# Constants in Chemistry

First we have to tell you what a constant is. Simply put, a constant is a number or measurement that is always the same — no matter where, when, or what condition. When you have a formula that asks for a constant, it is always the same number. Here are some examples...
 6.02 x 1023 NAME: Avogadro's Number WHAT: It tells you the number of atoms in a mole or the number of molecules in a mole of a substance. 9.1x10-31kg NAME: Mass of an Electron WHAT: We talk about electrons spinning around the nucleus of an atom. Well, the me is the mass of one of those electrons. 1.675x10-24g NAME: Mass of a Neutron WHAT: In the nucleus of an atom there are neutrons and protons. A neutron has this much mass. 1.673x10-24g NAME: Mass of a Proton WHAT: In the nucleus of an atom there are neutrons and protons. A proton has a mass of this amount. 6.63x10-34Js NAME: Planck's Constant WHAT: Max Planck figured out that energy can be gained and lost by an atom. He used this constant to help him figure out the amount of energy. The units are 'Joule seconds' which are an amount of energy and time. 3x108m/s NAME: Speed of Light (in a vacuum) WHAT: Scientists figured out that light always travels at the same speed in a vacuum. The number is really 299,792,458 meters per second, but we abbreviate it. 9.8 m/s2 NAME: Acceleration of Gravity of Earth WHAT: What if you drop a ball from a height? It speeds up as it falls. The amount it speeds up (acceleration) is because of the force of gravity on Earth. The gravity on the Moon is different so the acceleration of falling objects will be different up there. 1.66x10-27kg NAME: Atomic Mass Unit (also called a Dalton) WHAT: It is 1/12 the mass of a carbon-12 (C-12) atom. It is the basis for figuring out the mass all other atoms. 1.6x10-19C NAME: Charge of an Electron WHAT: This is the charge of one electron flying around the nucleus. The "C" represents the unit value Coulombs. Coulombs are units of electrical charge. .082 Latm/molK 8.31 J/molK or 8.31(m2kg)/(s2K) NAME: Universal Gas Constant WHAT: This constant is used in the Ideal Gas Law "PV=nRT". It has the same value for all gases. You use a different value depending on which units you have available. The choice with Joules is the one you will usually use in class. The "K" stands for the temperature in Kelvin. "J" the value for joules.

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