Chapter Four

Discovering the Atom

Modern Atomic Model A.K.A. the Electron Cloud Model

The modern model of the atom consists of a positively charged center, called the nucleus, surrounded by negatively charged particles called electrons. These electrons are located in a “cloud of probability”. The two types of particles in the nucleus are positively charged protons and neutrons (no charge). Overall the atom is uncharged.

Nucleus

Electron cloud. The area where electrons may be found.

The Atom and the Periodic TableAtomic number : is the number of

protons contained in the atom’s nucleus.

Since every atom of the same element has the same number of protons, therefore the element can be identified by it’s atomic number.

Atom (continued) The mass of the proton is relatively

equal to that of a neutron, but about 2000 times larger than an electron.

Since the atom is electrically neutral, it contains the same number of protons as electrons.

The Atom and the Periodic TableSince the atom is overall electrically

neutral (no charge), the number of positive particles must equal the number of negative particles.

Therefore, the number of positive ____________ must equal the number of negative _______________.

Daily Assignment

Explain what information you can obtain about an atom by looking at the information contained on the periodic table.

The Atom and the Periodic Table Atomic Mass tells you the number of

protons and neutrons in the nucleus Atomic Mass = Protons + Neutrons

* Due to isotopes, the number of neutrons in an atom of the same element can vary. You must always calculate the atomic mass.

To calculate the number of neutrons, manipulate the equation:

Neutrons = Atomic Mass - Protons

Isotopes An Isotope is an element that has a different

number of neutrons (but the same number of protons, obviously) which therefore changes the atomic weight.

If you are asked to determine if a particular element is an isotope, simply subtract the atomic number from the mass number and that will give you the number of neutrons.

If the number of neutrons is different than on your table, it’s an isotope.

The Atom and the Periodic Table Atomic number = 13 Atomic mass = 27

Element name ______ Element symbol ______ # protons ______ # neutrons ______ # electrons ______

Atomic number = 15 Atomic mass = 31

Element name ______ Element symbol ______ # protons ______ # neutrons ______ # electrons ______

The Atom and the Periodic Table Atomic number = Atomic mass =

Element name ______

Element symbol ______

# protons __26__

# neutrons __30__

# electrons __ _

Atomic number = Atomic mass =

Element name ______

Element symbol ______

# protons ______ # neutrons

______ # electrons ____

The Atom and the Periodic Table Atomic number = Atomic mass =

Element name _silver_

Element symbol ______

# protons ______ # neutrons

__61___ # electrons ______

Atomic number = Atomic mass =

Element name ______

Element symbol ______

# protons ___54_

# neutrons _77___

# electrons ______

More Practice Atomic number = Atomic mass =

Element name ______

Element symbol ______

# protons ______ # neutrons

______ # electrons ______

Atomic number = Atomic mass =

Element name ______

Element symbol ______

# protons ______ # neutrons

______ # electrons ______

Daily Assignment Below is a square taken from the periodic table. Identify the type of information

given by each number, word, or symbol listed.

28

Ni Nickel 58.71

Isotopes Isotopes are atoms of the same element

that have a different number of neutrons. Since the number of neutrons vary, the atomic masses of these atoms are different.

Isotopes are the reason for the average atomic mass listed on the periodic table.

IsotopesBoron has two isotopes, 5

11B and 510 B

1.What is the atomic number of each? 2.What is the atomic mass of each? 3.How many protons, neutrons and electrons does each isotope have?

Isotopes

Uranium, element 92, has three isotopes. One has 142 neutrons, the other has 144 neutrons, and the third has 147 neutrons.

1.What is the atomic mass of each isotope?

2.Which atomic mass appears on the periodic table?

Average Atomic Mass The average atomic mass listed on the periodic

table is a weighted average of all the isotopes of that element.

So…. What is a weighted average? Think of your grades I use to calculate your average. Each Homework = 5 points You score Each Test = 100 points You score Each Webquest = 200 points You score To calculate the average, use the formula, Total your scores x 100

Total points available

Isotopes and Average Atomic Mass

Uranium has three common isotopes. The atomic masses of each is U 234, U 235, and U 238, if 0.01% is U 234, 0.71% is U, 235, and 99.28% U 238 , calculate the average atomic mass of all the isotopes of uranium.

Isotopes and Average Atomic Mass Rubidium has two common isotopes,

85 Rb, and 87 Rb. If the abundance of 85 Rb is 72.2% and the abundance of 87Rb is 27.8%, what is the average atomic mass of all the isotopes of rubidium?

Energy Levels Electrons in atoms are arranged into energy levels The lower the energy the electron has, the lower

its energy level will be. Electrons can move up or down energy levels

depending on the energy of the atom Think of a staircase:

Energy Levels As the electrons gain energy, they move up the

staircase. When the atom loses energy, the electrons will

move back down and release heat or energy (light). When the electrons return, they are in ground

state. When they are moved up, they are in excited state.

Electrons can never be exactly located, instead we say they move in orbitals.

An orbital is an approximation of the location of the electron

Each orbital can only hold so many electrons… look at your periodic table…