If the half-life of a radioactive substance is 10 days, what fraction of the original substance will remain after 30 days? MCQ with Answer and Explanation

Explanation:
NaCl + H₂SO₄ (conc.) → NaHSO₄ + HCl↑, at low temperature. At high temperature, NaHSO₄ + NaCl → Na₂SO₄ + HCl. This is the laboratory preparation of HCl gas. Industrial production is by direct combination of H₂ and Cl₂. Nitric acid is an oxidizer and would produce chlorine. Sodium hydroxide gives salt and water (neutralization). Ammonia reacts with acid.

Explanation:
Magnesium (Z=12): configuration 2,8,2. Period = number of shells = 3; Group = 2 (2 valence electrons). Be is Group 2, Period 2; Ca is Group 2, Period 4; Sr Period 5.

Explanation:
Across a period, nuclear charge increases while electrons are added to the same shell, leading to stronger attraction, so atomic radius decreases and ionization energy generally increases (with some exceptions like N to O due to half-filled stability). Metallic character decreases, non-metallic character increases. Electron affinity becomes more negative (increases in magnitude). Exam note: Ionization energy trend: Increases across period, decreases down group.