How can the number of core electrons be determine from the periodic table?

Answer 1

By finding the atomic number minus the group number

Answer 2

Short answer; Summary:

Total number of electrons - number of valence electrons = number of core electrons

^^ Use electron configuration; MUST be used for transition elements as there are many exceptions to the simple rules of thumb given for these elements. Just stick w/ electron configurations here ;)

For MAIN Elemental groups ONLY:

Group Numbers (1=1 valence E, 2= 2 valence, 13=3.... 18=8 Valence electrons)

Long descriptive answer:

By core electons, I presume you mean the non valent electrons (or "inner electrons"), correct?

The total number of electrons can be determined by looking at an element's Atomic Number (Number of protons); the number of electrons is EQUAL to the number of protons in an atom. Ions are a different story; you'll have to add or take away electrons from the electron configuration to determine this.

So, let's take the element Argon. Ar has the atomic number 18. Argon is in Group 18, the Noble Gases Group (it has 8 valence electrons). Therefore, you would have 10 "core" electrons and 8 "valence" electrons for Argon. You could answer the question by determining the number of Valence electrons of your given Element, and subtract that from the Total number of electrons to find out how many core electrons you have.

Summary:

Total number of electrons - number of valence electrons = number of core electrons

Group 17 (one left) has 7 valence electrons, and the same goes for groups 1 (one Valence electron), 2 (two Valence electrons), 13-18 (18 has 8, subtract one valence electron until you get to group 13, which has 3 valence electrons). The Transition elements require the electron configuration, because some elements have different numbers of valence electrons despite their "group number".

The electron configuration is what will determine which electrons are "core" electrons and which electrons are "valence" electrons for EVERY element, including the "main group" elements, as well as the "transition elements" (middle of Periodic Table). The electron configuration of Ar is as follows:

1s2 2s2 2p6 3s2 3p6

Argon has 8 Valence electrons; valence electrons are the electrons that occupy the outermost shell of an electron cloud (up to 8). Valence electrons occupy the outermost S and P orbitals, while the core or ground electrons occupy f and d orbitals.

** Ignoring those outermost electrons, there you have it: your "core electrons"**

I hope this answer helps you >.<