Masses of Atoms

Chapter 19-2Pages 584-587

Atomic Mass- Nucleus of atom

contains most of the mass Protons and

neutrons are far more massive than electrons

Atomic Mass

- Atomic mass unit (amu) – unit of measurement used for atomic particles

Mass of a proton = 1 amu = mass of 1 neutron

1 amu ≈ 1/12 mass of a carbon atom containing 6p+ and 6 n

Numbers- Atomic number – number of protons in an atom

Each element has a unique number of protons (atomic number)

Numbers

- Mass number – the sum of the number of protons and number of neutrons in the nucleus

- Mass # = Atomic # + # of neutrons

-Isotopes – atoms of the same element with different number of neutrons

Isotopes of an element have slightly different properties

- Examples: Carbon-12 (C-12) most common and stableCarbon-14 (C-14) unstable and radioactive

- To identify isotopes, use the element name followed by the mass number Examples: Boron-10, Boron-11

- Average atomic mass – the weighted-average mass of the mixture of all isotopes of an element

4/5 x (11 amu) + 1/5 x (10 amu) = 10.8 amu

this is the average atomic mass ↑

Boron-11 80% (4/5) of all boron atoms are the isotope Boron-11

Boron-10 20% (1/5) of all boron atoms are the isotope Boron-10

** The average atomic mass is always very close to the mass number of the most common (most abundant) isotope, and can be rounded to the nearest whole number when using mass numbers.