Saturday, April 29, 2017

halacha - When does Shabbat officially start?



I am confused about the exact/official time of Shabbat starting. Is it 18 minutes before sunset, when the candles are lit, or at sunset?




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periodic trends - Comparing radii in lithium, beryllium, magnesium, aluminium and sodium ions

Apparently the of last four, $\ce{Mg^2+}$ is closest in radius to $\ce{Li+}$. Is this true, and if so, why would a whole larger shell ($\ce{...