Electron Configuration, Ion Formation, and Lewis Structures

Vocabulary:

1. valence shell
2. valence electrons
3. core electrons
4. electronegativity
5. anion
6. cation
7. Lewis structure (Lewis dot diagram)

Banksy, British, 2002

Important!

• Be sure to view these notes in slideshow mode in order to fully understand them.

Electronegativity

• The protons in the nucleus are positively charged and are what attract and hold the electrons in their orbitals.

Chlorine Atom

Electronegativity

• The protons in the nucleus are positively charged and are what attract and hold the electrons in their orbitals.
• Atoms are capable of gaining electrons from other atoms if:
• there is a strong enough pull from the nucleus

Chlorine Atom

Electronegativity

• The protons in the nucleus are positively charged and are what attract and hold the electrons in their orbitals.
• Atoms are capable of gaining electrons from other atoms if:
• there is a strong enough pull from the nucleus
• there is a vacancy in the valance shell (outer shell) of the atom

Chlorine Atom

Electronegativity

• Electronegativity: The relative attraction an atom has for gaining an extra electron.
• Linus Pauling developed the concept of electronegativity and assigned relative values to each element to quantify electronegativity.

Electronegativity

• A high electronegativity (2 or higher) indicates that an atom has a tendency to gain electrons.
• A low electronegativity (lower than 2) indicates that an atom has a tendency to lose electrons.

Draw the orbital diagram for

chlorine

​

4s __ 4p ___ ___ ___

​

3s ___3p ___ ___ ___

​

2s ___2p ___ ___ ___

​

1s ___

​

17 protons

potassium

​

4s __ 4p ___ ___ ___

​

3s ___3p ___ ___ ___

​

2s ___2p ___ ___ ___

​

1s ___

​

19 protons

Valence Electrons vs. Core Electrons

Valence electrons: the s and p electrons on the highest occupied energy level (the “action” electrons)

​

Core electrons: all of the other electrons on lower energy levels

Identify the valence electrons

chlorine

​

4s __ 4p ___ ___ ___

​

3s ___3p ___ ___ ___

​

2s ___2p ___ ___ ___

​

1s ___

​

17 protons

potassium

​

4s __ 4p ___ ___ ___

​

3s ___3p ___ ___ ___

​

2s ___2p ___ ___ ___

​

1s ___

​

19 protons

Identify the core electrons

chlorine

​

4s __ 4p ___ ___ ___

​

3s ___3p ___ ___ ___

​

2s ___2p ___ ___ ___

​

1s ___

​

17 protons

potassium

​

4s __ 4p ___ ___ ___

​

3s ___3p ___ ___ ___

​

2s ___2p ___ ___ ___

​

1s ___

​

19 protons

Consequences

chlorine

​

4s __ 4p ___ ___ ___

​

3s ___3p ___ ___ ___

​

2s ___2p ___ ___ ___

​

1s ___

​

17 protons

potassium

​

4s __ 4p ___ ___ ___

​

3s ___3p ___ ___ ___

​

2s ___2p ___ ___ ___

​

1s ___

​

19 protons

Lower energy valence shell = more attracted to nucleus

Higher energy valence shell = less attracted to nucleus

Consequences

chlorine

​

4s __ 4p ___ ___ ___

​

3s ___3p ___ ___ ___

​

2s ___2p ___ ___ ___

​

1s ___

​

17 protons

potassium

​

4s __ 4p ___ ___ ___

​

3s ___3p ___ ___ ___

​

2s ___2p ___ ___ ___

​

1s ___

​

19 protons

That means easy to gain 1 more electron here and hard to lose electrons

And easy to lose an electron here and hard to gain electrons

This why chlorine has a high electronegativity...

...and potassium has a low electronegativity.

