Which of the following reactions best represents the reaction between H3PO4
and water?
A) H3PO4 + H2O ⇌ H2PO4
- + H2O
B) H3PO4 + H2O ⇌ H2PO4
- + H3O+
C) H3PO4 + 3 H2O → PO4 3- + 3 H2O
D) H3PO4 + 3 H2O → PO4 3- + 3 H3O+

Option B,

Explanation:

In this reaction, a weak acid is reacting with  water. Thus, water is this case will act as a proton acceptor or a base as well as an acid. Hence water will be amphiprotic for this chemical process and can donate as well accept as a proton. Now when weak acid such as phosphoric acid loses a hydrogen ion it forms a weak conjugate base ie. H2PO4^-. Water being a weak base shall accept the proton and forms hydronium ion i.e H3O^+

The dihydrogen phosphate ion reacts with water:

H2PO4^- + H2O <----> HPO4^2- + H3O^+

After some time a proton is again transferred to the H2O molecule to produce  phosphate ion

HPO4^2- + H2O <----> H3O^+ + PO4^3-

assuming that your periodic table has eighteen columns, you should highlight   the four elements in the third column from the left of the table.

neutral atoms of elements after calcium contain d electrons. the energy of d orbitals of a particular main shell lies between the energy of the s and p orbitals of the next main shell. for example, electrons in the have energy higher than those in the orbital while lower than those of the orbital.

by the aufbau principle, electrons fill orbitals one at a time, starting from those of the lowest energy. the orbital is therefore filled right after orbitals and before orbitals.  

potassium has electron configuration and calcium . the s orbital of a main shell holds a maximum of two electrons. the next element would thus have the electron configuration and satisfy the requirements.

a neutral atom of the element contains three electrons on top of the most recent noble gas element. it should thus be placed in the third column of the periodic table. both scandium and yttrium would satisfy the conditions. an aufbau diagram indicates that the 4f orbital is filled before the 5d. however, it appears that exceptions exists for both lanthanum and actinium. neutral atoms of the two d-block elements also demonstrate the and should be highlighted as well.

6.4 cm (max) - 25 cm (min)