What Is The Electron Cloud Model?

The early 20th century was a very auspicious time for the sciences. In addition to Ernest Rutherford and Niels Bohr giving birth to the Standard Model of particle physics, it was also a period of breakthroughs in the field of quantum mechanics. Thanks to ongoing studies on the behavior of electrons, scientists began to propose theories whereby these elementary particles behaved in ways that defied classical, Newtonian physics.

One such example is the Electron Cloud Model proposed by Erwin Schrodinger. Thanks to this model, electrons were no longer depicted as particles moving around a central nucleus in a fixed orbit. Instead, Schrodinger proposed a model whereby scientists could only make educated guesses as to the positions of electrons. Hence, their locations could only be described as being part of a ‘cloud’ around the nucleus where the electrons are likely to be found.

Atomic Physics To The 20th Century:

The earliest known examples of atomic theory come from ancient Greece and India, where philosophers such as Democritus postulated that all matter was composed of tiny, indivisible and indestructible units. The term “atom” was coined in ancient Greece and gave rise to the school of thought known as “atomism”. However, this theory was more of a philosophical concept than a scientific one.

Various atoms and molecules as depicted in John Dalton's A New System of Chemical Philosophy (1808). Credit: Public Domain
Various atoms and molecules as depicted in John Dalton’s A New System of Chemical Philosophy (1808). Credit: Public Domain

It was not until the 19th century that the theory of atoms became articulated as a scientific matter, with the first evidence-based experiments being conducted. For example, in the early 1800’s, English scientist John Dalton used the concept of the atom to explain why chemical elements reacted in certain observable and predictable ways. Through a series of experiments involving gases, Dalton went on to develop what is known as Dalton’s Atomic Theory.

This theory expanded on the laws of conversation of mass and definite proportions and came down to five premises: elements, in their purest state, consist of particles called atoms; atoms of a specific element are all the same, down to the very last atom; atoms of different elements can be told apart by their atomic weights; atoms of elements unite to form chemical compounds; atoms can neither be created or destroyed in chemical reaction, only the grouping ever changes.

Discovery Of The Electron:

By the late 19th century, scientists also began to theorize that the atom was made up of more than one fundamental unit. However, most scientists ventured that this unit would be the size of the smallest known atom – hydrogen. By the end of the 19th century, his would change drastically, thanks to research conducted by scientists like Sir Joseph John Thomson.

Through a series of experiments using cathode ray tubes (known as the Crookes’ Tube), Thomson observed that cathode rays could be deflected by electric and magnetic fields. He concluded that rather than being composed of light, they were made up of negatively charged particles that were 1ooo times smaller and 1800 times lighter than hydrogen.

The Plum Pudding model of the atom proposed by John Dalton. Credit: britannica.com
The Plum Pudding model of the atom proposed by John Dalton. Credit: britannica.com

This effectively disproved the notion that the hydrogen atom was the smallest unit of matter, and Thompson went further to suggest that atoms were divisible. To explain the overall charge of the atom, which consisted of both positive and negative charges, Thompson proposed a model whereby the negatively charged “corpuscles” were distributed in a uniform sea of positive charge – known as the Plum Pudding Model.

These corpuscles would later be named “electrons”, based on the theoretical particle predicted by Anglo-Irish physicist George Johnstone Stoney in 1874. And from this, the Plum Pudding Model was born, so named because it closely resembled the English desert that consists of plum cake and raisins. The concept was introduced to the world in the March 1904 edition of the UK’s Philosophical Magazine, to wide acclaim.

Development Of The Standard Model:

Subsequent experiments revealed a number of scientific problems with the Plum Pudding model. For starters, there was the problem of demonstrating that the atom possessed a uniform positive background charge, which came to be known as the “Thomson Problem”. Five years later, the model would be disproved by Hans Geiger and Ernest Marsden, who conducted a series of experiments using alpha particles and gold foil – aka. the “gold foil experiment.”

In this experiment, Geiger and Marsden measured the scattering pattern of the alpha particles with a fluorescent screen. If Thomson’s model were correct, the alpha particles would pass through the atomic structure of the foil unimpeded. However, they noted instead that while most shot straight through, some of them were scattered in various directions, with some going back in the direction of the source.

A depiction of the atomic structure of the helium atom. Credit: Creative Commons
A depiction of the atomic structure of the helium atom. Credit: Creative Commons

Geiger and Marsden concluded that the particles had encountered an electrostatic force far greater than that allowed for by Thomson’s model. Since alpha particles are just helium nuclei (which are positively charged) this implied that the positive charge in the atom was not widely dispersed, but concentrated in a tiny volume. In addition, the fact that those particles that were not deflected passed through unimpeded meant that these positive spaces were separated by vast gulfs of empty space.

