Ch. 3-3a Distinguishing and Counting Atoms

POINT > Define Atomic Number POINT > Define Mass Number POINT > Describe and identify isotopes POINT > Determine the number of neutrons in an isotope POINT > Define the atomic mass unit (U) POINT > Calculate average atomic mass

The atomic number is the number of protons in the nucleus of an atom Ex. Carbon has 6 protons Each element contains a different number of protons. Therefore… The atomic number identifies each element C 6 Carbon

The Periodic Table is organized by increasing atomic number Hydrogen has 1 proton, helium has 2 protons, lithium has 3 protons, etc C 6 Carbon

Protons have a charge of 1+. Atoms are neutral in charge. the So the atomic number also tells you how many electrons there are (same as # of protons) C 6 Carbon

WB CHECK: What is the atomic number of aluminum (Al)? What is the atomic number of cesium (Cs)? How many electrons does an atom of oxygen have?

Recall that the mass of an atom is almost entirely the protons and neutrons The mass number is the total number of protons and neutrons in an atom The mass number usually isn’t shown on the Periodic Table

Mass number is represented one of two ways: or gold-197 (Nuclear symbol) (Hyphen notation)

WB CHECK: What is the mass number of this krypton atom? How many protons does krypton have?

Most elements have isotopes. Isotopes are atoms of an element with different numbers of neutrons.

Isotopes have different mass numbers due to different number of neutrons present Isotopes have the same atomic number since the number of protons is not different

WB CHECK: Carbon-12, carbon-13 and carbon-14 are the isotopes of carbon. How many protons does carbon-14 have? How many electrons does carbon-12 have?

The number of neutrons can be calculated: Mass number - atomic number = # of neutrons Ex. Mass# 48 Atomic # neutrons

WB CHECK: How many neutrons in this krypton atom? How many electrons does krypton have?

POINT > Define the atomic mass unit (U) The atomic mass unit (amu or U) is a relative quantity, with carbon-12 chosen as reference Carbon-12 = 12U Notice that 12 is the number of protons + neutrons Protons and neutrons are very close to 1.0 U

POINT > Define the atomic mass unit (U) Carbon-12 = 12U So, 1/12 of a carbon-12 atom = 1 U Atomic masses of other elements are determined by comparison to carbon -12

POINT > Calculate atomic mass (atomic weight) The average atomic mass (or atomic weight) is a value that takes into account the abundance of different isotopes. This number appears in the Table 1.We estimate the mass of protons and neutrons to be =1.0 U 2.We weight the isotopes of an element by their naturally occurring abundance

POINT > Calculate average atomic mass Multiply the mass of each isotope by its natural abundance (as a decimal). Then add the products. Example: Chlorine has two isotopes, chlorine-35 and chlorine % of all chlorine atoms are chlorine-35 and 24.23% are chlorine-37

Example – Chlorine Chlorine-35 Chlorine-37 (35u x ) + (37u x ) = = 35.48u Actual value using AMU = 35.45u Cl Chlorine

WB CHECK: What atom is the basis for the atomic mass unit? Lithium has two isotopes, Li-6 and Li % is Li-7 and 7.5% is Li-6. Calculate the atomic weight of lithium. 6.93u

 Read pages  Practice problems #1-3 page 76  Determining Atomic Mass W.S.