LecturePLUS Timberlake1 Chapter 2 Atoms and Elements Electron Arrangement and Periodic Law.

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Presentation transcript:

LecturePLUS Timberlake1 Chapter 2 Atoms and Elements Electron Arrangement and Periodic Law

What had we learned so far? Atomic Structure – –Nucleus –Electrons Essential question: how are those electrons surrounding nucleus arranged?

LecturePLUS Timberlake3 Characteristics of Electrons Extremely small mass Located outside the nucleus Moving at extremely high speeds in a sphere Have specific energy levels

LecturePLUS Timberlake4 Energy of Electrons When atoms are heated, bright lines appear called line spectra Electrons in atoms arranged in discrete levels. An electron absorbs energy to “jump” to a higher energy level. When an electron falls to a lower energy level, energy is emitted.

Experimental Evidence Discharging Tubes The Flame Test The light coming out of the excited atomic entities is very specific to particular element! Results are quite reproducible.

Experimental Evidence There has been no radioactive decay going on. Hence the nucleus does not change when the atomic entity gets excited either by electricity or heat. So the colored light must have come from those electrons. Light emitted from excited atomic entities is the tool used to probe how electrons are arranged.

LecturePLUS Timberlake7 Loss and Gain of Energy GaInGaIn LossLoss

What is light? Lights, both visible and invisible to human eyes, are electromagnetic waves. Time Out! Before we go any further, what is a Wave?

What is a wave? A wave is a means to transfer energy from point A to point B. Waves in water Sound waves Typical mechanical waves such as those in water and sound waves DO need medium in which they propagate. Water and air are the prerequisites for waves to travel.

Waves – in more the abstract form Note: great link to an online simulation of waves. on-a-string_en.html - Wavelength - distance from crest to crest abbreviated Greek letter,  pronounced “lambda”. Also can be defined as how far the wave travels in a cycle.

Waves – in more the abstract form Frequency – the number of complete waves passing any given point per second. SI unit for frequency is Hertz (Hz), or cycles/sec. Abbreviated Greek letter,  pronounced “nu”. The graph shows that the top wave passes any given point 4 complete wave forms every second; the middle one 2 complete wave forms; and the bottom one 1 complete wave form.

Radio waves Micro waves Infrared. Ultra- violet X- Rays Gamma Rays Low Energy High Energy Low Frequency High Frequency Long Wavelength Short Wavelength Visible Light

> decreasing energy  > decreasing frequency > > increasing wavelength >

Types of Spectra Continuous – all wavelengths within a given range are included. Electromagnetic – all electromagnetic radiation arranged according to increasing or decreasing wavelength. a.unit for wavelength ranges from meters to nanometers b.unit for frequency is hertz (Hz) (# waves per second)

Types of Spectra Visible spectrum - light you can see (ROY-G-BIV) a.Red has the longest wavelength and the smallest frequency. b.Violet has the shortest wavelength and the greatest frequency. Bright Line spectrum (emission spectrum)  Bands of colored light emitted by excited electrons when they return to the ground state.

Passing Light Through a Prism White light is made up of all the colors of the visible spectrum. Passing it through a prism separates the colors in white light.

If the light is not white, By heating a gas with electricity we can get it to give off colors. Passing this light through a prism does something different.

If the light is not white, Each element gives off its own characteristic colors. Can be used to identify the atom. This is how we know what stars are made of.

Spectroscopy 1.Emission spectra of a substance is studied to determine its identity. 2.Spectroscope – instrument that separates light into a spectrum. 3.Spectral lines – represent wavelength of light emitted when excited electrons fall back to the ground state.

How Does a Spectroscope Work?

Emission Spectrum (Line Spectrum)

Emission Spectrum

LecturePLUS Timberlake24 Do you get it? Answer with: 1) Energy absorbed2) Energy emitted 3) No change in energy A. What energy change takes place when an electron in a hydrogen atom moves from the first (n=1) to the second shell (n=2)? B. What energy change takes place when the electron moves from the third shell to the second shell?

LecturePLUS Timberlake25 Answers A.1) Energy absorbed B.2) Energy emitted

LecturePLUS Timberlake26 Bohr Model First model of the electron structure Gives levels where an electron is most likely to be found Incorrect today, but a key in understanding the atom