Posted on by Matt Williams

Atomic number

Fine Structure Constant

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Ever wonder why the periodic table of elements is organized the way it is? Why, for example, does Hydrogen come first? And just what are these numbers that are used to sort them all? They are known as the element’s atomic number, and in the periodic table of elements, the atomic number of an element is the same as the number of protons contained within its nucleus. For example, Hydrogen atoms, which have one proton in their nucleuses, are given an atomic number of one. All carbon atoms contain six protons and therefore have an atomic number of 6. Oxygen atoms contain 8 protons and have an atomic number of 8, and so on. The atomic number of an element never changes, meaning that the number of protons in the nucleus of every atom in an element is always the same.

Arranging elements based on their atomic weight began with Ernest Rutherford in 1911. It was he who first suggested the model for an atom where the majority of its mass and positive charge was contained in a core. This central charge would be roughly equal to half of the atoms total atomic weight. Antonius van den Broek added to this by formerly suggesting that the central charge and number of electrons were equal. Two years later, Henry Moseley and Niels Bohr made further contributions that helped to confirm this. The Bohr model of the atom had the central charge contained in its core, with its electrons circulating it in orbit, much like how the planet in the solar system orbit the sun. Moseley was able to confirm these two hypotheses through experimentation, measuring the wavelengths of photon transitions of various elements while they were inside an x-ray tube. Working with elements from aluminum (which has an atomic number thirteen) to gold (seventy nine), he was able to show that the frequency of these transitions increased with each element studied.

In short, the higher the atomic number (aka. the higher the number of protons), the heavier the element is and the lower it appears on the periodic table. The atomic number of an element is conventionally represented by the symbol Z in physics and chemistry. This is presumably derived from the German word Atomzahl, which means atomic number in English. It is not to be confused with the mass number, which is represented by A. This corresponds to the combined mass of protons and neutrons in the element.

We have written many articles about the atomic number for Universe Today. Here’s an article about the atomic nucleus, and here’s an article about the Atom Models.

If you’d like more info on the Atomic Number, check out NASA’s Atoms and Light Energy Page, and here’s a link to NASA’s Atomic Numbers and Multiplying Factors Page.

We’ve also recorded an entire episode of Astronomy Cast all about the Atom. Listen here, Episode 164: Inside the Atom.

Sources:
NDT Resource Center
Jefferson Lab
Wise Geek
Wiki Answers

Posted on by Jerry Coffey

What Is Atomic Mass

Faraday's Constant

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The answer to ‘what is atomic mass’ is this: the total mass of the protons, neutrons, and electrons in a single atom when it is at rest. This is not to be associated or mistaken for atomic weight. Atomic mass is measured by mass spectrometry. You can figure the molecular mass of an compound by adding the atomic mass of its atoms.

Until the 1960’s chemists and physicists used different atomic mass scales. Chemists used a scale that showed that the natural mixture of oxygen isotopes had an atomic mass 16. Physicists assigned 16 to the atomic mass of the most common oxygen isotope. Problems and inconsistencies arose because oxygen 17 and oxygen 18 are also present in natural oxygen. This created two different tables of atomic mass. A unified scale based on carbon-12 is used to meet the physicists’ need to base the scale on a pure isotope and is numerically close to the chemists’ scale.

Standard atomic weight is the average relative atomic mass of an element in the crust of Earth and its atmosphere. This is what is included in standard periodic tables. Atomic weight is being phased out slowly and being replaced by relative atomic mass. This shift in wording dates back to the 1960’s. It has been the source of much debate largely surrounding the adoption of the unified atomic mass unit and the realization that ‘weight’ can be an inappropriate term. Atomic weight is different from atomic mass in that it refers to the most abundant isotope in an element and atomic mass directly addresses a single atom or isotope.

Atomic mass and standard atomic weight can be so close, in elements with a single dominant isotope, that there is little difference when considering bulk calculations. Large variations can occur in elements with many common isotopes. Both have their place in science today. With advances in our knowledge, even these terms may become obsolete in the future.

We have written many articles about atomic mass for Universe Today. Here’s an article about the atomic nucleus, and here’s an article about the atomic models.

If you’d like more info on the Atomic Mass, check out NASA’s Article on Analyzing Tiny Samples, and here’s a link to NASA’s Article about Atoms, Elements, and Isotopes.

We’ve also recorded an entire episode of Astronomy Cast all about the Atom. Listen here, Episode 164: Inside the Atom.

Sources:
Wikipedia
Windows to Universe
NDT Resource Center

Posted on by Jerry Coffey

What Is An Electron

Faraday's Constant

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What is an electron? Easily put, an electron is a subatomic particle that carries a negative electric charge. There are no known components, so it is believed to be an elementary particle(basic building block of the universe). The mass of an electron is 1/1836 of its proton. Electrons have an antiparticle called a positron. Positrons are identical to electrons except that all of its properties are the exact opposite. When electrons and positrons collide, they can be destroyed and will produce a pair (or more) of gamma ray photons. Electrons have gravitational, electromagnetic, and weak interactions.

