Ionization energy
Ionization energy is the energy needed to pull one electron off a gas-phase atom. It rises across a period and falls down a group. A sudden big jump in successive values shows that the next electron comes from an inner shell.
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Analogy
Like the effort needed to pull a magnet off a fridge: stronger magnets (higher Zeff) take more effort.
Example
1st: 7382nd: 14513rd: 7733 (big jump)So Mg has 2 outer electrons.
In practice
Sodium reacts violently because its single outer electron is easy to remove. Neon barely reacts because its electrons are very hard to remove.
Practice
Flashcards
- What does a huge jump between two successive ionization energies tell you?The next electron comes from an inner, closer shell, so the atom has just used up its outer electrons.
Interview questions
- Why does the first ionization energy of oxygen fall slightly below that of nitrogen?
- Nitrogen has three unpaired 2p electrons (half-full)
- Oxygen has one paired 2p orbital
- Repulsion between paired electrons makes one easier to remove
Questions people ask
Why does ionization energy dip from Be to B?
Beryllium's outer electrons are in the 2s subshell. Boron's outer electron is in 2p, which sits slightly higher in energy and is shielded a little by the 2s electrons. So boron's electron is easier to remove, even though boron has more protons.
Why does the second ionization energy exceed the first?
After the first electron leaves, the ion is positive. The remaining electrons feel the same nuclear charge but less electron repulsion, so they are held more tightly. Removing a second electron from a positive ion always takes more energy than removing the first from the neutral atom.