Electrons Date your notes 10/31/2012 or 11/1/2012

How many electrons do atoms have? The number of protons and the number of electrons determine the charge of an atom Unless you are given a charge, assume that your atom is neutral Therefore, the number of electrons can be found…

Niels Bohr Danish physicist, 1913 Worked with Ernest Rutherford Studied the emission spectrum of Hydrogen to learn about placement of electrons in atoms

Emission Spectra When electricity is passed through a gas, the atoms will emit lines of different frequencies, which serves as its “fingerprint” This is very similar to when light is passed through a prism.

Electrons exist in orbits that contain a certain amount of energy (energy levels) Similar to planets orbiting the sun Bohr’s Theory

nucleus Energy levels/electron orbits get larger as you move farther away from the nucleus Electrons will exist in the lowest possible energy level—we call this “ground state” Bohr’s model

The first energy level is the smallest. It can only hold 2 electrons Bohr’s model

The second energy level is larger. It can hold up to 8 electrons Bohr’s model

The third energy level is slightly larger. It can hold up to 8 electrons Bohr’s model

The fourth energy level is much larger. It can hold up to 18 electrons Bohr’s model

1st energy level holds up to 2 electrons 2nd energy level holds up to 8 electrons 3rd energy level holds up to 8 electrons 4th energy level holds up to 18 electrons 5th energy level holds up to 18 electrons 6th energy level holds up to 32 electrons 7th energy level holds up to 32 electrons Summary of Energy Levels

Radon

Valence Electrons Energy levels do not have to be filled. If an atom has only 7 electrons: 2 go on the 1 st energy level 5 go on the 2 nd energy level. The electrons on the outermost energy level are called valence electrons

Sulfur 6 valence electrons

Practice Bohr Models – make a Bohr model for each element and identify how many valence electrons the atom has. 1.Oxygen 2.Aluminum 3.Potassium 4.Calcium

When electrons are in the lowest possible energy level, they are in their ground state Energy and Electrons

Electrons can absorb a quantum of energy and move to a higher energy level. This is called the excited state.

A quantum is a unit of energy. It is the amount needed to send an electron to the next higher energy level

This is unstable, and electrons quickly return to ground state When electrons return to the ground state, they release the quantum of energy they absorbed. It is released as a photon of visible light.

This is unstable, and electrons quickly return to ground state When electrons return to the ground state, they release the quantum of energy they absorbed. It is released as a photon of visible light.

Electrons can absorb a larger quantum of energy and move to an even higher energy level to a higher excited state.

When electrons return to the ground state, they release the quantum of energy they absorbed. It is released as a photon of visible light. This is unstable, and electrons quickly return to ground state

This electron absorbed a greater amount of energy, so it releases a photon of light with higher energy. It will be a different color.

Flame Test We can see the result of electrons moving back down from an excited state to a ground state, when we provide the energy to move them to an excited state. What you will do: You will not finish today—test two or three. We will finish on Tuesday.

Your Turn Brainstorm for 2 minutes – what do you know about light?

Properties of Light Light travels as a wave Light travels at the speed of light 3.00x10 8 m/s Light is electromagnetic radiation Visible light is a very small part of the EMR spectrum ROYGBIV Blue light has the most energy; red light has the least energy

Properties of Light Light is made of photons –It is a particle –It travels as a wave of energy –It is both at the same time—wave-particle duality Speed of light = wavelength of light x frequency of light c = fλor c = λν (lambda x nu) c = 3.00x10 8 m/s Energy of a photon = Planck’s constant x frequency of light E photon = hν or E photon = hf h=Planck’s constant = 6.63x J*s

Light and Electrons Electrons can absorb varying sized quanta and “jump” to varying excited states They also “drop” from excited state to ground state and release a different amount of energy and a different color of light In any given sample of an element, all possible jumps and drops are taking place Not all photons are in the visible light range—UV and infrared

Practice Radiation from outer space is detected that has a frequency of 94.7 MHz. What is the wavelength of this radiation, and what type is it?

Practice What is the energy of a photon of red light that has a frequency of 4.48 x Hz?

Practice A student has both a red laser pointer and a green laser pointer. Which has the greater amount of energy? Red light wavelength = 670 nm Green light wavelength = 530 nm