Chemistry: Inside the Atom

Part 3: The Electron

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Electron Arrangement

• Remember in a neutral atom there are the same number of electrons as protons!!!– Electrons: Travel around the nucleus in paths

called orbits (they are negatively charged)– All the electron paths put together make and

electron shell (electron cloud)

Electron Arrangement

• Bohr Model

1 P0 N

Nucleus

Electron

Electron Shell

Bohr Model

• This model focuses on the electrons:– Electrons move in paths around the nucleus of the

atom called Energy Levels.– Electrons are found ON energy levels– The farther the energy level from the nucleus, the

greater the energy of the electrons there.

Bohr Model Continued…

• Atoms can give off or absorb energy– Electrons can move from 1 energy Level to

another– Adding energy (absorbing) = Electrons moving

farther from the nucleus– Giving off energy = Electrons moving closer to the

nucleus.

Bohr Model

Nucleus

Adding Energy- Electrons moving farther from the

nucleus

ELECTRON

Bohr Model

Nucleus

Giving Off Energy- Electrons moving

closer to the nucleus

ELECTRON

Electron ShellsPeriod on the Table of Elements

Number of Electron Shells for an atom in that period

1 12 23 34 45 56 67 7

Example: The element Nitrogen is in the 2nd period so it has two electron shells

Electron Shells• These shells are represented with a letter– K through Q• Look at diagram on the board

– Each Electron shell can hold only a certain number of electrons:

Electron Shell How many electrons it can hold

K 2L 8M 18N 32

Inner and Outer Shells

• A “K” shell is complete when it has 2 Electrons. A

“L” shell is complete when it has 8 electrons.• “M” and “N” shells can hold 18 and 32 electrons

when they are inner shells. When an “M” or “N” shell is an outer shell it can hold only 8 electrons.

• An outer shell can never hold more than 8 electrons.• The electrons in the outer shell are called Valence

Electrons