: loses 2 electrons from the neutral configuration 2. Ti²⁺: loses 2 electrons from the neutral configuration 3. Se²⁻: gains 2 electrons from the neutral configuration 4. Ni²⁺: loses 2 electrons from the neutral configuration 5. Br⁻: gains 1 electron from the neutral configuration 6. Mn³⁺: loses 3 electrons from the neutral configuration

Step 3: Identify the ions with noble-gas configurations

To determine which ions possess noble-gas configurations, we will compare their electron configurations to the known noble gases (He, Ne, Ar, Kr, Xe, and Rn). If the ion's configuration is the same as one of the noble gases, it possesses a noble-gas configuration. 1. Sr²⁺: \([Kr]\) 2. Ti²⁺: \([Ar]3d^2\) 3. Se²⁻: \([Kr]\) 4. Ni²⁺: \([Ar]3d^8\) 5. Br⁻: \([Kr]\) 6. Mn³⁺: \([Ar]3d^4\) Based on the above electron configurations, we can see that Sr²⁺, Se²⁻, and Br⁻ possess noble-gas configurations as their electron configurations match with noble gases.

Key Concepts

Noble-Gas Configuration

In chemistry, the noble-gas configuration refers to the electron configuration of noble gases. These are elements that are characterized by having their outer electron shell completely filled. This complete filling grants them high stability and low reactivity, which is why they are termed "noble."

Noble gases include Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn). These elements have electron configurations ending in \(ns^2np^6\), except for Helium which ends in \(1s^2\).

Many ions aim to achieve this stable configuration through the loss or gain of electrons. When an ion's electron configuration matches that of a noble gas, it is said to have a noble-gas configuration. This is often sought by elements as they undergo chemical reactions.

Ions

An ion is an atom or molecule that has gained or lost one or more electrons, resulting in a net electrical charge. Ions can be either positively charged (cations) when electrons are lost, or negatively charged (anions) when electrons are gained.

The formation of ions is a fundamental concept in chemistry and is crucial in the formation of ionic compounds. The charge on an ion depends on the number of electrons lost or gained compared to the neutral atom. For instance:
- Strontium (Sr), when it loses two electrons, becomes \(\text{Sr}^{2+}\), a cation.
- Bromine (Br), when it gains an electron, becomes \(\text{Br}^{-}\), an anion.

Understanding ions is essential for predicting the chemical behavior and reactions of different elements and compounds.

Electron Loss and Gain

Electron loss and gain is an important mechanism through which atoms achieve more stable electronic configurations.

- **Cations**: Formed by the loss of electrons, typically by metals. For example, \(\text{Sr}^{2+}\) is formed from neutral strontium by losing two electrons from its outer shell.
- **Anions**: Formed by the gain of electrons, typically by non-metals. \(\text{Se}^{2-}\) and \(\text{Br}^{-}\) gain additional electrons to achieve a complete outer shell, mimicking the noble gases.

Electrons are added to or removed from the outermost shell, known as the valence shell. This process is driven by the desire of atoms to achieve the electron configuration of the nearest noble gas. This change can alter the chemical properties of the element significantly.

Periodic Table and Atomic Numbers

The periodic table is a fundamental tool in chemistry as it organizes all known elements based on their atomic number and similar properties.

- **Atomic Number**: It represents the number of protons in an atom and, for a neutral atom, this also equals the number of electrons. Strontium, for instance, has an atomic number of 38, indicating it has 38 protons and 38 electrons.
- **Position and Properties**: The periodic table helps determine electron configurations and predict chemical behavior. Elements in the same group (vertical column) often have similar electron configurations and exhibit similar chemical properties.

Using the periodic table, one can predict how elements will react, which ions they might form, and their tendency to assume noble-gas configurations. This is vital for understanding the underlying patterns and trends in chemistry.