Section 2.3 Notes
Periodic Table and Atomic Theory
(Text Pages 60 – 67)
Key Terms:
Bohr-Rutherford Diagram
Energy Levels
Valence Energy Level
Valence Electrons
Learning Objectives:
Draw Bohr-Rutherford diagrams for elements 1-18
Know how many electrons can be placed in each of the first four energy levels (2, 8, 8, 18)
Be able to identify the valence energy level and number of valence electrons for elements 1-18
Recognize patterns in energy level diagrams for elements in the same period and elements in the same group.
_____________________________________________________________________________________ Portfolio Items:
Worksheet #11 – “Electron Diagrams” - Text Questions:
_____________________________________________________________________________________
Recall:
- The atomic # for an element tells you the number of protons (p+) and electrons (e-) in an atom of that element
- Electrons are found outside the nucleus of an atom - Neils Bohr said that electrons are found in energy levels.
In this section we will learn about the way that electrons are organized in energy levels and how this influences the reactivity of different elements.
Structure of an Atom: Energy Levels
Energy Level: a region surrounding the nucleus of an atom that may be occupied by one or more electrons They are also called electron shells.
Important points about Energy Levels
The period # of an element tells you how many energy levels its atoms have. Ex: Carbon has ______ energy levels with electrons in them.
Ex: Sulfur has ______ energy levels with electrons in them.
Energy levels can be numbered from 1 to 7.
Electrons in higher energy levels are further from the nucleus. Electrons in higher energy levels have more energy.
The highest energy level in an atom that has electrons in it is known as the valence energy level. Any electrons in that level are known as valence electrons.
Not all energy levels can hold the same number of electrons. The rules below outline how many electrons each level can hold below.
Energy Level Max. Number of Electrons
1st Level 2
2nd Level 8
3rd Level 8
4th Level 18
*You only need to know about four levels for our purposes.
Electrons enter these energy levels starting closest to the nucleus.
Bohr-Rutherford Diagrams
Bohr-Rutherford Diagram: shows the arrangement of the subatomic particles in an atom. It shows the number of protons and neutrons in the nucleus. It also shows the number of electrons in each energy level.
Drawing Bohr-Rutherford Diagrams
Step 1: Draw a circle to represent the nucleus. Write the element symbol inside at the top of the circle.
Step 2: Find atomic number gives # of electrons (e-) and # of protons (p+)
Step 3: Calculate # of neutrons (no) using Mass# - Atomic#
Step 4: Label inside the nucleus with # of p+AND # of no (write these below the element symbol!)
Step 5: Fill the energy levels using the 2,8,8, 18 energy level pattern for electrons. Each energy level looks like a horizontal line with # of e- written inside it: --5e--
Sample Bohr-Rutherford Diagram: Sodium
**Complete Worksheet #11 up to and including #5.**
Energy levels. Must
have total of 11 e-
same as atomic #
This leaves 1 e- to go in the valence energy
level. There is only 1 valence electron
1e
---- 8e ----
The next 8 e- go in the 2
ndenergy level
2e
--Fil the lowest energy level first. Up to 2
e- can go in here.
Na
p
+ =11
Neutrons = Mass # - Atomic #
= 23 – 11
= 12
Valence Electrons and Reactivity
There are patterns in the periodic table for the number of energy levels and valence electrons found in different elements.
Elements in the same family (group) have the same number of valence electrons. This explains why they have similar properties.
The number of energy levels in the atoms of an element increases as you move down a group. Elements in the same period have electrons in the same valence energy level.
Valence electrons are very important because the number of valence electrons determines how reactive an element will be.
If an element has a full valence energy level, it will not be reactive. We say that it is stable. This helps explain why the Noble Gases are stable.
Example: Neon
Elements that do not have full valence energy levels will react with other elements in order to become stable! These reactions involve gaining, losing, or sharing electrons!
1 2
3 4 5 6 7 8 9 10
11 12 13 14 15 16 17 18
1. What is similar about the valence energy level for the elements of the Alkali Metals Family? _
2. What is similar about the valence energy level for the elements of the Alkaline Earth Metal Family?
_
3. What is similar about the valence energy level for the elements of the Halogen Family?
_
4. What is similar about the valence energy level for the elements of the Noble Gases?
_
5. Describe the change in valence electrons as you move across a period of elements. _
6. Which of the elements below is stable? Which is unstable? Explain how you know. o Magnesium, Argon, Carbon
_
7. How many electrons can each of the first three energy levels hold? ___________________________ 8. Give an example of an element with
9. Which element has a full valence energy level without having 8 electrons in it? ________________ 10. Identify each element based on the description provided:
a. Period 2 element with 3 valence electrons: ______Boron_________ b. Alkali metal with valence electrons in the third energy level: ______Sodium_________ c. Period 3 element with 7 valence electrons: ______Chlorine_________ d. Alkaline earth metal with two energy levels: ______Beryllium_________ e. Has 6 valence electrons in the second energy level: ______Oxygen_________
f. A stable element in the third period: ______Argon_________
g. Has one complete energy level: ______Helium_________
11. Identify the valence energy level and number of valence electrons for each:
Valence Energy Level # of Valence Electrons
a. Silicon _____ _____
b. Lithium _____ _____
c. Hydrogen _____ _____
d. Nitrogen _____ _____
e. Calcium _____ _____
f. Krypton _____ _____
g. Iodine _____ _____
