Energy Levels
- Each electron in an atom has a particular average amount of energy.
- The further away an electron is from the nucleus, the more energy it has.
- Each principal energy level (symbol n) corresponds to an electron shell at a certain distance from the nucleus.
- Energy levels are divided into sub‑levels named s, p, d, f.
Electronic Configuration
- Electronic configuration represents the arrangement of electrons in atoms, showing principal quantum shells, sub‑shells, and the number of electrons present.
- Electrons may also be represented in boxes.
- The energy change that accompanies electron removal is called ionisation energy.
Ionisation Energy
- First ionisation energy (IE₁): The energy needed to remove 1 mole of electrons from 1 mole of atoms in the gaseous state to form 1 mole of gaseous ions.
- Successive ionisation energies: The energy required to remove electrons one by one from a gaseous atom.
- Ionisation energy increases as proton number increases.
- The further the outer electron shell is from the nucleus, the lower the ionisation energy.
- Ionisation energy decreases when more full electron shells shield the outer electrons from the nucleus.
Shielding
- Inner shell electrons reduce the effect of nuclear charge on outer shell electrons.
Sub‑shells
- Sub‑shells are regions within principal quantum shells where electrons exist with specific energies.
- Named s, p, d, f.
Atomic Orbitals
- Orbitals are regions of space outside the nucleus that can hold a maximum of 2 electrons.
- Orbitals differ in shape:
- s: 1 orbital → 2 electrons
- p: 3 orbitals → 6 electrons
- d: 5 orbitals → 10 electrons
Blocks in the Periodic Table
- Group 1 and Group 2 elements: outer electrons in s subshell → s‑block.
- Group 13 to 18 (except Helium): outer electrons in p subshell → p‑block.
- Elements adding electrons to d subshell → d‑block (mostly transition elements).
Spin‑Pair Repulsion
- Electrons in the same orbital repel each other due to their charge.
- Pairing electrons with opposite spins reduces repulsion.
- Repulsion is stronger in paired electrons than in single electrons in separate orbitals.
- This explains why electrons in p orbitals occupy separate orbitals before pairing.
Free Radicals
- A species with one or more unpaired electrons.
Atomic Radius
- The covalent atomic radius is half the distance between the nuclei of two covalently bonded atoms of the same type.
- This is a useful measure for comparing elements across a period.
Electron Shells and Subshells
- Electron shells can be divided into s, p, d, and f subshells.
- Maximum electrons: s = 2, p = 6, d = 10.
- Each subshell has orbitals that can hold 2 electrons each.
Using Ionisation Energy Data
- The outer‑shell electronic configuration and the position of an element in the periodic table can be deduced using successive ionisation energy data.
Factors Affecting Ionisation Energy
- Spin‑pair repulsion.
- Distance of electron from nucleus.
- Number of positive charges in the nucleus.
- Degree of shielding of outer electrons by inner shells.
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