Bellringer1. Who is credited with the development of

the periodic table?

2. What is the difference between Mendeleev’s version of the periodic table and the current version of the periodic table?

How Atoms Differ

Objectives:3.2 Calculating the number of protons, neutrons, and electrons in an isotope

Atomic Number• Def.: the number of PROTONS in an atom• What is the atomic number of:

– Hydrogen?– Lead?– Chlorine?

• How many protons are in an atom of:– Hydrogen?– Lead?– Chlorine?

Atomic Number (cont.)• What is the charge of a proton?• Where are protons found? • What else is in the nucleus?• What would be the charge of the

NUCLEUS of the following elements:– Hydrogen?– Lead?– Chlorine?

Atomic Number (cont.)• What is needed to BALANCE this positive

nuclear charge, so the atom is neutral?• So how many electrons?• Atomic number = number of protons =

number of electrons• How many electrons are in an atom of:

– Hydrogen?– Lead?– Chlorine?

Practice & Assignment• P.99: 11-13• Study Guide workbook: p.21 (1-12)

Isotopes and Mass Number

• How Dalton’s atomic theory was wrong:– Atoms ARE divisible. (Who is credited with

first splitting an atom?)– All atoms of a particular element are NOT

identical.• All atoms of an element do have the same

number of protons and electrons, so what could differ?

Isotopes and Mass Number (cont.)

• Isotopes: atoms with the same number of protons (same element), but with different numbers of neutrons.

• Examples: – Potassium (K): K-39, K-40, K-41– Chlorine (Cl): Cl-35 and Cl-37– Uranium: U-235 and U-238

Isotopes and Mass Number (cont.)

• If atoms have the same number of protons and electrons, but different numbers of neutrons, what else will be different?

• Answer: MASS• Mass number = number of protons +

number of neutrons

Isotopes and Mass Number (cont.)

• Examples:– K-39 = 19 protons + 20 neutrons– K-40 = ___ protons + ___ neutrons– K-41 = ___ protons + ___ neutrons

• Isotope symbolic notation: mass number Element Symbol = 39K atomic number 19

• Isotope name is potassium-39

Isotopes and Mass Number (cont.)

• Review: 1. Number of protons is the same for all atoms

of a particular element

2. Number of neutrons can vary ( = isotopes)• What is the mass number?• What is the atomic number?• Number of neutrons = ?

Isotopes and Mass Number (cont.)

• Number of neutrons = mass number – atomic number

• Example: If an atom of neon has an atomic number of 10, and a mass number of 22:1. How many protons? Neutrons? Electrons?

2. What is the NAME of the isotope?

3. What is the SYMBOL of the isotope?

Practice & Assignment

• Practice: p.101 (14)• Study Guide Workbook: pp. 21-22 (13-23)

Mass of Individual Atoms• Review:

– Mass of proton = 1.673 x 10-24 g– Mass of neutron = ?– Mass of electron = 9.11 x 10-28 g

• An atom of K-39, has ____ protons, _____ neutrons, and ______ electrons.

• Who wants to add up that actual mass?!?!

Mass of Individual Atoms (cont.)

• We ALL want to work with whole numbers, so scientists came up with the atomic mass unit (amu).

• Def.: 1/12 the mass of a carbon-12 atom (ALMOST equal to the mass of 1 proton or 1 neutron)

Mass of Individual Atoms (cont.)

• Review: What does the mass number of an element tell you?

• The mass recorded on the periodic table for an element is called atomic mass.

• Atomic mass is determined by the weighted average mass of the isotopes of that element.

Mass of Individual Atoms (cont.)

• What is the atomic mass for chlorine?• Chlorine exists naturally as a mixture of

75.770% Cl-35 and 24.230% Cl-37.• What do Cl-35 and Cl-37 mean?• The ACTUAL masses of these isotopes

would be 34.969 amu and 36.966 amu.• The math:

(75.770% x 34.969) + (24.230% x 36.966) = 35.453 amu

• What was the atomic mass of chlorine?

Practice & Assignment• Remember, to calculate atomic mass:

1. Calculate mass contribution of each isotope by multiplying its mass x its % abundance.

2. Add together all the mass contributions.

• P.104: 15-16 (Practice)• Assignment: P. 113 (59-68)