By 1911, physicist Ernest Rutherford interpreted the Geiger-Marsden experiments and rejected Thomson’s model of the atom. Instead, he proposed a model where the atom consisted of mostly empty space, with all its positive charge concentrated in its center in a very tiny volume, that was surrounded by a cloud of electrons. This came to be known as the Rutherford Model of the atom.

Subsequent experiments by Antonius Van den Broek and Niels Bohr refined the model further. While Van den Broek suggested that the atomic number of an element is very similar to its nuclear charge, the latter proposed a Solar-System-like model of the atom, where a nucleus contains the atomic number of positive charge and is surrounded by an equal number of electrons in orbital shells (aka. the Bohr Model).

The Electron Cloud Model:

During the 1920s, Austrian physicist Erwin Schrodinger became fascinated by the theories Max Planck, Albert Einstein, Niels Bohr, Arnold Sommerfeld, and other physicists. During this time, he also became involved in the fields of atomic theory and spectra, researching at the University of Zurich and then the Friedrich Wilhelm University in Berlin (where he succeeded Planck in 1927).

Artist's concept of the Electron Cloud model, which described the likely location of electron orbitals. Credit: prezi.com
Artist’s concept of the Electron Cloud model, which described the likely location of electron orbitals over time. Credit: Pearson Prentice Hall

In 1926, Schrödinger tackled the issue of wave functions and electrons in a series of papers. In addition to describing what would come to be known as the Schrodinger equation – a partial differential equation that describes how the quantum state of a quantum system changes with time – he also used mathematical equations to describe the likelihood of finding an electron in a certain position.

This became the basis of what would come to be known as the Electron Cloud (or quantum mechanical) Model, as well as the Schrodinger equation. Based on quantum theory, which states that all matter has properties associated with a wave function, the Electron Cloud Model differs from the Bohr Model in that it does not define the exact path of an electron.

Instead, it predicts the likely position of the location of the electron based on a function of probabilities. The probability function basically describes a cloud-like region where the electron is likely to be found, hence the name. Where the cloud is most  dense, the probability of finding the electron is greatest; and where the  electron is less likely to be, the cloud is less dense.

These dense regions are known as “electron orbitals”, since they are the most likely location where an orbiting electron will be found. Extending this “cloud” model to a 3-dimensional space, we see a barbell or flower-shaped atom (as in image at the top). Here, the branching out regions are the ones where we are most likely to find the electrons.

Thanks to Schrodinger’s work, scientists began to understand that in the realm of quantum mechanics, it was impossible to know the exact position and momentum of an electron at the same time. Regardless of what the observer knows initially about a particle, they can only predict its succeeding location or momentum in terms of probabilities.

At no given time will they be able to ascertain either one. In fact, the more they know about the momentum of a particle, the less they will know about its location, and vice versa. This is what is known today as the “Uncertainty Principle”.

Note that the orbitals mentioned in the previous paragraph are formed by a hydrogen atom (i.e. with just one electron). When dealing with atoms that have more electrons, the electron orbital regions spread out evenly into a spherical fuzzy ball. This is where the term ‘electron cloud’ is most appropriate.

This contribution was universally recognized as being one of the cost important contributions of the 20th century, and one which triggered a revolution in the fields of physics, quantum mechanics and indeed all the sciences. Thenceforth, scientists were no longer working in a universe characterized by absolutes of time and space, but in quantum uncertainties and time-space relativity!

We have written many interesting articles about atoms and atomic models here at Universe Today. Here’s What Is John Dalton’s Atomic Model?, What Is The Plum Pudding Model?, What Is Bohr’s Atomic Model?, Who Was Democritus?, and What Are The Parts Of An Atom?

For more information, be sure to check What Is Quantum Mechanics? from Live Science.

Astronomy Cast also has episode on the topic, like Episode 130: Radio Astronomy, Episode 138: Quantum Mechanics, and Episode 252: Heisenberg Uncertainty Principle

What Are The Parts Of An Atom?

Since the beginning of time, human beings have sought to understand what the universe and everything within it is made up of. And while ancient magi and philosophers conceived of a world composed of four or five elements – earth, air, water, fire (and metal, or consciousness) – by classical antiquity, philosophers began to theorize that all matter was actually made up of tiny, invisible, and indivisible atoms.

