Structure of matter: atoms, ions and the periodic table
Domain 2 - aine ehitus - asks you to move between the symbol of an element, the structure of its atom, and its position in the periodic table (perioodilisustabel), then to explain why the substance behaves as it does. Typical tasks: complete an atom table, write an ion symbol, decide whether a formula unit is ionic or molecular.
Atom and ion
- Z (atomic number, järjenumber) = protons = electrons in a neutral atom; A (mass number) = protons + neutrons; isotopes differ only in neutron count.
- Electrons fill shells 2, 8, 8 for the first 20 elements; the outer-shell (valent) electrons decide the group and the chemistry.
- Metals lose electrons (Na → Na⁺, Mg → Mg²⁺); non-metals gain them (Cl → Cl⁻, O → O²⁻), each heading for a noble-gas configuration.
- An ion's protons and electrons are unequal, so the charge = protons − electrons.
Periodic table
- Period number = number of shells; group number (IA–VIIIA) = number of outer electrons.
- Group I alkali metals react violently with water; Group VII halogens form salts with metals; Group VIII noble gases are unreactive because their shells are full.
- Across a period, atomic radius decreases and non-metallic character increases; down a group, radius increases and metallic character increases.
Bonding
- Ionic: metal + non-metal, ions in a lattice, conducts when molten or dissolved. Covalent: non-metal atoms share electron pairs, forming molecules (H₂O, CO₂, CH₄). Metallic: positive ions in a sea of delocalised electrons, conducts as a solid.
Worked example
An atom has Z = 17 and A = 35. Give protons, neutrons, electrons, shell configuration, group, period and the ion it forms. Protons 17, neutrons 35 − 17 = 18, electrons 17. Configuration 2, 8, 7 → period 3, group VII (halogen); it is one electron short of 2, 8, 8, so it gains one electron and becomes Cl⁻ with 18 electrons and charge 17 − 18 = −1.
Traps that cost marks
- Neutrons = A − Z, not A and not Z.
- A neutral atom has equal protons and electrons - neutrons are irrelevant to the charge.
- Mixing up period and group numbers.
- Writing Cl⁺ or Cl²⁻ from habit instead of counting electrons.
- Saying 'molecules' for NaCl - it is an ionic lattice, its formula unit is NaCl.