1 Why Learn about Atomic Structure? Knowing the structure of atoms tells us about their –chemical properties –light-emitting properties –light-absorbing properties From this information we can learn about galaxies, stars, planets, asteroids, based on the light they emit or reflect. An example of absorption spectra from many different types of stars.

2 Atomic Structure An atom is composed of a dense core called a nucleus and surrounding this nucleus one or more negatively charged electrons. The nucleus is composed of positively charged protons and neutral neutrons. The electron is also 1,800 times lighter than a proton. Protons and neutrons however weigh about the same. The electric force of attraction between the positive protons and negative electrons keeps the electrons bound to the nucleus.

3 Atomic Structure An atom is mostly empty space because the electron moves around the nucleus at such a great distance. If the proton were 1 cm wide a hydrogen atom would be larger than a football field! A chemical element is determined based on how many protons the nucleus contains (Hydrogen has 1, Carbon has 6, Oxygen has 8 protons). When two atoms with the same number of protons have different numbers of neutrons the two atoms are isotopes of one another (Carbon has 6 protons but can have 6, 7, or 8 neutrons).

4 Ions An atom normally has the same number of electrons as protons. With the same number of positive and negative charges, an atom normally has no net charge. If an atom loses or gains one or more electrons it has become ionized. With too few electrons, the atom has a net positive charge, too many electrons and it has a net negative charge.

5 Quantum Structure of an Atom An electron does not orbit a nucleus like a planet orbits the Sun. For a given atom there are only a select few orbits that an electron can occupy. This means that the orbits are quantized. Electrons may shift between these quantum levels with either the emission or absorption of a photon of electromagnetic radiation.

6 Quantum Structure of an Atom If an electron absorbs a photon of light it can shift to a higher quantum level If an electron emits a photon of light it can shift to a lower quantum level The energy and wavelength of the photon in both cases depends on the energy difference between the two quantum levels.

7 Spectra and Atomic Structure Each type of atom has a unique set of wavelengths of light that it can absorb and emit –Hydrogen emits red light at 656 nm, blue light at 486 nm and other lines –Hydrogen will also absorb only red light at 656 nm, blue light at 486 and a few other lines We use this to identify the atoms present by studying the spectrum of an object

8 Emission or Absorption Spectra The brightness of an emission line or the darkness of an absorption line indicates how many atoms are absorbing or emitting that color. The number of atoms absorbing or emitting depends on the number present and on the temperature of the gas.

9 Types of Spectra Continuous Spectra - from hot, dense or solid objects. Emission Line Spectra - from hot, tenuous (thin) gas. Absorption Line Spectra - from cold, tenuous gas through which light from a hot, dense object passes.

10 Types of Spectra Emission and absorption spectra are sometimes shown as graphs of intensity versus frequency. Several molecules have been identified in this spectra of a comet.

11 Doppler Shift If a source of light is moving towards or away from an observer its spectral lines are shifted based on the speed and direction The faster the object is moving the greater the shift Shifts to longer wavelengths means object is moving away. Shorter, coming closer.

12 Doppler Shift Radar guns used by police to catch speeders use Doppler shift to determine speed. Astronomers refer to an increase in wavelength (object moving away) as a redshift. A decrease in wavelength (object moving closer) is called a blueshift. This technique is also used to search for planets around other stars.

13 The Motion of Stars Stars are rarely ever moving exactly towards or away from you. The doppler shift only gives one component of the motion. Sometimes photographs of stars taken many years apart can reveal the other components of stellar motion. This star is moving away from Earth. The light from this star will appear redshifted.