Since that time, scientists have engaged in a process of ongoing discovery with the atom, hoping to discover its true nature and makeup. By the 20th century, our understanding became refined to the point that we were able to construct an accurate model of it. And within the past decade, our understanding has advanced even further, to the point that we have come to confirm the existence of almost all of its theorized parts.

Continue reading “What Are The Parts Of An Atom?”

Why Is the Solar System Flat?

It’s no mystery that the planets, moons, asteroids, etc. in the Solar System are arranged in a more-or-less flat, plate-like alignment in their orbits around the Sun.* But why is that? In a three-dimensional Universe, why should anything have a particular alignment at all? In yet another entertaining video from the folks at MinutePhysics, we see the reason behind this seemingly coincidental feature of our Solar System — and, for that matter, pretty much all planetary systems that have so far been discovered (not to mention planetary ring systems, accretion disks, many galaxies… well, you get the idea.) Check it out above.

Video by MinutePhysics. Created by Henry Reich
Continue reading “Why Is the Solar System Flat?”

Earth is the Most Exotic Place In The Universe

I’m often asked by students in my community education astronomy classes whether any new elements have been found in outer space unknown on Earth. The answer to the question is no – nature uses the same 98 natural elements to fashion everything from the familiar stars and planets to those in the farthest galaxies we can see. Outside of an occasional compound or mineral, Earth is the place where you’ll find more exotic elements than anywhere else in the universe.

An element is a pure substance made of just one type of atom. What sets one element apart from another is the number of protons in the nuclei of its atoms. Any atom with six protons will always be carbon, 79 protons gold, 94 protons plutonium and 1 proton hydrogen. The proton number is also the element’s atomic number on the periodic table of elements. Elements in the table are arranged according to their atomic number.

Atoms are made of protons, neutrons and orbiting electrons. The number of protons in atom's nucleus makes it unique from all the others. Hydrogen, the simplest element, has one while carbon has six.
Atoms are made of protons, neutrons and orbiting electrons. The number of protons in atom’s nucleus makes it unique from all the others. Hydrogen, the simplest element, has one; carbon has six.

The two most common elements are hydrogen and helium, numbers 1 and 2 in the periodic table; together they make up 98% of all the visible matter in the universe. The remaining 2% includes everything else from lightweight lithium (number 3) all the way up to californium (98), the heaviest natural element found on Earth and in the stars. Californium is unstable and “decays” into simpler elements. Although scientists make it in the lab by bombarding berkelium (97) with neutrons, trace amounts of this very rare element are found naturally in rich uranium deposits.

When I was in high school studying chemistry, the periodic table of elements ended at Lawrencium (103). At present there are 118 elements, the most recent one created in the lab being ununoctium (you-nah-NOC-tee-um). Matter of fact, all the elements beyond 98 are artificial, brought to life in nuclear reactors or in particle accelerator experiments. They live very short lives. With so many positively-charged protons pushing against one another in their nuclei, these elements quickly break apart into simpler ones in a process called radioactive decay.

Artist's concept of the first stars in the Universe turning on some 200 million years after the Big Bang. These first suns were made of almost pure hydrogen and helium. They and later generations of stars cooked up the heavier elements from these simple ones. Credit: NASA/WMAP Science Team
Artist’s concept of the first stars in the Universe turning on some 200 million years after the Big Bang. These first suns were made of almost pure hydrogen and helium. They and later generations cooked up heavier elements that were later incorporated into the sun and us. Credit: NASA/WMAP Science Team

Back at the time of the Big Bang, when the universe sprang into existence, only the simplest elements – hydrogen, helium and trace amounts of lithium – were cooked up. You can’t build a planet from such fluffy stuff. It took the first generation of stars, which formed from these basic building blocks, to synthesize more complicated elements like carbon, oxygen, sulfur and the like via nuclear fusion in their cores.

When the stars exploded as supernovae, not only were these brand new elements blasted into space, but the enormous heat and pressure during the blast built even heavier elements like gold, copper, mercury and lead. All became incorporated in a second generation of stars. And a third.

The 2% of star-made elements, which include carbon, oxygen, nitrogen and silicon among others, went to build the planets and later became essential for life. We’re made of highly processed material you and I. The atoms of our beings have been in and out of the cores of several generations of stars. Think about this good and hard and you might just get in touch with your own “inner star”.

