Atom - 3

Atomic mass is the total mass of an atom, determined mainly by the combined mass of its protons and neutrons. Because individual atoms are too tiny to weigh on a standard scale, scientists measure their masses relative to a single reference standard, Carbon-12.

The atomic mass unit

Electrons are so light that it takes about 1835 electrons to equal the mass of a single proton. Because an electron's mass is negligible, an atom's mass comes almost entirely from its protons and neutrons, which each weigh about 1 atomic mass unit (amu, or unified unit u). One Carbon-12 atom has a mass of 1.99 times 10 to the power of minus 23 grams, and dividing this by twelve gives 1.66 times 10 to the power of minus 24 grams, which equals 1 amu.

What isotopes are

• Atoms of the same element that have the same number of protons but different numbers of neutrons are called isotopes.
• Carbon-12 contains six protons and six neutrons in its nucleus.
• Carbon-13 contains six protons and seven neutrons, giving it a slightly larger mass of 13.003 u.
• Carbon-14 contains six protons and eight neutrons, though it is extremely rare in nature.

How mass spectrometry works

• A machine called a mass spectrometer heats a sample until it vaporizes into a gas.
• The machine removes electrons from the vaporized atoms, turning them into positively charged ions.
• These ions travel through a magnetic field, which bends their flight paths.
• Lighter ions curve more sharply than heavier ones, allowing the machine to separate each isotope and record its abundance.

Calculating average atomic mass

Natural carbon is made of roughly 99 percent Carbon-12 and 1 percent Carbon-13. Multiplying each isotope's mass by its percentage abundance and adding the results gives an average atomic mass of 12.01 u. This average value is what is listed on the periodic table, and mass spectrometry is used to analyze elements in everything from crime scene evidence to meteorites.