Atomic mass is defined as the number of protons and neutrons in an atom, where each proton and neutron has a mass of approximately 1 amu (1.0073 and 1.0087, respectively). A chlorine atom has a mass number of 35. Subtracting 17 from 35 equals 18, which is the number of neutrons in a chlorine atom. In the periodic table, all elements are represented by their chemical symbol with the mass number written on the top-left portion and the atomic number on the bottom-left.
Mass Number Isotope
The weight of an object is defined as the force of gravity on the object and may be calculated as the mass times the acceleration of gravity, w = mg. Since the weight is a force, its SI unit is the newton.
For an object in free fall, so that gravity is the only force acting on it, then the expression for weight follows from Newton's second law.
You might well ask, as many do, 'Why do you multiply the mass times the freefall acceleration of gravity when the mass is sitting at rest on the table?'. The value of g allows you to determine the net gravity force if it were in freefall, and that net gravity force is the weight. Another approach is to consider 'g' to be the measure of the intensity of the gravity field in Newtons/kg at your location. You can view the weight as a measure of the mass in kg times the intensity of the gravity field, 9.8 Newtons/kg under standard conditions.
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Is 0.3 Equal To 0.30
At the Earth's surface, where g=9.8 m/s2 :
The kilogram is the SI unit of mass and it is the almost universally used standard mass unit. The associated SI unit of force and weight is the Newton, with 1 kilogram weighing 9.8 Newtons under standard conditions on the Earth's surface. However, in the US common units, the pound is the unit of force (and therefore weight).The pound is the widely used unit for commerce. The use of the pound force constrains the mass unit to an inconveniently large measuring unit called a 'slug'. The use of this unit is discouraged, and the use of exclusively SI units for all scientific work is strongly encouraged. |
What is a slug? |
The mass of an atom is almost entirely due to the mass of its neutrons and protons. The mass of an electron (9.11 x 10–31 kg) is 1/1800th of the mass of both a proton (1.673 x 10-27 kg) and a neutron (1.675 x 10-27 kg). As a result, the mass of the electrons factors little in the the total mass of the atom (it would take over 1800 electrons to equal the mass of 1 neutron).
Mass Number Isotope
Themass of the protons + the mass of the neutrons is an element's mass number. Since isotopes of an element have different numbers of neutrons, they have different mass numbers.
For example: Two naturally occurring isotopes of chlorine are chlorine-35 & chlorine-37. Thirty-five and thirty-seven are the mass numbers for the two isotopes. Both isotopes have the same number of protons (17).
# protons | |
chlorine-35 | 18 |
17 |
The number of neutrons is determined by subtracting the number of protons from the mass number; (35-17 = 18 & 35-17 = 20 respectively).
Isotopes are also written using their symbol with the mass number (to the upper left) and atomic number (to the lower left).