Chronon

As the Seventh Doctor once told Mel, chronons were "discrete particles of time." (TV: Time and the Rani) They were also a unit of time measurement. In River Song's World, Kent described the effect of the presence of the Eleventh Doctor at Area 52 as "caus[ing] the [time] loop to extend by four chronons". (TV: The Wedding of River Song)

Knowledge about chronons was sketchy at best. It was known, for instance, that chronons were released by time travel, but time travel also needed chronons to work. (AUDIO: The Time of the Daleks) Exact details about this interrelationship were uncertain.

When weaponised
Time Lords were adept at using chronons against their enemies. They trapped the Mad Mind of Bophemeral in a single chronon following the Millennium War. (PROSE: The Quantum Archangel) Also the Tenth Doctor implied that the Daleks' temporal prison was in some way fuelled by chronons, when he called it a "chronon loop". (TV: Journey's End) Furthermore, the Rani weaponised chronons when she created a planetary shell of chronons in the atmosphere of the planet Lakertya in order to create a Time Manipulator. (TV: Time and the Rani)

Beneficial usage
Chronons could be blocked, either by a TARDIS or by a special-purpose device. Doing so could prevent a person or object from being erased from the present, even if their past no longer existed. (GAME: City of the Daleks)

In an alternative timeline, Rose Tyler and UNIT used mirrors to bounce chronon energy off Donna Noble, allowing her a one-way time jump to correct the change made to her time stream. Rose also implied the concentration of chronons used was directly relevant to how far the jump went. (TV: Turn Left)

Negative effects
Chronon particles were deleterious to the health of time travellers. After a Dahensa ranger crash-landed near Zarechny, Siberia in 1903 and began to emit chronon particles, the First Doctor and his granddaughter Susan Foreman both exhibited signs of radiation poisoning. (AUDIO: The Wanderer)

While the Fifth Doctor was travelling through the Time Vortex, the TARDIS detected dangerous levels of chronon particles, which were causing temporal distortions in local space, emanating for the Jariden Vault of Stellar Curios. He materialised his TARDIS in the Vault and offered his assistance to the Jaridens in order to correct the problem.

Considerably later in the Doctor's personal timeline, his eighth incarnation once again detected the emission of the particles from the Vault. Although his intended destination was the Petrachi Connection, his TARDIS nevertheless materialised in the facility as well. He remembered his younger self's visit, though his memories were initially rather vague. (AUDIO: The Four Doctors)

Behind the scenes

 * Chronons are used as a in-game currency in the MMORPG, Worlds in Time. They must be purchased with real money.
 * One reading of The Time Monster is that chronons are what TOMTIT are based upon, when it speaks of "atoms of time". This feeling is reinforced by the presence of Chronovores. However that script never mentions chronons directly, leaving it up to the viewer to decide what the technobabble might mean.