ELECTRONS IN THE ATOM

UNIT 4

OBJECTIVES

1. Explain how atomic emission spectra can be used to identify elements

2. Describe Bohr’s model of the atom.3. Describe the Quantum Mechanical

model of the atom4. Write elements’ electron

configurations.

HOW DO WE KNOW WHAT THE STARS ARE MADE OF?

ATOMIC EMISSION SPECTRA• When an element is heated, its atoms absorb

energy and become excited• To become stable again, these excited and

unstable atoms then release the energy as light• If this light is passed through a prism the

element’s atomic emission spectrum is produced

ATOMIC EMISSION SPECTRA• An element’s atomic emission spectrum is the

set of wavelengths (colors) of light given off when atoms of that element are excited (e.g. heated)

• Each element’s emission spectrum is unique and can be used to identify the element– It is the element’s “fingerprint”

HOW DO WE KNOW WHAT THE STARS ARE MADE OF?

• Scientist analyze the light from a star using spectroscopes (similar to powerful prisms)

• Match the frequencies of light to the known spectra of the elements

• Stars are made of the same stuff as the rest of the Universe: 73% hydrogen, 25% helium, and the last 2% is all the other elements

LIGHT• Visible light is a type of electromagnetic radiation• All other electromagnetic radiation is invisible

• Electromagnetic (EM) radiation is energy that travels through space in the form of electromagnetic waves

• The electromagnetic spectrum encompasses all forms of electromagnetic radiations

increasing energy

BOHR’S MODEL OF THE ATOM• Bohr studied the emission spectrum

of hydrogen and developed his model of the atom

• The Bohr model describes the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around it

THE BOHR MODEL OF THE ATOM

• Each orbit or “ring” has a distinct energy levels or quantum number (n)– the bigger the number

the higher the energy

• Electrons in smaller orbits closer to the nucleus have less energy than electrons found in larger orbits farther from the nucleus

BOHR’S ATOM CONTINUED• The lowest energy state of an atom is its ground

state• When an atom gains energy (through heating for

example) it is in an excited state• in an excited state the electron absorbs the energy &

jumps to higher energy level • when it falls back down to its ground state it releases

excess energy in the form of light

• Because electrons jump between orbitals that have specific energy levels only certain colors can be given off

• This is how Bohr explained hydrogen’s emission spectrum

BOHR MODEL CONTINUED

Transitioncolor of light

emitted

n = 3 to n = 2 red

n = 4 to n = 2 blue-green

n = 5 to n = 2 blue

n = 6 to n = 2 violet

Wait!• Bohr’s model explained the emission

spectrum of Hydrogen, but it did not explain the emissions of any other element!

THE QUANTUM MECHANICAL MODEL OF THE ATOM

1. Electrons behave like waves2. It is impossible to know the

exact location or the velocity of an electron in an atom • (they don’t travel in circular

orbits around the nucleus)

3. Although it’s impossible to describe the exact location or describe how they are moving, the model describe the probability that electrons will be found in certain locations around the nucleus

ATOMIC ORBITALS• An atomic orbital is

a three-dimensional pocket of space around the nucleus that the electron is most likely to be found

• An electron has a 90% chance of being found within that space

• That is the best we can do!

ATOMIC ORBITALS

ORGANIZATION OF ATOMIC ORBITALS1. Principal energy level

(n)

2. Energy Sublevel

3. Orbitals

value: n = 1-7 s, p. d, f 1, 3, 5, 7

description:

-(n) indicates relative size and energy of orbital -As (n) increases so do energy and

size

-sublevels are labeled according to

shape:s: spherical p: dumbbell d/f: varied

-each sublevel has

a certain number of orbitals:

s = 1p =3d =5f = 7

-each orbital can hold two

electrons

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ELECTRON CONFIGURATION

• An atoms electron configuration is the way an atom’s electrons are distributed among the orbitals of an atom

• The most state stable electron configuration is an atom’s ground state– Ground state: all electrons are in the lowest

possible energy state

• Electron configuration represented by writing symbol for the orbital and a superscript to indicate the number of electrons in the orbital Li: 1s2 2s1

increasingenergy

Energy

Each orbital can hold two electrons

2p

3p

1s

2s

3s

4s

5s4p

3d

4d

He Helium

2

4.003H

Hydrogen

1

1.008Li Lithium

3

6.941Be

Beryllium

4

9.012B

Boron

5

10.81C

Carbon

6

12.01N

Nitrogen

7

14.01O

Oxygen

8

16.00F

Fluorine

9

19.00Ne

Neon

10

20.18

The Pauli Exclusion Principle• The two electrons in an orbital

must spin in opposite directions

1s 2p2s

3s 3p

3d4s

HUND’S RULE• Negatively charged electrons repel each other, so:

– Electrons won’t pair up unless they have to– Once there is one electron in every orbital…the

pairing will begin!

2s1s

2p

1.

2s1s

2p

4.

2s1s

2p

2.

2s1s

2p

3.

Add an electron:

Add an electron:

Add an electron:

DRAW THE ORBITAL DIAGRAM AND WRITE THE

ELECTRON CONFIGURATION FOR:• Carbon

• Helium• Potassium

ELECTRON CONFIGURATION• The periodic table can be divided into four

distinct blocks based on valence electron configuration

• electron configuration explain the recurrence of physical and chemical properties

SHORTHAND (NOBLE GAS) NOTATION

• Shows electron filling starting from previous noble gas:– Na: 1s22s22p63s1

– Noble gas configuration: [Ne]3s1

WRITE THE FOLLOWING ELECTRON CONFIGURATIONS

IN NOBLE GAS NOTATION:• Fluorine• Titanium• Beryllium