And click on the Time Machine icon. Click on âSelect Diskâ. Choose your external drive and click on âUse Backup Diskâ. Time Machine will copy everything (OS, Software, Files, and Preferences) to the external hard drive.
The standard enthalpy of formation of a pure element is in its reference form its standard enthalpy formation is zero. Carbon naturally exists as graphite and diamond. The enthalpy difference between graphite and diamond is too large for both to have a standard enthalpy of formation of zero. To determine which form is zero, the more stable form of carbon is chosen. This is also the form with the lowest enthalpy, so graphite has a standard enthalpy of formation equal to zero. Table 1 provides sample values of standard enthalpies of formation of various compounds.
There are three naturally occurring isotopes of carbon: 12, 13, and 14. 12 C and 13 C are stable, occurring in a natural proportion of approximately 99:1. 14 C is produced by thermal neutrons from cosmic radiation in the upper atmosphere, and is transported down to earth to be absorbed by living biological material.arbon-15 redirects here. For the firearm, see Carbon 15. Carbon (C) has 15 known isotopes, from 8 C to 22 C, 2 of which (12C and 13C) are stable. The longest-lived radioisotope is 14C, with a half-life of 5,700 years.