2.35. Atomic Number http://www.ck12.org
Mass Number
There is another number in theFigure2.66 for helium. That number is themass number, which is the mass of the
atom in a unit called theatomic mass unit (amu). One atomic mass unit is the mass of a proton, or about 1.67×
10 −^27 kilograms, which is an extremely small mass. To better appreciate how small an atomic mass unit is, go to
this URL: http://www.colorado.edu/physics/2000/periodic_table/atomic_mass.html.
A neutron has just a tiny bit more mass than a proton, so its mass is often assumed to be one atomic mass unit as
well. Because electrons have virtually no mass, just about all the mass of an atom is in its protons and neutrons.
Therefore, the total number of protons and neutrons in an atom determines its mass in atomic mass units.
Consider helium again. Most helium atoms have two neutrons in addition to two protons. Therefore the mass of most
helium atoms is 4 atomic mass units (2 amu for the protons + 2 amu for the neutrons). However, some helium atoms
have more or less than two neutrons. Atoms with the same number of protons but different numbers of neutrons are
called isotopes. Because the number of neutrons can vary for a given element, the mass numbers of different atoms
of an element may also vary. For example, some helium atoms have three neutrons instead of two. Therefore, they
have a different mass number than the one given in theFigure2.66.
Q:What is the mass number of a helium atom that has three neutrons?
A:The mass number is the number of protons plus the number of neutrons. For helium atoms with three neutrons,
the mass number is 2 (protons) + 3 (neutrons) = 5.
Q:How would you represent this isotope of helium to show its atomic number and mass number?
A:You would represent it by the element’s symbol and both numbers, with the mass number on top and the atomic
number on the bottom:
5
2 HeSummary
- Elements are pure substances that make up all matter, so each one is given a unique name. The names of
elements are also represented by unique one-, two-, or three- letter symbols. - The number of protons in an atom is called its atomic number. This is also unique for each element.
- An atom’s mass number is its mass in atomic mass units (amu), which is about equal to the total number of
protons and neutrons in the atom. Different isotopes of an element have different mass numbers because they
have different numbers of neutrons.
Vocabulary
- atomic mass unit (amu): SI unit for the mass of an atom, where 1 amu equals the mass of a proton (about 1.7
× 10 −^24 grams). - atomic number: Number of protons in an atom.
- mass number: Number of protons plus neutrons in an atom; equal to the atom’s mass in atomic mass units
(amu).
Explore More
Use the periodic table of the elements at the first URL below to fill in the blanks in the worksheet at the second URL.