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Why does graphite leave dark marks on paper?

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representations of molecular structure — molecule Several methods of representing a molecule's structure. In Lewis structures, element symbols represent atoms, and dots represent electrons surrounding them. A pair of shared electrons (covalent bond) may also be shown as a single dash. The ball-and-stick model better illustrates the spatial arrangement of the atoms. For aromatic compounds, the Kekulé structure is common, in which each bond is represented by a dash, carbon atoms are implied where two or more lines meet, and hydrogen atoms are usually omitted. Bond-line formulas, similar to the Kekulé structure, are often used for complex nonaromatic organic compounds. Sugars are often drawn as Fischer projections, in which the carbon “backbone” is drawn as a straight vertical line, with carbon atoms implied where horizontal lines intersect the vertical one.

Graphite leaves dark marks on paper because of its unique crystal structure, which consists of parallel layers of carbon atoms arranged in fused hexagonal rings.12

Within each layer, the carbon atoms are strongly bonded, but the bonding between the layers is very weak—relying on van der Waals forces—which makes the layers slide easily over one another. This property, known as lubricity, allows thin flakes or particles of graphite to shear off the main body of the material when it is rubbed against a surface like paper.12

When you write with a pencil, these small particles of graphite are deposited onto the paper fibers. The apparent darkness of the mark is determined by the number and size of these deposited particles. Because the layers are not tightly bound to each other, the material is soft enough to leave these visible traces behind, a characteristic that led to its name from the Greek verb graphein, meaning "to write".32