Common table salt is formed of tight-fitting atoms of sodium and chlorine. Each elements has its own individual character - one's a flammable metal (sodium), the other a dangerous gas. Put them together and you create a safe, tasty edible. Credit: Wikipedia
Common table salt is formed of tight-fitting atoms of sodium and chlorine. Each elements has its own individual character – one’s a flammable metal (sodium), the other a dangerous gas. Put them together and out comes a safe, tasty seasoning. Credit: Wikipedia

Let’s reframe the question about exotic materials in space not present on Earth. Instead of elements, if we look at compounds, we hit paydirt. A compound is also a pure substance but consists of two or more chemical elements joined together. Familiar compounds include water (two hydrogens joined to one oxygen) and salt (one sodium and one chlorine).

A selection of molecules (chemical compounds) including water found in the Orion Nebula detected by the Herschel Space Telescope. Credit: ESA, HEXOS and the HIFI Consortium E. Bergin
A selection of molecules (chemical compounds) including water found in the Orion Nebula detected by the Herschel Space Telescope. Credit: ESA, HEXOS and the HIFI Consortium E. Bergin

Astronomers have found about 220 compounds or molecules in outer space many of them with siblings on Earth but some alien. We don’t have to look far to find them since a few have been delivered right to our doorstep as rocky packages called meteorites.  Here’s a short list of new minerals that formed within asteroids (where meteorites originate) under conditions very different from those found on Earth:

Barringerite – a metallic compound made of iron, nickel and phosphorus
Oldhamite – brown mineral made of calcium, magnesium and sulfur
Kosmochlor  – green mineral containing calcium, chromium, silicon and oxygen

How about new stuff on planets and comets? Astronomers have discovered compounds in the atmospheres of the giant planets Jupiter, Saturn, Uranus and Neptune like silane (silicon-hydrogen), arsine (arsenic-hydrogen) and phosphine (phosphorus-hydrogen) that don’t exist naturally on Earth. Humans have created all three in the lab and put them to good use in various industries including the manufacture of semi-conductors.

Helium was first discovered in the spectrum of the sun by French astronomer Pierre Janssen in 1868 by observing sunlight through a prism during an eclipse. It was discovered on Earth in 1895. Credit: Bob King
Helium, the gas that floats our balloons, was first discovered in the spectrum of the sun by French astronomer Pierre Janssen in during a solar eclipse in 1868. Scientists finally found it on Earth in 1895. Click to learn the helium story. Credit: Bob King

And then there’s Brownleeite, a manganese silicide found in 2003 in a dust particle shed by comet 26P/Grigg-Skjellerup. Moving beyond the solar system, astronomers see unusual long-chained carbon molecules in space that couldn’t form on Earth because oxygen would tear them apart. Space is their safe haven.

So, Earth is the location in the Universe where you’ll find more exotic elements than anywhere else. Thanks to human activity and the complicated molecules that wound together to form life, Earth’s the most exotic place in the universe.

What are Photons

Faraday's Constant

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When we think about light we don’t really think about what it is made of. This was actually the subject one of the most important arguments in physics. For the longest time physicists and scientist tried to determine if light was a wave or a particle. There were the physicists of the eighteenth century who strongly believed that light was made of basic units , but certain properties like refraction caused light to be reclassified as a wave. It would take no less than Einstein to resolve the issue. Thanks to him and the work of other renowned physicists we know more about what are photons.

To put it simply photons are the fundamental particle of light. They have a unique property in that they are both a particle and a wave. This is what allows photons unique properties like refraction and diffusion. However light particles are not quite the same as other elementary particles. They have interesting characteristics that are not commonly observed. First, as of right now physicists theorize that photons have no mass. They have some characteristics of particles like angular momentum but their frequency is independent of the influence of mass They also don’t carry a charge.

Photons are basically the most visible portion of the electromagnetic spectrum. This was one of the major breakthroughs Einstein and the father of quantum physics, Planck made about the nature of light. This link is what is behind the photoelectric effect that makes solar power possible.Because light is another form of energy it can be transferred or converted into other types. In the case of the photoelectric effect the energy of light photons is transferred through the photons bumping into the atoms of a giving material. This causes the atom that is hit to lose electrons and thus make electricity.

As mentioned before photons played a key role in the founding of quantum physics. The study of the photons properties opened up a whole new class of fundamental particles called quantum particles. Thanks to photons we know that all quantum particles have both the properties of waves and particles. We also know that energy can be discretely measured on a quantum scale.

Photons also played a big role in Einstein’s theory of relativity. without the photon we would not understand the importance of the speed of light and with it the understanding of the interaction of time and space that it produced. We now know that the speed of light is an absolute that can’t be broken by natural means as it would needs an infinite amount of energy something that is not possible in our universe. So without the photon we would not have the knowledge about our universe that we now possess.

