Answer:
All groups have the same number of valence electrons.
Explanation:
groups are the columns.
Answer:
He used too few significant digits
Explanation:
The answer is D
the oldest fossils on earth are 3.5 billion years
Answer:
The ratio [A-]/[HA] increase when the pH increase and the ratio decrease when the pH decrease.
Explanation:
Every weak acid or base is at equilibrium with its conjugate base or acid respectively when it is dissolved in water.
⇄ 
This equilibrium depends on the molecule and it acidic constant (Ka). The Henderson-Hasselbalch equation,
![pH = pKa + Log \frac{[A^{-}]}{[HA]}](https://tex.z-dn.net/?f=pH%20%3D%20pKa%20%2B%20Log%20%5Cfrac%7B%5BA%5E%7B-%7D%5D%7D%7B%5BHA%5D%7D)
shows the dependency between the pH of the solution, the pKa and the concentration of the species. If the pH decreases the concentration of protons will increase and the ratio between A- and AH will decrease. Instead, if the pH increases the concentration of protons will decreases and the ratio between A- and AH will increase.
It's only a small difference (103 degrees versus 104 degrees in water),
and I believe the usual rationalization is that since F is more
electronegative than H, the electrons in the O-F bond spend more time
away from the O (and close to the F) than the electrons in the O-H bond.
That shifts the effective center of the repulsive force between the
bonding pairs away from the O, and hence away from each other. So the
repulsion between the bonding pairs is slightly less, while the
repulsion between the lone pairs on the O is the same -- the result is
the angle between the bonds is a little less.
Hope this helps!