What is defined as the number of atoms surrounding an atom in a crystal lattice and indicates how tightly the atoms are packed together

The coordination number.

Explanation:

The coordination number is defined as the number of atoms surrounding an atom in a crystal lattice and indicates how tightly the atoms are packed together.

Coordination number is also known as ligancy in chemistry and it basically refers to the total or overall number of neighboring ions, atoms or molecules with respect to a central atom in a crystal lattice. The term coordination number was first defined by the notable and amiable Swiss chemist by the name, Alfred Werner in 1893.

Generally, the coordination number of an atom in a crystal lattice can have a minimum value of two (2) and a maximum value of sixteen (16). It is usually determined by the size of the ligands, central atom and the charge associated with the electronic configuration of an ion. For example, the coordination number of carbon in methane (CH4) is four (4) because an atom of carbon has four (4) atoms of hydrogen bonded to it.

the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula

(that's what i know about formula mass)