Unit II—Part 3

Atom: the smallest unit of matter that retains the identity of the substance

First proposed by Democritus around 420 B.C.

“Atomos” in Greek means indivisible

Atoms are composed of 2 regions: Nucleus: the center of the atom that

contains the mass of the atom Electron cloud: region that surrounds the

nucleus that contains most of the space in the atom

NucleusElectron

Cloud

The nucleus contains 2 of the 3 Subatomic Particles: Protons: positively (+) charged subatomic

particles Neutrons: neutrally ( ) charged subatomic

particles

The 3rd Subatomic Particle resides outside of the nucleus in the electron cloudElectron: the subatomic particle

with a negative charge and relatively no mass

Protons and neutrons live compacted in the tiny positively charged nucleus accounting for more than 99% of the mass of the atom

The negatively charged electrons are small and have a relatively small mass but occupy a large volume of space outside the nucleus

In an atom:

The # of protons = the # of electrons If 20 protons are present in an atom then

20 electrons are there to balance the overall charge of the atom—atoms are neutral

The neutrons have no charge; therefore they do not have to equal the number of protons or electrons

Atomic number: this number indicates the number of protons in an atom Ex: Hydrogen’s atomic number is 1

So hydrogen has 1 proton Ex: Carbon’s atomic number is 6

So carbon has 6 protons

**The number of protons identifies the atom. Ex. 2 protons = He, 29 protons = Cu

Atomic Math Challenge

8

O

Oxygen

15.99

Atomic Number

Element Symbol

Element Name

Atomic Mass

http://www.videopediaworld.com/video/16322/Chemistry-of-Life-Atoms-and-Elements

http://www.videopediaworld.com/video/16322/Chemistry-of-Life-Atoms-and-Elements

Mass number: the number of protons and neutrons in the nucleus

Ex: Hydrogen can have a mass of 3. Since it has 1 proton it must have 2 neutrons

# of neutrons = mass # - atomic #

Li has a mass number of 7 and an atomic number of 3 Protons = 3 (same as atomic #) Neutrons= 7-3 = 4 (mass # - atomic #)

Ne has a mass number of 20 and an atomic number of 10 Protons = 10 Neutrons = 20 - 10= 10

The electrons are equal to the number of protons So e- = p+ = atomic #

Ex: He has a mass # of 4 and an atomic # of 2 p+ = 2 no = 2 e- = 2

Cl has a mass # of 35 and an atomic # of 17 p+ = 17, no = 18, e- = 17

K has a mass # of 39 and an atomic # of 19 P+ = 19, no = 20 e- = 19

H:\Chemistry Websites\lsps07_int_theatom\Shortcut to lsps07_int_theatom.lnk

Pass out the student worksheet from “The Atoms Family” Lesson

Atomic Math Challengeat this time and complete in class

Bohr Model of the atom:Reviewers think this could lead to misconceptions!

All of the protons p+ and neutrons n are within the nucleus

The 1st energy ring can hold up to 2 e-

The 2nd energy ring can hold up to 8 e-

The 3rd energy ring can hold up to 18 e-

The 4th energy ring and any after can hold up to 32 e-

Mass # = 12 atomic # = 6

p+ = 6 no = 6 e- = 6

6 p and 6 n live in the nucleus

Bohr Model Practice

Lithium

Atomic # 3

Atomic Mass 7

Protons 3

Neutrons 4

Electrons 3 # Electrons

1st energy ring / level 2

2d energy ring / level 1

3d energy ring / level

4th energy ring / level

P+ = 3

N = 4

Bohr Model Practice

Aluminum

Atomic #

Atomic Mass

Protons

Neutrons

Electrons

# Electrons

1st energy ring / level

2d energy ring / level

3d energy ring / level

4th energy ring / level

13

27

13

14

13

2

8

3none

P+ = 13

N = 14

Hand out the Bohr Model Practice worksheet and

assign 8 different atoms for students to complete their

models