Question

iron (fe): ar4s²3d⁶
core electrons
valence electrons

Explanation:

Core electrons are the electrons in an atom that are not valence electrons (not involved in bonding). The electron configuration [Ar] represents the core electrons (since Ar is a noble gas and its electrons are core - like in this context). Valence electrons are the electrons in the outermost shell that can participate in bonding. For Fe with electron configuration [Ar]4s²3d⁶, the 4s and 3d electrons (total of 2 + 6 = 8) are in the outer - most relevant shells for chemical behavior (even though 3d is a lower energy level than 4s in some filling order concepts, in terms of chemical reactivity, these are the electrons that can be involved in bonding).

Answer:

core electrons: [Ar]; valence electrons: 4s²3d⁶