How many electrons does each of the following elements have in its outermost electron shell?
(a) Magnesium
(b) Molybdenum
(c) Selenium
The Correct Answer and Explanation is :
To determine how many electrons are in the outermost shell of an element, we need to consider its position in the periodic table and its electron configuration. Elements are arranged in periods (rows) and groups (columns) based on their atomic structure. The outermost electrons, also known as valence electrons, are located in the highest energy level (shell) of an atom.
(a) Magnesium (Mg)
- Atomic number: 12
- Electron configuration: 1s² 2s² 2p⁶ 3s²
Magnesium has an atomic number of 12, meaning it has 12 electrons. These electrons are arranged in different shells. The first shell (1s²) holds 2 electrons, the second shell (2s² 2p⁶) holds 8 electrons, and the third shell (3s²) holds 2 electrons. Therefore, the outermost electron shell (the third shell) has 2 electrons.
(b) Molybdenum (Mo)
- Atomic number: 42
- Electron configuration: [Kr] 4d⁵ 5s¹
Molybdenum has an atomic number of 42, meaning it has 42 electrons. These electrons are arranged in the following electron configuration: [Kr] 4d⁵ 5s¹. The electron configuration shows that molybdenum has its outermost electrons in the 5th shell. In particular, the outermost electrons are in the 5s and 4d orbitals. There are 6 electrons in the outermost shell (1 in 5s and 5 in 4d).
(c) Selenium (Se)
- Atomic number: 34
- Electron configuration: [Ar] 3d¹⁰ 4s² 4p⁴
Selenium has an atomic number of 34, meaning it has 34 electrons. The electron configuration for selenium is [Ar] 3d¹⁰ 4s² 4p⁴. This configuration shows that selenium’s outermost electrons are in the 4th shell, specifically in the 4s and 4p orbitals. The outermost shell has a total of 6 electrons (2 in 4s and 4 in 4p).
Summary:
- Magnesium (Mg): 2 electrons in the outermost shell.
- Molybdenum (Mo): 6 electrons in the outermost shell.
- Selenium (Se): 6 electrons in the outermost shell.