Matter All matter is composed of atoms. Atoms are made up of electrons, protons and neutrons. Electrons and protons are electrically charged, neutrons are, well, neutral. The unit of electric charge is the charge of one electron (1.6x10-19 Coulombs). This charge is negative. The charge of a proton is of the same magnitude, but the proton is positively charged. Electrons, protons and neutrons are all made up of smaller particles called quarks, however, these quarks do not exist independently, so the smallest divisible parts of matter are electrons and protons. The mass of an electron is 9x10-31 kg. The proton is 1836 times heavier, 1.67x10-27 kg. In further calculations, the mass of a proton will be considered 1u, where 1u = 1.67x10-27 kg. Atoms can have any number of electrons, protons and neutrons. But not all combinations are stable, and many combinations do not exist in nature.

Bohr Atom Niels Bohr (1913) was the first one to explain all the experimental evidence about the atoms in a single model. The positively-charged protons and neutral neutrons are found in the nucleus. The electrons move around the nucleus in certain specific orbits. In a neutral atom the number of electrons = the number of protons. All atoms with the same number of protons in the nucleus are called an element. When an atom has an extra positive or negative charge, than it is called an ion. Bohr model is only a model to illustrate to students how the atomic structure works, and it is only an illustration, not how nature really works.

Isotopes Elements are divided into sub-groups called isotopes based on the number of protons AND neutrons in the nucleus. All atoms of an element with the same number of neutrons in the nucleus are of the same type of isotope. There are 339 naturally occurring isotopes on Earth, 268 of these are stable. There are more than 3100 isotopes known, most of them are created artificially.

Stability of Isotopes Many of the artificially created isotopes decay immediately. Stable isotopes do not decay into something else for a long time. Depending on our purposes, long may mean years, it may also mean billions of years. 40K + e- → 40Ar + νe

Bohr Atom - continued In this model electrons have only certain energies allowed corresponding to particular distances from nucleus. As long as the electron is in one of those energy orbits, it will not lose or absorb any energy. The orbits closer to the nucleus have lower energy. Electrons want to be in the lowest possible energy state called the ground state. However, the electron can go to a higher orbit after absorbing energy, or the electron can go to a lower orbit after emitting energy. When you heat the atoms, electrons go to higher orbits. When they cool off, they give the energy back by radiating electromagnetic waves.

Why do we care? Earth receives almost all of its energy from the Sun in the form of electromagnetic radiation. Earth also radiates its energy also in the form of electromagnetic waves. To be able to say anything about the climate, we must understand how this radiation is created and how it is radiated.

Electric and Magnetic Fields Electrical charges and magnets alter the region of space around them so that they can exert forces on distant objects. James Clerk Maxwell proposed that if a changing magnetic field can make an electric field, then a changing electric field should make a magnetic field. A consequence of this is that changing electric and magnetic fields should trigger each other and these changing fields should move at a speed equal to the speed of light. Maxwell also said that light is an electromagnetic wave.

Electromagnetic Radiation Light, electricity, and magnetism are manifestations of the same thing called electromagnetic radiation. This energy also comes in many forms that are not detectable with our eyes such as infrared (IR), radio, X-rays, ultraviolet (UV), and gamma rays.

Properties of EM Radiation It can travel through empty space. Other types of waves need some sort of medium to move through: water waves need liquid water and sound waves need some gas, liquid, or solid material to be heard. The speed of light is constant in space. All forms of light have the same speed of 300,000 kilometers/second in space (often abbreviated as c). The forms of light are Gamma rays, X-rays, Ultraviolet, Visible, Infrared, Radio. A wavelength of light is defined similarly to that of water waves---distance between crests or between troughs. Visible light (what your eye detects) has wavelengths 4000-8000 Ångstroms. 1 Ångstrom = 10-10 meter. Visible light is sometimes also measured in nanometers: 1 nanometer = 10-9 meter. Radio wavelengths are often measured in centimeters. The abbreviation used for wavelength is the greek letter lambda .

Intensity and Energy The energy of the EM radiation depends only on the wavelength (frequency), the shorter the wavelength, the higher the energy (blue is more energetic than red). The type of EM radiation produced by an object will also depend on its energy (temperature). Temperature is a measure of the random motion (or energy) of a group of particles. Higher temperature (T) means more random motion (or energy).

Spectrum of visible light Hot objects give thermal spectrum (continuous spectrum). White light has a continuous spectrum. Even though all the colors are present in the spectrum we can still see a different color. Continuous spectrum Discrete spectrum Emission spectrum Absorption spectrum

Emission Spectrum An emission line is produced by an atom in an “excited” energy state. An electron in an excited energy state is not where it wants to be. It wants to lose energy and go to a lower energy orbit. In order to go to a lower energy orbit, the electron must lose energy of a certain specific amount, because only energy states with a specific energy are available to be filled. The energy of photon = the difference in energy of these energy orbits. The intensity depends on the density and temperature of the gas.

Absorption Spectrum An absorption line is produced when a photon of just the right energy is absorbed by an atom, kicking an electron to a higher energy orbit. Other photons moving through the gas with the wrong energy will pass right on by the atoms in the thin gas. They make up the rest of the continuous spectrum you see. The more atoms undergoing a particular absorption transition, the darker the absorption line. The strength of the absorption line depends on the density and temperature.

Blackbody Radiation A black body is an object that absorbs all light that falls on it. No electromagnetic radiation passes through it and none is reflected. Because no light is reflected or transmitted, the object appears black when it is cold. If the black body is hot, these properties make it an ideal source of thermal radiation. Thermal radiation: Gives out electromagnetic radiation proportional to its temperature. The warmer the object, the more radiation it gives out. Every object we know is to some extent a blackbody. However, we can only see these objects when the radiation they give out is in the region where our eyes can see -> when they are very hot.

What we cannot see! Much of a person's energy is radiated away in the form of infrared energy. Some materials are transparent to infrared light, while opaque to visible light (note the plastic bag). Other materials are transparent to visible light, while opaque or reflective to the infrared (note the man's eyeglasses). This is basically the reason for the Greenhouse Effect. Greenhouses are covered with glass and just like the man’s glasses, they let the visible light through, but does not allow the infrared to pass (not entirely correct, but we will assume this for the moment).

Temperature Dependence of Spectra The shape of the spectrum depends only on the temperature of the object. As the temperature of an object increases, more light is produced at all wavelengths than when it was cooler. As the temperature of an object increases, the peak of thermal spectrum curve shifts to smaller wavelengths. When you add up all of the energy of all of the square meters on the object's surface, you get the luminosity---the total amount of energy emitted every second by the object. Luminosity is proportional to the fourth power of temperature.