Atomic structure / Struktur atom
Atomic structure tests subatomic particles, proton and nucleon number, isotopes, electron configuration and how ions form. You must distinguish atomic number from mass number and use electron arrangement to predict group and period.
Core rules
- Proton number Z is the number of protons; it defines the element.
- Nucleon number A is protons plus neutrons; neutrons = A − Z.
- A neutral atom has electrons = protons. For an ion, electrons = protons − charge for a positive ion, and electrons = protons + charge for a negative ion.
- Isotopes have the same Z but different A; they have the same chemical properties but different physical properties and contribute to relative atomic mass.
- For the first 20 elements, shells fill 2,8,8; valence electrons determine chemical properties. Relative atomic mass is a weighted average of isotope masses, not the mass of one atom.
Worked example
An ion of ²⁴Mg²⁺ has Z = 12 and A = 24. Protons = 12; neutrons = 24 − 12 = 12. Neutral Mg has 12 electrons; Mg²⁺ has lost 2, so it has 10 electrons. Configuration: 2.8. It is isoelectronic with Ne. Reasoning: the charge tells you electron loss or gain; nucleon number does not change during ion formation.
Traps
For anions, add electrons: O²⁻ has 10 electrons, not 8. Do not use nucleon number as electron number. Isotopes have identical electron configuration, so identical chemical reactions, but different neutron numbers. In notation, mass number is superscript before the symbol and atomic number is subscript; do not swap them.