Question

Three elements ' \(X\), ' \(Y\) ' and ' \(Z\) 'have atomic numbers 18, 19 and 20 respectively. How many electrons are present in the \(M\) shells of these elements? (a) \(8,9,10\) (b) \(8,10,13\) (c) \(8,8,8\) (d) \(8,9,12\)

Short answer

(a) 8,9,10

Step-by-step solution

Understand the Electron Configuration of Elements

The atomic number of an element is equal to the number of protons and also the number of electrons in a neutral atom. The electron shells are filled in a specific order: K, L, M, N, etc. The M shell is the third shell and can hold a maximum of 18 electrons. However, it is filled only after the K shell (which can hold 2 electrons) and the L shell (which can hold 8 electrons) are full.

Determine the Electron Configuration for Element X

Element X has an atomic number of 18, which means it has 18 electrons. The electron configuration for X would be 2 electrons in the K shell, 8 electrons in the L shell, and the remaining 8 electrons will fill the M shell. So, the M shell of element X has 8 electrons.

Determine the Electron Configuration for Element Y

Element Y has an atomic number of 19, so it has 19 electrons. Following the electron configuration order: 2 electrons will fill the K shell, 8 will fill the L shell. This leaves us with 9 electrons to place in the M shell. So, element Y has 9 electrons in the M shell.

Determine the Electron Configuration for Element Z

Element Z has an atomic number of 20, indicating it has 20 electrons. The first 2 shells will be fully occupied with 2 electrons in the K shell and 8 in the L shell. The remaining 10 electrons will be placed in the M shell. This means element Z has 10 electrons in the M shell.

Match with the Given Options

Now we can match the M-shell electrons of elements X, Y, and Z with the given options. X has 8, Y has 9, and Z has 10 electrons in their respective M shells. Therefore, the correct option is (a) 8,9,10.

Key concepts

Electron Configuration

Understanding the electron configuration is crucial when studying atomic structure. It refers to the arrangement of electrons in an atom's electron shells or energy levels. Electrons tend to fill the lowest energy levels first before moving to higher ones, following a set of rules known as the Aufbau principle.

Each shell can hold a specific number of electrons: the first shell (K shell) can hold up to 2, the second (L shell) up to 8, and the third (M shell), generally up to 18. However, before the M shell starts filling up, K and L shells must be full. This electron configuration influences the chemical properties of an element and plays a critical role in determining how elements interact during a chemical reaction.

Applying Electron Configuration Rules

• The K shell (1st shell) is filled with 2 electrons.
• The L shell (2nd shell) can hold up to 8 electrons.
• The M shell (3rd shell) can start filling once the first two shells are complete.

With this in mind, we can figure out the electron configuration for an element by knowing its atomic number, which tells us the number of electrons in a neutral atom.

Atomic Number

The atomic number is a fundamental characteristic of an element that determines its place in the periodic table. It is denoted by the symbol 'Z' and represents the number of protons found in the nucleus of an atom. Since atoms are neutral in charge, the atomic number also equals the number of electrons in a neutral atom.

For example, if an element has an atomic number of 8, it means the atom has 8 protons and, therefore, 8 electrons. The atomic number is crucial for determining the electron configuration of an element, as it tells us how many electrons we need to distribute among the electron shells according to the energy level distribution rules.

Importance of the Atomic Number

• Determines the identity of an element.
• Indicates the number of protons in the nucleus.
• Helps figure out the number of electrons for neutral atoms.

With the atomic number, we can deduce how these electrons will occupy the various electron shells, which is central to chemical reactivity and bonding.

Electron Shells

Electron shells are regions surrounding the nucleus of an atom where electrons are likely to be found. These shells are labeled starting from the nucleus outwards as K, L, M, N, and so on. Each shell can only accommodate a certain number of electrons and must be filled in order before moving to the next shell.

When discussing electron shells, we often refer to them in terms of energy levels, with electrons in shells closer to the nucleus having lower energy than those in outer shells. This organization is fundamental in determining how atoms interact with one another and the types of chemical bonds they form.

Rules of Electron Shell Filling

• Shells closer to the nucleus fill up first.
• Electron capacity increases with shell level (2 for K, 8 for L, 18 for M, etc.).
• Electrons occupy the lowest available energy level.

The deals with the number of electrons in the 'M' shell for elements with atomic numbers 18, 19, and 20, and by applying knowledge of the electron shells, we can ascertain these distributions accurately.