Really basic question - what exactly are core electrons? ENC (effective nuclear charge) has a formula:
# of protons - # of core electrons = ENC
Mg has 12 protons and 10 core electrons, giving it a ENC of 2+.
How do I know that Mg has 10 core electrons?
12Mg is 1s2 2s2 2p6 3s2. Those 3s2 electrons are the valence electrons. The other 10 are the core electrons.
To determine the number of core electrons in an atom, you need to understand the concept of electron configuration.
Electron configuration represents the distribution of electrons in an atom's energy levels or electron shells. The electron shells are labeled as 1, 2, 3, and so on, starting from the closest shell to the nucleus (shell 1) and moving outward. Each shell has a maximum number of electrons it can hold:
- Shell 1 can hold a maximum of 2 electrons.
- Shells 2 and higher can hold a maximum of 8 electrons.
The core electrons of an atom are the inner-shell electrons, meaning they are found in the shells closer to the nucleus. These electrons do not participate in chemical reactions and are shielded from the influence of outer electrons by them.
To determine the number of core electrons for an element like magnesium (Mg), you need to look at its electron configuration. The electron configuration of magnesium is 1s² 2s² 2p⁶ 3s².
In the electron configuration, the numbers and letters represent the shells (1s, 2s, 2p, 3s) and the superscript numbers represent the number of electrons in each shell.
So, adding up the superscript numbers for the shells that are closer to the nucleus (1s, 2s, and 2p), we find that magnesium has 2 + 2 + 6 = 10 core electrons.
Therefore, knowing the electron configuration and understanding the arrangement of electrons within an atom helps determine the number of core electrons.