Answer:
the valence electrons of atoms in a pure metal can be modeled as a sea of electrons
One of the causes of polarity is the unequal forces of the dipole forces due to differences in electronegativity. The more electronegative molecule tend to attract electrons, hence the polarity arrows point towards it. The electronegativity of Phosphorus and Fluorine is 2.19 and 3.98, respectively. Thus, it points outward towards the fluorine atoms.
If the crucible wasn't covered with a lid the reactants may have produced a gas that was released into the surroundings, or mass may have been lost in the form of water vapour.
A. Reactants, products
In a chemical reaction, reactants interact to form products
