2.3 – The Periodic Table and Atomic Theory

Key Ideas - Outcomes

• You will need to know how to draw a Bohr diagram for the first 18 elements on the periodic table

• Electrons are arranged in a specific pattern

• The outermost shells are called valence electrons

• Determine the difference between an atom and an ion

• Use atomic structure to explain why elements behave differently

Valence Electrons

• We will go into more detail but you will need to know:

• Alkali metals have one valence electron

• Alkaline Earth Metals have 2 valence electrons

• Halogens have 7 valence electrons

• Noble gases have filled valence shells (8 valence electrons)

Periodic Table

• The periodic table is full of patterns

• Elements that share properties line up in columns because they share the same arrangement of electrons

• One way to show the arrangement is with a Bohr diagram

Bohr Diagrams

• A Bohr model shows how many electrons are in each shell surrounding the nucleus

• The regions surrounding the nucleus of an atom are often called electron shells

Bohr Diagrams

• Niels Bohr described how the electrons are arranged in the shells around a nucleus in an atom

• His theory has 3 ideas:

1. Electrons move around the nucleus in shells

2. Each shell is a certain distance away from the nucleus and can hold a definite number of electrons

3. After the shell closest to the nucleus is full, electrons start filling the next shell

• Draw a circle and put the symbol and number of protons and neutrons inside of it

• Add the appropriate number of shells (this is determined by the period number)

• EX: Magnesium is in the 3rd period so it will have 3 shells

How to draw an atom:

Mg 12p+

12no

There are now 3 shells in total

Filling up Electron Shells

1. Maximum of 2 electrons in the first shell

2. Maximum of 8 electrons in the next 2 shells

3. Maximum of 18 electrons in the 4th shell and all shells after that (we won’t be making these!!)

4. Electrons occur in pairs

5. There must be 4 single electrons before the electrons will occur in pairs

• Remember the pattern: 2, 8, 8, 18

• This pattern applies to all atoms although not all atoms have that many electrons

Electron Shells: 3-D Outside the Nucleus

• The way an element chemically reacts depends on the number of electrons in it’s outer shell.

• Atoms are stable when their outer shells are full of electrons.

• If a shell is not full, the atom is reactive, and it either wants to fill its outer orbital or get rid of it altogether.

Shell

(orbital)

Maximum

# of e-

1 2

2 8

3 8

4 18

Just use this table for our purposes in grade 9

Magnesium example

• EX: Magnesium has 12 electrons (from atomic number)

• So, 2 electrons in the first shell, 8 in the second and 2 in the last

• Watch how the electrons are added (up, down, side to side)

Examples when outer shell is not full

Look at the outer shell of each:

• Outer shells not full.

• Reactive!

Fluorine

Lithium

Lithium will donate 1 e- and Fluorine borrow 1 e-

Bohr Models of the First 18 Elements

Valence Electrons

• Valence electrons are found in the outermost shell.

• Most elements in the same family have the same number of valence electrons

Examples: Halogens = seven valence electrons Alkali = one valence electron Alkali Earth metals = two valence electrons Noble Gases - are stable and non-reactive

because they are full in the outer most valence shell.

• This is a fancy way of saying “outer shell electrons”

• If you are ever asked for the number of valance electrons, they are just asking how many electrons are found in the last, outermost, shell only

• EX: in the previous slide, the magnesium atom has 2 valence electrons

Valence Electrons

• Atoms are elements as you see them on the Periodic Table: with equal amounts of p+ and e-

• EX: sodium atom has 11 p+ and 11 e-

• Ions are elements that have gained/lost electrons and now have a charge

• EX: sodium ion has 1+ charge because it has lost 1 e-

Atoms VS Ions

State if the following are Atoms or Ions

Na K H+ Cl- Ca

O2- Ne Be2+ Al N3-

Anything with a charge is an ion, all the others are atoms

Practice!

To determine the number of electrons in an ion you need to subtract the charge to the atomic number

◦ EX: Calcium has an atomic number of 20 and a 2+ charge.......so, its ion has 20 - 2 = 18 electrons

◦ EX: Fluorine has an atomic number of 9 and a 1- charge.......so, its ion has 9 – (-1) = 10 electrons (remember subtracting a negative is like adding!)

Ions and Electrons