Atomic mass and weight are two terms but mean the same thing. That is the mass of an atom at rest. Unlike in normal everyday life where "mass" is how much matter a physical body contains, and "weight" is a bodies force due to gravity. Weight= mass x acceleration due to gravity, which is why on the moon we are lighter because of the smaller force of gravity.

Technically mass is a measure of the amount of matter something contains and weight is a measure of the amount of a force acting on something. Most often, when people think of weight, they are thinking of the affect of Earth's gravity on their mass (though they may not actually realize that is what they are thinking about). Most people don't recognize the difference between mass and weight. As a result of this, many people use the word "weight" when they really should use the word "mass". This confusion has also made its way into the terms used to describe atoms. So, the term "atomic mass" should be used when describing the sum total of the number of protons and neutrons in an atom. However, often "atomic weight" is improperly used instead, but they really are the same thing as they are used in common conversation.

Although unlike everyday life, modern physics makes a distinction between mass and weight, "the use of "weight" for "mass" also persists in some scientific terminology – for example, in the chemical terms "atomic weight", "molecular weight", and "formula weight", can still be found rather than the preferred "atomic mass" etc.": "In the physical sciences, weight is a measurement of the gravitational force acting on an object. Near the surface of the Earth, the acceleration due to gravity is approximately constant; this means that an object's weight is roughly proportional to its mass." "In commerce and in many other applications, weight means the same as mass as that term is used in physics. In modern scientific usage, however, weight and mass are fundamentally different quantities: mass is an intrinsic property of matter, whereas weight is a force that results from the action of gravity on matter: it measures how strongly gravity pulls on that matter. However, the recognition of this difference is, historically, a relatively recent development and in many everyday situations the word "weight" continues to be used when "mass" is meant. For example, we say that an object "weighs one kilogram", even though the kilogram is a unit of mass. The distinction between mass and weight is unimportant for many practical purposes because the strength of gravity is very similar everywhere on the surface of the Earth. In such a constant gravitational field, the gravitational force exerted on an object (its weight) is directly proportional to its mass. So, if object A weighs, say, 10 times as much as object B, then object A's mass is 10 times that of object B. This means that an object's mass can be measured indirectly by its weight (for conversion formulas see below). For example, when we buy a bag of sugar we can measure its weight (how hard it presses down on the scales) and be sure that this will give a good indication of the quantity that we are actually interested in, which is the mass of sugar in the bag. Nevertheless, the Earth's gravitational field can vary by as much as 0.5% at different locations on Earth (see Earth's gravity). These variations alter the relationship between weight and mass, and must be taken into account in high precision weight measurements that are intended to indirectly measure mass. To eliminate this variation, when the weight of objects is used in commerce, the value given is what they would weigh at a nominal standard gravitational acceleration of 9.80665 m/s2 (approx. 32.174 ft/s2) Spring scales, which measure local weight, must be calibrated at the location at which they will be used to show this standard weight, to be legal for commerce. The use of "weight" for "mass" also persists in some scientific terminology – for example, in the chemical terms "atomic weight", "molecular weight", and "formula weight", can still be found rather than the preferred "atomic mass" etc." Source and further information: http://en.wikipedia.org/wiki/Weight Further information: http://www.answerbag.com/a_view/2429890

Atomic weight (symbol: Ar) is a dimensionless physical quantity, the ratio of the average mass of atoms of an element (from a given source) to 1/12 of the mass of an atom of carbon-12. Atomic Mass:The atomic mass is the average mass of an element in atomic mass units ("amu"). Though individual atoms always have an integer number of atomic mass units, the atomic mass on the periodic table is stated as a decimal number because it is an average of the various isotopes of an element. Isotopes can have a weight either more or less than the average. The average number of neutrons for an element can be found by subtracting the number of protons (atomic number) from the atomic mass.

atomic mass unit= 1/12 the mass of a carbon-12 atom atomic weight= 6.022E23 atoms of an elements (in grams)