Answer:
1. Electron geometry and molecular shape: tetrahedral;
The bonds are polar;
The molecule is nonpolar.
2. Because in NH3 the central atom has lone pairs.
Explanation:
The electron geometry of a molecule is how the bonds are disposal in the central atom. According to the VSEPR theory, the bonds are done in a way to maximize the distance between then, because of the repulsive forces. When there are lone pairs of electrons in the central atom, they occupy a huge space, and force the bonds to be more close, so, the molecule shape change.
Carbon tetrabromide (CBr4) has carbon as a central atom (that has 4 valence electrons) and 4 bromides bonded to it, each on sharing one pair of electrons with carbon. Thus, there'll be no lone pair in the central atom, and the electron geometry and the molecular shape will be the same --tetrahedral -- because it's the shape of 4 sharing electrons.
The bromine has a huge electronegativity, and carbon has a small one, thus, the bond between then, will be polar, the bromide intends to have the shared electron to itself, and has a negative partial charge. Because the bonds are done with the same elements, and they are opposite in the molecule, the dipole moment is canceled and equal to 0, thus the molecule is nonpolar.
2. NH3 has as central atom nitrogen, which has five electrons in its valence shell. When it does 3 simple bonds with the hydrogens, it remains 1 lone pair of electrons in it, so, it forces the bonds, and the molecular geometry of it is pyramidal trigonal.
On the other hand, H2CO has as central atom carbon, which has 4 valence electrons. The carbon does 1 double bond with oxygen, and two simple bonds with the hydrogens, so it has no lone pairs and has a trigonal geometry.
