Hint: A unit cell is the most basic structure of any crystal can be broken down to. A crystal is repetition of this basic structure in three dimensions. Line structure joining the individual molecules and atoms which makes the complete crystal is called the space lattice.
Complete step by step solution: Atoms, ions or molecules make up any matter. When these are arranged in a regular pattern consistently throughout the crystalline substance then it is called a crystal. A line structure pattern joining these units followed throughout the substance describes the space lattice structure of the substance. (b) A space lattice is an array of points showing how particles are arranged at different sites in three dimensions. Now based on these dimensions, we can come up with many different space lattices of the system of particles making up the substance. Now these systems of particles can be grouped with a combination of particles which when repeated again and again make up the whole crystal. This group of particles is called (a) unit cell. Similar to space lattice, unit cells also have different types.
Note: These terminologies of space lattices and unit cells are different ways to break down millions of molecules comprising the substance into a simpler unit, we can focus on. These unit cells and space lattices describe a lot about the behaviour of material to the environment. For example, Mica is rock forming material which has a sheet like structure to visible eyes. This also related to the elementary structure of molecules of the rock.
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