Ions

• When an atom gains or loses electrons, it becomes an ion
• If an atom loses one or more electrons, it becomes positively charged and is called a “cation” (pronounced cat - ion )
• If an atom gains one or more electrons, it becomes negatively charged and is called an “anion” (pronounced an - ion)

Summary:

• Elements that have a nearly full valence shell, like chlorine, have high electronegativities (ability to gain an electron) and can gain electrons easily.

chlorine

​

4s __ 4p ___ ___ ___

​

3s___3p ___ ___ ___

​

2s___2p ___ ___ ___

​

1s ___

​

17 protons and 17 electrons = 0 charge

Summary:

• Elements that have a nearly full valence shell, like chlorine, have high electronegativities (ability to gain an electron) and can gain electrons easily.

chlorine

​

4s __ 4p ___ ___ ___

​

3s___3p ___ ___ ___

​

2s___2p ___ ___ ___

​

1s ___

​

17 protons and 18 electrons = 1- charge

Summary:

• Atoms that have gained one or more electrons are now negative ions and are called anions.

chlorine

​

4s __ 4p ___ ___ ___

​

3s___3p ___ ___ ___

​

2s___2p ___ ___ ___

​

1s ___

​

17 protons and 18 electrons = 1- charge

Summary:

• Elements that have a nearly empty valence shell, like potassium, have low electronegativities and can lose electrons easily.

potassium

​

4s __ 4p ___ ___ ___

​

3s___3p ___ ___ ___

​

2s___2p ___ ___ ___

​

1s ___

​

19 protons + 19 electrons = 0 charge

Summary:

• Atoms that have lost one or more electrons are now positive ions and are called cations.

potassium

​

4s __ 4p ___ ___ ___

​

3s___3p ___ ___ ___

​

2s___2p ___ ___ ___

​

1s ___

​

19 protons + 18 electrons = 1+ charge

Notice

chlorine anion (1- charge)

​

4s __ 4p ___ ___ ___

​

3s___3p ___ ___ ___

​

2s___2p ___ ___ ___

​

1s ___

​

potassium cation (1+ charge)

​

4s __ 4p ___ ___ ___

​

3s___3p ___ ___ ___

​

2s___2p ___ ___ ___

​

1s ___

​

When each atom gained or lost electrons, the new outer shell (energy level) is full, like the noble gases.

The Octet Rule

Atoms lose or gain electrons in order to have a full valence shell of 8 electrons (s²p⁶, a noble gas electron configuration).

Short Cut

• Using periodic trends, we can predict the number of valence electrons for the main group elements.

1

2

3

4

5

6

7

8

An easier way!

Rather than draw orbital diagrams, we can use Lewis Structures (Lewis Dot Diagrams) to show numbers of valence electrons.

Cl

1. Write symbol of element.
2. Determine number of valence electrons by position on periodic table
3. Use dots to represent electrons around symbol (one at a time on each side and then pair them up.)

7 valence electrons because in group 17

An easier way!

Cl

So when chlorine forms an ion, will it gain or lose electrons? How many?

It needs one more to have an octet.

What will its charge be now?

1-

Lewis Structure Practice: potassium

K

1. Write symbol of element.
2. Determine number of valence electrons by position on periodic table
3. Use dots to represent electrons around symbol (one at a time on each side and then pair them up.)

1 valence electron because in group 1

Lewis Structure Practice: potassium

K

So when potassium forms an ion, will it gain or lose electrons? How many?

It will lose one electron to have a full valence

What will its charge be now?

1+

Lewis Structure Practice: nitrogen

N

1. Write symbol of element.
2. Determine number of valence electrons by position on periodic table
3. Use dots to represent electrons around symbol (one at a time on each side and then pair them up.)

5 valence electrons because in group 15

Lewis Structure Practice: nitrogen

So when nitrogen forms an ion, will it gain or lose electrons? How many?

It will gain three electrons to have a full valence

What will its charge be now?

3-

N

Lewis Structure Practice: nitrogen

When a sulfur atom forms an ion, what charge will it have?

It will gain two electrons to have a full valence

What will its charge be now?

2-

S

Short Cut

• Using periodic trends, we can predict the common ion charges for the main group elements.

1+

2+

3+

3-

2-

1-

0