2.3
Your guide: Miss SanaCounts atoms the way you count rupees: carefully.
How do we know electrons sit in shells? We never see them.
Two kinds of evidence answer this.
Smart Syllabus 2026 leaves out the atomic spectra part (2.3.1) for the 2026 exam. The ionization evidence part stays. Learn both, because they explain shells.
Heat a gas or pass electric discharge through it. It gives out light of certain wavelengths.
This spectrum has coloured lines. It is the atomic emission spectrum.
Pass white light through a cold gas. Some wavelengths are absorbed. They show as dark lines. This is the atomic absorption spectrum.
The dark lines sit at exactly the same wavelengths as the coloured lines.
Each element has its own fixed energy levels. So each has its own spectrum. The book calls spectra the finger prints of elements.
Remove the electrons of one atom one by one. The energies are the successive ionization energies.
The energy rises as we go from the valence shell to inner shells.
For magnesium, the first two electrons come from the outer shell. They need less energy.
A big jump comes at the third electron. It lies in a shell much closer to the nucleus.
A much bigger jump comes at the eleventh and twelfth electrons. They lie in the first shell.
Two big jumps show that electrons of magnesium sit in three shells.
All ionization energies are strongly endothermic. It takes energy to remove an electron.
Down a group the energy falls. More shells weaken the hold on the valence electrons.
Across a period it rises. The shell number is the same, but the proton number grows.
Alkali metals have the lowest value in each period. Noble gases have the highest.
Step 1 / 7
How do we know electrons sit in shells? We never see them.
Two kinds of evidence answer this.
Smart Syllabus 2026 leaves out the atomic spectra part (2.3.1) for the 2026 exam. The ionization evidence part stays. Learn both, because they explain shells.
Heat a gas or pass electric discharge through it. It gives out light of certain wavelengths.
This spectrum has coloured lines. It is the atomic emission spectrum.
Pass white light through a cold gas. Some wavelengths are absorbed. They show as dark lines. This is the atomic absorption spectrum.
The dark lines sit at exactly the same wavelengths as the coloured lines.
Each element has its own fixed energy levels. So each has its own spectrum. The book calls spectra the finger prints of elements.
Remove the electrons of one atom one by one. The energies are the successive ionization energies.
The energy rises as we go from the valence shell to inner shells.
For magnesium, the first two electrons come from the outer shell. They need less energy.
A big jump comes at the third electron. It lies in a shell much closer to the nucleus.
A much bigger jump comes at the eleventh and twelfth electrons. They lie in the first shell.
Two big jumps show that electrons of magnesium sit in three shells.
All ionization energies are strongly endothermic. It takes energy to remove an electron.
Down a group the energy falls. More shells weaken the hold on the valence electrons.
Across a period it rises. The shell number is the same, but the proton number grows.
Alkali metals have the lowest value in each period. Noble gases have the highest.
Q1. Quick Check 2.2 (c): which element is a noble gas, and which is a metal giving AX2?
Element I has the highest i1 = 2372, so it is the noble gas. Element III has i2 = 1760, about 2 × 900 = 1800, so two outer electrons: a metal for AX2.
✗ “Every element has the same spectrum.”
✓ Each element has its own fixed energy levels, so its own spectrum.
✗ “Ionization energy releases energy.”
✓ Removing an electron takes energy. It is endothermic.
1. In the magnesium plot, where does the first big jump come?
(c) Two electrons are in the outer shell. The third lies in the shell below.
2. Which group has the lowest ionization energy in each period?
(a) The book says so.
3. Dark lines in an absorption spectrum sit at:
(a) The book says they match exactly.
4. The ionization energy goes down a group because:
(a) More shells weaken the hold on valence electrons.
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