The temperature of the surface of the Sun is about 6000 Degrees Kelvin. The surface of the Sun is a plasma, and photons are repeatedly absorbed and emitted by electrons in atoms - photons and matter are in thermal equilibrium, both at the temperature of the surface of the Sun. When radiation from the Sun reached the Earth, it is still at this temperature, and when it passes through the Earth's atmosphere, the temperature is maintained. The Earth's atmosphere though is at a temperature of between 200 - 300 Degrees Kelvin, so light and matter have different temperatures inside the same space. This phenomena is possible because interaction of radiation and matter is limited, and is possible wherever interaction is limited]]>

**A Planetary Model For The Atom**

In the Bohr Model the neutrons and protons (symbolized by red and blue balls in the adjacent image) occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun.

This similarity between a planetary model and the Bohr Model of the atom ultimately arises because the attractive gravitational force in a solar system and the attractive Coulomb (electrical) force between the positively charged nucleus and the negatively charged electrons in an atom are mathematically of the same form - both gravitational and electric forces obey inverse square laws - but the intrinsic strength of the Coulomb interaction is much larger than that of the gravitational interaction; in addition, there are positive and negative electrical charges so the Coulomb interaction can be either attractive or repulsive, but gravitation is always attractive.

**Angular Momentum and Energy are Quantized**

The basic feature of quantum mechanics that is incorporated in the Bohr Model and that is completely different from the analogous planetary model is that the angular momentum is quantized – it can only come in multiples of a quantitycalled Planck's constant,This means that only certain orbits are allowed, illustrated above. This leads also to the quantization of energy.

The adjacent figure shows such quantized energy levels for the hydrogen atom. These levels are labelled by an integer n that is called a quantum number. The lowest energy state is generally termed the ground state. The ground state of hydrogen has energy of -13.6eV. The states with successively more energy than the ground state are called the first excited state, the second excited state, and so on. Beyond an energy called the ionization potential the single electron of the hydrogen atom is no longer bound to the atom. Then the energy levels form a continuum. The energy of each excited state is given by

**Atomic Excitation and De - excitation**

Atoms can make transitions between the orbits allowed by quantum mechanics by absorbing or emitting exactly the energy difference between the orbits. The following figure shows an atomic excitation cause by absorption of a photon and an atomic de-excitation caused by emission of a photon.

In each case the wavelength of the emitted or absorbed light is exactly such that the photon carries the energy difference between the two orbits. This energy us given by the formulawhere E is the energy difference between which the electron makes a transition, h is Planck's constant and f is the frequency of the emitted or absorbed radiation. Becausewe may also write

]]>where

of a particle or photon

For a photonand for a particle with mass(at much smaller speeds than the speed of light).

Consider what this means for an alpha particle approaching a gold atom. The diameter of a gold atom is aboutand the size of an alpha particle may be consider to be related to the wavelength. If the wavelength is small then so is the particle.

For an alpha particleSuppose the alpha particle has an energy ofThis is the kinetic energy of the alpha particle, equal toso that

The wavelength of wave associated with the alpha particle is

This is about the same size as a gold atom.

Suppose now that the alpha particle has an energy 10 MeV. The procedure above gives a speed of 2200000 m/s and a wavelength ofThis is much smaller than the size of an atom – about the same size as a nucleus. The alpha particle will now see a gold atom as mostly empty space and will be much more likely to pass through undeflected by the nucleus.

]]>The spectrum of hydrogen falls into distinct series. When a hydrogen atom emits or absorbs radiation, the electron jumps from one energy level to another. The lower of these energy levels determines the series of the spectrum in which the emitted or absorbed radiation falls.

The wavelengths of the radiation in each group are described by an equation.

Wavelengths in the Lyman series are given by the formula

is a constant called the Rydberg constant.

Wavelengths in the Balmer series are given by the formula

Wavelengths in the Paschen series are given by the formula

Wavelengths in the Brackett series are given by the formula

Wavelengths in the Pfund series are given by the formula

Other series exist. The general formula is given by

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