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Physics - Concepts - Atoms

Atoms are very, very small. With a nucleus of protons (or protons and neutrons) and orbiting electrons, atoms make up the fundamental elements of the Universe. While atoms can be broken down to their own fundamental parts, elements like Hydrogen or Sulfur cannot be separated to "lesser" elements - thus are fundamental.

All of the elements make up what we call the Periodic Table of Elements. This tool describes in detail information about each element such as how many components of protons, neutrons and electrons are.

The best source for an online Periodic Table of Elements is Web Elements.

A molecule is a collection of elements. An example is a water molecule: H2O - two parts hydrogen, one part oxygen.

More information on the specifics of the elements is found on the Web Elements website. The purpose of this section is to visit a typical atom. We will briefly visit:

Energy Levels

Energy Levels:

An atom consists of at least one part, a proton. This positively charged particle resides at the center of the atom. The atomic weight of an element is dependant on how many protons an atom has.

This is a model of the Bohr Atom, named after Niels Bohr. The Bohr model is designed to demonstrate how absorption and emission lines appear in spectra taken from the Sun.

An atom consists of several possible orbits for the electron. The orbit depends on the energy of the electron:

When an electron absorbs a photon (a packet of light), the electron gains energy and moves up an orbit. The image above demonstrates a hydrogen atom with 4 possible orbits (there can be more). Only one electron can be in one orbit at one time (actually, two electrons can occupy an orbit as long as they have different "spin" - see the Quantum Physics section).

When an electron gains energy and moves up an orbit, an absorption spectra occurs. When an electron looses energy and moves back down an orbit, an emission spectra occurs (see Spectroscopy).

These energy levels follow a pattern:

There are actually three specific series of emission lines:

  • The Balmer series - emission that occurs within the visible spectrum
  • The Lyman series - emission that occurs within the ultraviolet spectrum
  • The Paschen series - emission that occurs within the infrared spectrum

Here is a look at the frequencies of the Balmer lines:

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Sometimes if the energy of the photon absorbed by the electron is greater than the allowed energy levels of the orbits, the electron will be striped away by that photon. This is called ionization. For the hydrogen atom, the infinite orbit has an allowed energy of 13.6eV. If a photons energy is greater than this, ionization occurs:

With regard to hydrogen, a hydrogen atom in its normal state is also called H I. A hydrogen atom that has been stripped of its electron is also called H II. Astronomers use H I and H II to help study interstellar debris and dust clouds.

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An isotope is an element (an atom) that has additional neutrons. The Hydrogen atom, that is your standard hydrogen complete with electron, only has a proton. However, hydrogen can have added to it a neutron. A neutron is a particle that has no charge, so it does not add to the atomic weight. A hydrogen atom that has one neutron is called Deuterium.

Deuterium = 2H

According to Wikipedia deuterium is a naturally occurring element - approximately 1 out of every 6500 hydrogen atoms is deuterium. Deuterium is considered a stable isotope, in that the neutron will not decay. Any element that looses neutrons is unstable and is said to be decaying - also know as radioactivity.


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