The atom of an element has 3 electrons in the M-shell. What is the atomic number of this element? 

The correct answer is 13.

Key Points

• The atomic number of an element is equal to the number of protons in its nucleus and also to the number of electrons in a neutral atom.
• The M-shell can hold a maximum of 18 electrons, but it is the third shell to be filled according to the 2n² rule (where n is the shell number).
• For an element with 3 electrons in the M-shell, the first and second shells (K and L) must be filled first.
• The K-shell holds 2 electrons, and the L-shell holds 8 electrons, making a total of 10 electrons in the first two shells.
• Adding the 3 electrons in the M-shell gives a total of 13 electrons, which corresponds to an atomic number of 13.

Additional Information

• Electron Configuration
  • Electron configuration describes the distribution of electrons among the various orbitals of an atom.
  • It follows the Aufbau principle, Hund's rule, and the Pauli exclusion principle.
• Periodic Table
  • The periodic table arranges elements by increasing atomic number and groups elements with similar chemical properties.
  • Elements in the same group have the same number of valence electrons.
• Valence Electrons
  • Valence electrons are the outermost electrons of an atom and are involved in chemical bonding.
  • The number of valence electrons determines the chemical reactivity and bonding behavior of the element.