Question

Write the full ground-state electron configuration for each: (a) Cl (b) Si (c) Sr

Short answer

\( \text{Cl: } [1s^2 2s^2 2p^6 3s^2 3p^5] \) \( \text{Si: } [1s^2 2s^2 2p^6 3s^2 3p^2] \) \( \text{Sr: } [1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2] \)

Step-by-step solution

- Determine the Atomic Number

Identify the atomic number for each element from the periodic table. (a) Cl has an atomic number of 17. (b) Si has an atomic number of 14. (c) Sr has an atomic number of 38.

- Use the Aufbau Principle

According to the Aufbau principle, electrons fill the lowest energy orbitals first. Use the order of orbital filling: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, etc.

- Write Electron Configuration for Cl

Add electrons to the orbitals for Cl until you reach the atomic number:\[1s^2 2s^2 2p^6 3s^2 3p^5\]

- Write Electron Configuration for Si

Add electrons to the orbitals for Si until you reach the atomic number:\[1s^2 2s^2 2p^6 3s^2 3p^2\]

- Write Electron Configuration for Sr

Add electrons to the orbitals for Sr until you reach the atomic number:\[1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2\]

Key concepts

Atomic Number

The atomic number of an element is a fundamental property that defines the number of protons in the nucleus of an atom. It also indicates the number of electrons in a neutral atom. For example, chlorine (Cl) has an atomic number of 17, silicon (Si) has an atomic number of 14, and strontium (Sr) has an atomic number of 38. With these values, we can determine how many electrons each element has, which is the first step in writing their electron configurations.

• Cl: 17 electrons
• Si: 14 electrons
• Sr: 38 electrons

Aufbau Principle

The Aufbau principle is a key guideline in chemistry for determining the electron configuration of an atom. According to this principle, electrons occupy the lowest energy orbitals available. This principle translates into a specific order in which orbitals are filled:

• 1s
• 2s
• 2p
• 3s
• 3p
• 4s
• 3d
• 4p
• 5s
and so on.

For example, to write the electron configuration for chlorine, knowing it has 17 electrons, you fill the orbitals starting from the lowest energy level:

• 1s (2 electrons)
• 2s (2 electrons)
• 2p (6 electrons)
• 3s (2 electrons)
• 3p (5 electrons, totaling 17 electrons)

Electron Orbitals

Electron orbitals are regions around the nucleus where electrons are likely to be found. They are sub-divided into different types based on their shape: s, p, d, and f orbitals.

• s orbitals: Spherical in shape, each s orbital can hold a maximum of 2 electrons.
• p orbitals: Dumbbell-shaped, each set of p orbitals can hold a total of 6 electrons (2 in each of the 3 p orbitals).
• d orbitals: More complex shapes, can hold up to 10 electrons (in 5 d orbitals).
• f orbitals: Even more complex shapes, can hold up to 14 electrons (in 7 f orbitals).

Understanding this breakdown helps in filling out the electron configurations. For Strontium (Sr) with 38 electrons, the electron configuration extends to the 5s orbital: 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2.