We have written many articles about photons for Universe Today. Here’s an article about how the sun shines, and here’s an article about why stars shine.

If you’d like more info on Photons, check out the Mass of the Photon. And here’s a link to an article about How Gravity Affects Photons.

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

Source:
Wikipedia

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

What are Electrons

Fine Structure Constant

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If you have heard of electrons you know that they have something to do with electricity and atoms. If so you are mostly right in describing what are electrons. Electrons are the subatomic particles that orbit the nucleus of an atom. They are generally negative in charge and are much smaller than the nucleus of the atom. If you wanted a proper size comparison the size of the earth in comparison to the sun would be a pretty close visualization.

Electrons are known to fall into orbits or energy levels. These orbits are not visible paths like the orbit of a planet or celestial body. The reason is that atoms are notoriously small and the best microscopes can only view so much of atoms at that scale. Even if we could view electrons they would move too fast for the human eye. As a matter of fact scientists still can’t calculate the exact position of electrons. They can only estimate their locations. That is why the modern model of the atoms has an electron cloud surrounding the nucleus of an atom instead of a defined system of electrons in concentric orbits.

Electrons are also important for the bonding of individual atoms together. With out this bonding force between atoms matter would not be able to interact in the many reactions and forms we see every day. This interaction between the outer electron layers of an atom is call atomic bonding. It can occur in two forms. One is covalent bonding where atoms share electrons in their outer orbits. The other is ionic bonding where an atom gives up electrons to another atom. In either case bonding must meet specific rules. We won’t go into great detail, but each electron orbit or electron energy level can only hold so many electrons. Atoms can only bond if there is room to share or receive extra electrons on the outermost orbit of the atom.

Electrons are also important to electricity. Electricity is basically the exchange of electrons in a stream called a current through a conducting medium. In most cases the medium is an acid, metal, or similar conductor. In the case of static electricity, a stream of electrons travels through the medium of air.

The understanding of the electron has allowed for a better understanding of some of the most important forces in our universe such as the electromagnetic force. Understanding its workings has allowed scientist to work out concepts such potential difference and the relationship between electrical and magnetic fields.

We have written many articles about electrons for Universe Today. Here’s an article about the atom diagram, and here’s an article about the electron cloud model.

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

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

Atomic Mass Unit

Faraday's Constant

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Believe it or not, there are actually several atomic mass units … however, the one that’s standard – throughout chemistry, physics, biology, etc – is the unified atomic mass unit (symbol u). It is defined as 1/12 (one-twelfth) of the mass of an isolated carbon-12 atom, in its ground state, at rest. You’ll still sometime see the symbol amu – which stands for atomic mass unit – but that’s actually two, slightly different, units (and each is different from the unified atomic mass unit!) … these older units are defined in terms of oxygen (1/16th of an isolated oxygen-16 atom, and 1/16th of an ‘average’ oxygen atom).

As it’s a unit of mass, the atomic mass unit (u) should also have a value, in kilograms, right? It does … 1.660 538 782(83) x 10-27 kg. How was this conversion worked out? After all, the kilogram is defined in terms of a bar of platinum-iridium alloy, sitting in a vault in Paris! First, it is important to recognize that the unified atomic mass unit is not an SI unit, but one that is accepted for use with the SI. Second, the kilogram and unified atomic mass unit are related via a primary SI unit, the mole, which is defined as “the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12“. Do you remember how many atoms there are in a mole of an element? Avogadro’s number! So, work out the Avogadro constant, and the conversion factor follows by a simple calculation …

The Dalton (symbol D, or Da) is the same as the unified atomic mass unit … why have two units then?!? In microbiology and biochemistry, many molecules have hundreds, or thousands, of constituent atoms, so it’s convenient to state their masses in terms of ‘thousands of unified atomic mass units’. That’s far too big a mouthful, so convention is to use kDa (kilodaltons).

Find out more on the (unified) atomic mass unit, from the Argonne National Laboratory, from the International Union of Pure and Applied Chemistry, and from the National Institute of Standards and Technology (NIST).

Learning to Breathe Mars Air and Mini-Detector Could Find Life on Mars or Anthrax at the Airport are two Universe Today articles relevant to the atomic mass unit.

Energy Levels and Spectra and Inside the Atom are two Astronomy Cast episodes related to the atomic mass unit.

Sources:
Wikipedia
Newton Ask a Scientist
Wise Geek