 # Section 2: Masses of Atoms

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Section 2: Masses of Atoms
Chapter 17: Properties of Atoms

Warm-Up: Which of the following can change within an element and not change the identity of that element? Proton Neutron Electron

Learning Goals Compute the atomic mass and mass number of an atom.
Identify the components of isotopes. Interpret the average atomic mass of an element.

Atomic Mass The nucleus contains most of the mass of the atom.
Protons and neutrons are far more massive than electrons The mass of a proton and a neutron is almost 1 atomic mass unit (amu)

Atomic Mass Different elements are distinguished by the number of protons that their atoms have Every carbon atom has six protons Every oxygen atom has eight protons Atomic Number: the number of protons in an atom

Atomic Mass Mass Number: the sum of the number of protons and the number of neutrons in a single atom If you know the mass number and the atomic number, you can calculate the number of neutrons # of neutrons = mass number - atomic number

Atomic Mass Example: An atom of potassium (K) has a mass number of 40. How many neutrons does it have?

Isotopes Not all atoms of an element have the same number of neutrons.
Isotopes: atoms of the same element that have different numbers of neutrons

Isotopes Suppose you have a sample of boron (B):
Naturally occurring atoms of boron have mass numbers of 10 or 11. How many neutrons are in each isotope of boron?

Isotopes Because most elements have more than one isotope, each element has an average atomic mass. Average atomic mass: the weighted average of mass of the mixture of its isotopes The average atomic mass for each element is listed in the periodic table!

Isotopes Four out of every five atoms of boron are boron-11 and one out of every five are boron-10. 11 11 11 11 10 4/5(11 amu) + 1/5(10 amu) = 10.8 amu

Check-In: What is the mass number of an isotope of neon (Ne) that has 11 neutrons? What is the average atomic mass of neon?