The determination of whether an electron belongs to the valence shell or the core (inner) electron shell depends on the electron configuration of the atom.
The valence shell is the outermost electron shell of an atom and contains the valence electrons. These are the electrons involved in chemical bonding and interactions with other atoms. The number of valence electrons determines an atom's chemical properties and its ability to form bonds.
On the other hand, the core electrons are the electrons that occupy the inner electron shells and are not involved in bonding or chemical reactions. Core electrons are generally closer to the nucleus and shield the valence electrons from the full positive charge of the nucleus.
To determine if an electron belongs to the valence shell or the core electron shell, you can follow these general guidelines:
Identify the atomic number of the element. The atomic number represents the number of protons in the nucleus and is equal to the number of electrons in a neutral atom.
Determine the electron configuration. The electron configuration describes how electrons are distributed in the different energy levels (shells) around the nucleus.
Locate the outermost principal energy level (shell) that contains electrons. This is the valence shell. The electrons in this shell are the valence electrons.
Any electrons in the inner shells (closer to the nucleus) beyond the valence shell are considered core electrons.
For example, let's take the element oxygen (O) with an atomic number of 8. The electron configuration of oxygen is 1s² 2s² 2p⁴. The valence shell is the second energy level (2s² 2p⁴), which contains six electrons (2s² + 2p⁴). These six electrons are the valence electrons. The 1s² electrons are the core electrons.
Note that the valence shell and core electrons can vary for different elements, so it's important to consult the electron configuration of the specific element you are interested in.