Describing elements by their place in the periodic system and atomic structure. Importance of the periodic law
Textbook: pp. 51–62
GoalDescribe an element by its place in the periodic system and its atomic structure; explain the importance of the periodic law.
New words
atomic radius · atom radiusihighest oxide and hydroxide · yuqori oksid va gidroksidvolatile hydrogen compound · uchuvchan vodorodli birikmapredicted element · bashorat qilingan element
Explanation
To describe an element, follow this plan: 1) period, group and subgroup; 2) nuclear charge, protons, neutrons (N = A – Z) and electrons; 3) electron formula and number of outer electrons; 4) metal or nonmetal; 5) formula and nature of the highest oxide, hydroxide and hydrogen compound; 6) comparison with neighbours. Along a period from left to right the atomic radius shrinks, electrons are held more strongly, metallic character weakens, nonmetallic character and electronegativity grow. Down a main subgroup the number of shells and the atomic radius grow and metallic character strengthens. The periodic law made it possible to predict new elements: Mendeleev foretold ekaboron (scandium), ekaaluminium (gallium) and ekasilicon (germanium); they were found in 1879, 1875 and 1886. For germanium the predicted Aᵣ was 72, the measured one 72.6. The law also helped to correct atomic masses (for example beryllium, Aᵣ = 9, valency II).
Worked examples
Bromine (Z = 35, A = 80): period 4, main subgroup of Group VII; 35 protons, 35 electrons, N = 80 – 35 = 45 neutrons; [Ar] 3d¹⁰ 4s² 4p⁵ (7 outer electrons). A nonmetal: highest oxide Br₂O₇ (Mᵣ = 2 · 80 + 7 · 16 = 272), hydroxide HBrO₄ — an acid, hydrogen compound HBr (valency I). A weaker nonmetal than chlorine, stronger than iodine.
Comparison: Mg and Ca are in one subgroup; Ca is lower, has more shells and a larger atom, so Ca(OH)₂ is the stronger base and Ca the more active metal. Na and Mg are in one period; Na is on the left, so it is the more metallic.
Class activity
“Element passport”: each group describes one element (for example Z = 17, 20, 33) following the plan and writes a passport. Passports are pinned to the wall and groups check each other. Notebooks and the table only.
Practice
1
Position, highest oxide and hydroxide of potassium (Z = 19)?
Period 4, main subgroup of Group I; K₂O and KOH (an alkali).
2
How many neutrons are in the nucleus of ⁶⁹Ga (Z = 31)?
38
3
Which is the stronger metal: Na or Mg? Why?
Na: it is further left in the period, with a larger radius, and gives up its outer electron more easily.
4
Why did Mendeleev leave empty cells in his table and what came of it?
He trusted the regular repetition of properties; gallium, scandium and germanium were found later and confirmed his predictions.