In 1913, Niels Bohr postulated that electrons resided in quantized energy states, with the energy determined by the spin(angular momentum)of the electron’s orbits and that the electrons could move between these orbits by the emission or absorption of photons. These orbits explained the spectral lines of the hydrogen atom. The Bohr model failed to account for the relative intensities of the spectral lines and it was unsuccessful in explaining the spectra of more complex atom. Gilbert Lewis proposed in 1916 that a ‘covalent bond’ between two atoms is maintained by a pair of shared electrons. In 1919, Irving Langmuir improved on Lewis’ static model and suggested that all electrons were distributed in successive “concentric(nearly) spherical shells, all of equal thickness”. The shells were divided into a number of cells containing one pair of electrons. This model was able to qualitatively explain the chemical properties of all elements in the periodic table.

The invariant mass of an electron is 9.109×10-31 or 5.489×10-4 of the atomic mass unit. According to Einstein’s principle of mass-energy equivalence, this mass corresponds to a rest energy of .511MeV. Electrons have an electric charge of -1.602×10 coulomb. This a standard unit of charge for subatomic particles. The electron charge is identical to the charge of a proton. In addition to spin, the electron has an intrinsic magnetic moment along its spin axis. It is approximately equal to one Bohr magneton. The orientation of the spin with respect to the momentum of the electron defines the property of elementary particles known as helicity. Observing a single electron shows the upper limit of the particle’s radius is 10-22 meters. Some elementary particles decay into less massive particles. But an electron is thought to be stable on the grounds that it is the least massive particle with non-zero electric charge.

Understanding what is an electron is to begin to understand the basic building blocks of the universe. A very elementary understanding, but a building block to great scientific thought.

We have written many articles about the electron for Universe Today. Here’s an article about the Electron Cloud Model, and here’s an article about the charge of electron.

If you’d like more info on the Electron, check out the History of the Electron Page, and here’s a link to the article about Killer Electrons.

We’ve also recorded an entire episode of Astronomy Cast all about the Composition of the Atom. Listen here, Episode 164: Inside the Atom.

Posted on by Jean Tate

Electron Volt

Fermi mapped GeV-gamma-ray emission regions (magenta) in the W44 supernova remnant. The features clearly align with filaments detectable in other wavelengths. This composite merges X-ray data (blue) from the Germany/U.S./UK ROSAT mission, infrared (red) from NASA’s Spitzer Space Telescope, and radio (orange) from the Very Large Array near Socorro, N.M. Credit: NASA/DOE/Fermi LAT Collaboration, NASA/ROSAT, NASA/JPL-Caltech, and NRAO/AUI
Fermi mapped GeV-gamma-ray emission regions (magenta) in the W44 supernova remnant. The features clearly align with filaments detectable in other wavelengths. This composite merges X-ray data (blue) from the Germany/U.S./UK ROSAT mission, infrared (red) from NASA’s Spitzer Space Telescope, and radio (orange) from the Very Large Array near Socorro, N.M. Credit: NASA/DOE/Fermi LAT Collaboration, NASA/ROSAT, NASA/JPL-Caltech, and NRAO/AUI

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From the name, electron volt, you might guess that this has something to do with electricity. Well, you’d be right, it does … but did you know that the electron volt is actually a unit of energy, like the erg or joule?

The symbol for the electron volt is eV – lower case e, upper case V. Like the meter, and parsec, the electron volt can have a prefix, so lots of electron volts can be written easily, so there’s a kilo-electron volt (keV, one thousand eV), mega-electron volt (MeV, one million eV), giga-electron volt (GeV, one thousand million eV), and so on.

About the energy the electron volt represents: if you accelerate an isolated electron through an electric potential difference of one volt, it will gain one electron volt of kinetic energy. Now a volt is a joule per coulomb, so an electron volt is one electric charge times one, or approx 1.6 x 10-19 joules (J).

Astronomers use electron volts to measure the energy of electromagnetic radiation, or photons, in the x-ray and gamma-ray wavebands of the electromagnetic spectrum, and also use electron volts to describe the difference in atomic or molecular energy states which give rise to ultraviolet, visual, or infrared lines, or limits. So, for example, the Lyman limit – which corresponds to the energy to just ionize an atom of hydrogen – is both 91.2 nm and 13.6 eV.

Now particle physicists use the electron volt, as a unit of energy too; however, confusingly, they also use it as a unit of mass! They do this by using the famous E = mc2 equation, so 1 eV – the unit of mass – is equal to 1 eV (the unit of energy) divided by c2 (c is the speed of light). So, for example, the mass of the proton is 0.938 GeV/c2, which makes the GeV/c2 a very convenient unit (= 1.783 x 10-27 kg). By convention, the c2 is usually dropped, and masses quoted in GeV.

Oh, and in some branches of physics, the eV is also a unit of temperature!

Would you like to read more on the electron volt? Try Energetic Particles (NASA), and How Big is an Electron Volt? (Fermilab).

Universe Today has many stories in which the electron volt features; here is a sample: Is a Nearby Object in Space Beaming Cosmic Rays at Earth?, Gamma-ray Afterglow reveals Pre-Historic Particle Accelerator, and Gamma Ray Bursts May Propel Fast Moving Particles.

The Astronomy Cast episode Gamma Ray Astronomy is a good example of electron volts in action.

Sources:
Wikipedia
NASA Science
GSU Hyperphysics