PRELIMINARY STAGE 2023
Contest 34
Yolo Kobo SHS: 52 points (Winner, qualified to the one-eighth stage; won 800 Cedis in Goil Riddle Bonanza)
St. Monica's SHS: 36 points
Atebubu SHS: 35 points
ROUND 4 - TRUE OR FALSE
PREAMBLE
1. Rubidium+ and Selenium2- are isoelectronic.
ANSWER: TRUE
2. Cesium+ and Iodine- are isoelectronic.
ANSWER: TRUE
3. Strontium2+ and Arsenic3+ are isoelectronic.
ANSWER: FALSE
EXPLANATION:
$Rb^+$ (37−1=36e⁻) and $Se^{2-}$ (34+2=36e⁻): isoelectronic. $Cs^+$ (55−1=54e⁻) and $I^-$ (53+1=54e⁻): isoelectronic. $Sr^{2+}$ (38−2=36e⁻) and $As^{3+}$ (33−3=30e⁻): not isoelectronic.
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PRACTICE QUESTIONS
1. QUESTION: $K^+$ and $Cl^-$ are isoelectronic.
ANSWER: TRUE
EXPLANATION:
$K^+$ has 19 − 1 = 18 electrons; $Cl^-$ has 17 + 1 = 18 electrons.
2. QUESTION: $Ca^{2+}$ and $Na^+$ are isoelectronic.
ANSWER: FALSE
EXPLANATION:
$Ca^{2+}$ has 20 − 2 = 18 electrons; $Na^+$ has 11 − 1 = 10 electrons.
3. QUESTION: $Mg^{2+}$ and Ne are isoelectronic.
ANSWER: TRUE
EXPLANATION:
$Mg^{2+}$ has 12 − 2 = 10 electrons, the same as neon.
4. QUESTION: $Al^{3+}$ and $F^-$ are isoelectronic.
ANSWER: TRUE
EXPLANATION:
$Al^{3+}$ has 13 − 3 = 10 electrons; $F^-$ has 9 + 1 = 10 electrons.
5. QUESTION: $K^+$ and $Na^+$ are isoelectronic.
ANSWER: FALSE
EXPLANATION:
$K^+$ has 19 − 1 = 18 electrons; $Na^+$ has 11 − 1 = 10 electrons.
6. QUESTION: $S^{2-}$ and $Cl^-$ are isoelectronic.
ANSWER: TRUE
EXPLANATION:
$S^{2-}$ has 16 + 2 = 18 electrons; $Cl^-$ has 17 + 1 = 18 electrons.