It takes more energy to remove core electrons than valence electrons.
- This is because core electrons, being closer to the nucleus, experience greater effective nuclear charge (ENC) because of fewer filled shells shielding them from the nucleus.
What is ionisation energy (IE)?
- the minimum amount of energy needed to remove a valence electron from a gaseous atom or ion forming a gaseous cation of a higher oxidation number
- A greater attraction between the electron and the nucleus results in a greater ionisation energy.
- 1st IE is the minimum energy needed to remove the 1st electron from a gaseous atom, while the 2nd IE is the minimum energy needed to remove the 2nd electron form a gaseous ion (note that it is not the removal of 2 electrons)
Example 2
Knowing that the difference 4th and 5th ionisation energies is much larger, we can imply that this difference is caused by removing a core electron instead of a valence electron. This means that there are 4 valence electrons (as removing the 1st to 4th electrons are valence electrons while the 5th electron is a core electron).
Elements with 4 valence electrons lie in group IVA (also known as group 14) of the periodic table (such as carbon and silicon). We do not know which period the element is in thus the valence electronic configuration would be written as ns²np², where s² represents the 2 elements in the s block before the element, and p² shows that it is the 2nd element in the p block. n is the period number which we do not know.
In the attached periodic table, the s block, d block, and p block are in yellow, blue and purple respectively.
Writing electronic configuration
Let's look at an example! The valence electronic configuration of Arsenic would be 4s²3d¹⁰4p³. On the other hand, the electronic configuration would be 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p³ or [Ar]4s²3d¹⁰4p³, since Argon is the closest noble gas that comes before Arsenic. Writing electronic configuration is done according to Aufbau principle, and this is simplified in the periodic table attached where you can count accordingly reading and counting from left to right of the periodic table then down. The number of elements in a certain block is written as a superscript.
Aufbau principle
- states that electrons fill orbitals of lowest sub energy level to capacity before filling orbitals of higher sub energy levels (see the 2nd attached picture)
Maximum number of electrons each subshell can hold
- s subshell ➮ 2
- p subshell ➮ 6
- d subshell ➮ 10
