Electrons in AtomsChapter 2 Section 2

Objectives How does the electron relate to the

modern atomic theory? How do electron energy levels in an atom

differ from one another? How are Lewis electron dot diagrams used

to illustrate valence electrons?

Important Vocabulary Electromagnetic spectrum Emission spectrum Energy level Electron cloud Valence electrons Lewis dot diagrams Wavelength Frequency

Electrons Motion & Energy Electrons have enough energy to keep

them in constant motion around the nucleus

This enables them to overcome the attraction of the positive nucleus

Electrons occupy orbitals of only certain amounts of energy

For them to move up a level of energy, energy must be added

For them to move down a level of energy, energy must be released in the form of light or heat

Waves Transfer Energy Energy is the ability to exert a force over a certain

distance It is also the ability to do work Waves carry energy because they can do work For example:

Water waves can transfer energy to a leaf, to a boat, or onto a beach

Sound waves can transfer energy to your eardrum

Light waves can transfer energy to your eye The bigger the wave the _______ energy it carries

Electromagnetic Radiation Electromagnetic radiation travels

in the form of waves that have both electric and magnetic properties

Electromagnetic waves travel through a vacuum at the speed of light (300 million m/s)

Two properties of waves are frequency and wavelength

Wavelength & Frequency• Wavelength is the distance from one crest to

the next• Frequency is the number of waves per second• A low frequency results in a long

wavelength and a high frequency results in a shorter wavelength

Electromagnetic Spectrum

The electromagnetic spectrum consists of electromagnetic radiation waves at all possible energies, frequencies, and wavelengths

The spectrum ranges from 103 m to 10-12 meters

Each part of the electromagnetic spectrum has unique properties

Radio Waves

Heinrich Hertz proved the existence of radio waves in the late 1880s

Radio waves have wavelengths that range from 200 to 600 m

Have the longest wavelengths and the lowest energy

They are used as TV signals, AM and FM radio signals, and for radar equipment

Radio telescopes view planets, comets, giant clouds of gas and dust, stars, and galaxies

Microwaves

Are low energy, low frequency radiation waves

They are used in Doppler radar for weather forecasting and to cook your food

Microwaves are also used to carry telecommunication signals

Most mobile phones use microwaves to transmit information, and space probes transmit signals back to Earth with microwaves

Different wavelengths of microwaves (grouped into "sub-bands") provide different information to scientists.

Infrared Waves In 1800, William Herschel discovered

them Have less energy than visible light Are given off by the human body

and other warm objects We experience infrared rays as heat

from fires and electric heaters Through night-vision goggles and

infrared thermal cameras we can see infrared waves

Visible Spectrum Cone-shaped cells in our eyes act as

receivers tuned to the wavelengths in this narrow band of the spectrum

A typical human eye will respond to wavelengths from about 380 to 750 nm

The spectrum does not contain all the colors that the human eye and brain can distinguish

Unsaturated colors such as pink, and purple colors such as magenta, are absent because they can only be made by a mix of multiple wavelengths

When white light shines through a prism, the white light is broken apart into the colors of the visible light spectrum

Water vapor in the atmosphere can also break apart wavelengths creating a rainbow

Ultraviolet Waves In 1801, Johann Ritter discovered them Have shorter wavelengths than visible

light UV waves are invisible to the human

eye, but some insects, such as bumblebees, can see them. 

The Sun is a source of the full spectrum of ultraviolet radiation, which is commonly subdivided into UV-A, UV-B, and UV-C

X-rays X-rays were first observed and documented in

1895 by German scientist Wilhelm Conrad Roentgen

X-rays have very small wavelengths, between 0.03 and 3 nanometers

X-rays are used by doctors to see the internal structures of the body

X rays have very high energies, so they may kill living cells or turn them into cancer cells when exposed to too much of this type of radiation

Gamma Rays Have the highest energy and shortest

wavelengths They are produced by the hottest and

most energetic objects in the universe, such as neutron stars and pulsars, supernova explosions, and regions around black holes

On Earth, gamma waves are generated by nuclear explosions, lightning, and the less dramatic activity of radioactive decay

Gamma rays can be used to treat cancer by killing the diseased cells

Electrons and Light When electrons become excited they give

off light The spectrum of light released is called the

emission spectrum Each element has a different emission

spectrum So it serves as evidence of energy levels

within atoms

Energy Levels There are seven levels of energy

available for electrons to occupy Electrons can move between energy

levels like the rungs of a ladder absorbing or releasing energy

The number of filled energy levels depends on the number of electrons

Lower levels are filled first

Energy Levels 1st energy level = 2 electrons 2nd energy level = 8 electrons 3rd energy level = 8 electrons 4th energy level = 18 electrons

For example: Na has 11 electrons 2 in level 1 8 in level 2 1 in level 3

Valence Electrons Every atom has between 1 and 8 valence electrons Valence electrons are electrons in the outermost

energy level of an atom They determine an atom’s chemical properties and

its ability for form bonds For example: Neon

Has 10 electrons2 electrons in the lowest level8 electrons in the 2nd level

Thus, it has 8 valence electrons!

Lewis Dot Diagrams Valence electrons are usually the only

electrons used in chemical reactions We represent valence electrons as

Lewis dot diagrams, which illustrate the valence electrons of an element as dots

Normally, all the elements within a group have the same electron dot structure with the exception of Helium

Lewis Dot Diagrams