Introduction to Research: Atoms

I’d like to share aspects of my research, but it requires a little scientific background to understand. For those of you whose ears went numb or eyes glazed over with that statement, do not stray away just yet! This information is intended for a general audience. I will try not to make any assumptions about reader’s knowledge and will include definitions of new words. It will be sparse with esoteric knowledge and abundant with useful information and interesting history. Most importantly, it will allow an insight into the research process, such as how long it takes for research to reach application and why scientific discovery should be highly valued.  To start off, I’m going to introduce the basic building blocks of all materials in the universe: the atom. The quote below from the eminent Richard Feynman should elucidate why this is a good starting point.

“If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generation of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis that all things are made of atoms — little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. In that one sentence, you will see, there is an enormous amount of information about the world, if just a little imagination and thinking are applied.” Richard Feynman

How We Discovered the Atom

  • Hypothesis – An educated prediction based on previous observations.

The discovery of the atom can be traced back to Ancient Greece. In fact, the word atom comes from the word atomos, which means “that which can’t be split”. Democritus, who likely lived in the same era as Socrates, developed the first atomic hypothesis and proposed that all matter is comprised of something that can’t be physically divided. He said that atoms come in different sizes and shapes, atoms are indestructible, and atoms are constantly in motion.

Modern Atomic Theory

  • Element – Basic substances that compose all things. Each element is distinct. (e.g., carbon, nitrogen, hydrogen)
  • Chemical compounds – Combination of elements to form new substances (e.g., carbon is an element, but carbon combines with hydrogen to make methane, which has properties that are different compared to isolated carbon and hydrogen)

Modern atomic theory was first proposed over 2000 years later, when John Dalton modified some of Democritus’ original ideas and outlined the following six points that composed his new theory.

  1. All matter is made of atoms.
  2. All atoms of a single element are the same.
  3. Atoms can’t be divided, created, or destroyed.
  4. Different elements that combine to form chemical compounds do so in whole-number ratios (e.g., 1 oxygen and 2 hydrogens make water)
  5. Atoms can be combined, separated, and rearranged in chemical reactions.
  6. When two elements form only a single compound, the combination is binary (e.g., 1 sodium atom and 1 chlorine atom to form table salt).

The sixth rule, which Dalton dubbed the “rule of greatest simplicity”, caused controversy. It could not be confirmed, and it led Dalton to believe that water consisted of one oxygen atom and one hydrogen atom, since hydrogen peroxide (two hydrogen atoms and two oxygen atoms) had not been discovered. It was later proven to be one of the major flaws of Dalton’s atomic theory.

Mass of an Atom

  • Liter – a measurement of volume. A bottle of water (16 oz) is about 1/2 liter
  • Volume – the amount of space matter occupies
  • Mass – how much “stuff” is in something. Mass is often synonymous with weight, but your mass never changes (as opposed to the moon, where your weight is less because there is less gravity)

After Dalton’s seminal work, other famous scientists started discovering aspects of the atom that are taken for granted today. Amedeo Avogadro noticed that 1 liter of nitrogen gas and 3 liters of hydrogen gas combined to form ammonia gas. Ammonia gas has the same ratio (1 nitrogen atom for every 3 hydrogen atoms), so Avogadro concluded that equal volumes of gasses must have equal number of molecules. He was able to use this to determine relative atomic ratios, which later enabled other scientists to quantify the mass of each atom.

Avogadro mixed gases and set a the groundwork for discovering atomic masses.

Avogadro mixed gases and set a the groundwork for discovering atomic masses.

Plum Pudding Model

J.J. Thomson performed experiments that demonstrated the existence of what is now known as the electron. He observed a negatively charged particle, which led him to what is referred to as the “plum pudding” model of the atom. A watermelon would be a more geographically-relevant analogy. A neutral atom has no charge, so the negative charge from the electron must be canceled out by a positive charge. The watermelon seeds would represent the electron and the red, fleshy watermelon would be a positive charge.

Alternatively, the watermelon model. The seeds would be randomly distributed particles with negative charges.

Alternatively, the watermelon model. The seeds would be randomly distributed particles with negative charges.

Planetary Model

This model was later shown to be incorrect, but pioneers in nuclear physics and chemistry first had to discover the atom’s nucleus. First, Ernest Rutherford took a piece of gold foil and bombarded it with particles. He had screens in front of and behind the gold foil serving as detectors, which the particles would burn an image into upon colliding with the screen. Rutherford discovered that the majority of the particles went straight through the solid gold foil. He also saw that some deflected to the front screen.

Astoundingly, he also saw that some particles were scattered by the gold and hit the rear detector in random spots. Rutherford concluded that atoms must be mostly empty space. This led to the planetary model of the atom, which many people today would picture when hearing the word “atom” or “nucleus”. The planetary model proposes a hard positively charged center, called the nucleus, that has negatively charged electrons zipping around in orbit.

Schematic of the gold foil experiment. As it turns out, the empty space observed was associated with an electron density (see more on this below).

Schematic of the gold foil experiment. As it turns out, the empty space observed was associated with an electron density (see more on this below).

Rutherford later did other experiments, which demonstrated the existence and mass of a proton. The proton has the opposite charge of an electron (it’s positive), but it is over 1000 times more massive. James Chadwick, among others, noticed that the number of protons could not account for the entire mass of an atom. He performed experiments similar to Rutherford’s and determined that a particle lacking a charge with a similar mass to a proton must exist. This was deemed a neutron.

The often-seen planetary model of the atom.

The often-seen planetary model of the atom.

Bohr Model

There were problems with the planetary model. An electron encircling the nucleus experiences an attractive force from the nucleus, similar to the gravitational force Earth exerts on the moon. To remain in orbit, the electron would have to lose energy by constantly emitting light; otherwise, it would crash into the nucleus. Neils Bohr attempted to solve this issue. Bohr observed how light interacts with matter. He noticed that elements had a response to light that was similar to a fingerprint. An element could be identified by what colors (or wavelengths) of light it allows to pass through.

Bohr postulated that the orbits (called orbitals) where electrons exist must be at a fixed, defined level, and each level is separated by defined spacing. In other words, Bohr’s model said that orbitals were quantized (restricted to a specific number, as in quantum mechanics). There were still problems with the Bohr model: it only worked for small atoms, and it didn’t exactly explain why what he proposed was true.

Schematic of Bohr Model

Schematic of Bohr Model

Heisenberg’s Uncertainty Principle

In an attempt to better explain the atom, Louis de Broglie said that, like light, all particles are also waves. This is an interesting postulate, which will be touched upon later, but for now it’s important in that it led Werner Heisenberg to his famous principle. Heisenberg’s Uncertainty Principle says that properties (like position and velocity) can’t be measured exactly without affecting the measurement. A poor experimentalist could understand this well; for example, someone weighs a sample and breaths on it, changing the weight by blowing small amounts of the sample off of the scale. However, the Uncertainty Principle does not refer to limitations of the researcher or the instrument. It is a physical principle that has not been changed no matter how precise scientific analysis has become.

The consequence of this was that atoms are now accurately described, not by Bohr’s model, which would define an exact position of the electron, but by an electron density. This is like a cloud that surrounds the nucleus. Within the cloud, there is a high probability of finding the electron, but the exact position is unknown. For my research, it is important to keep this in mind. More importantly, the Bohr model should be considered. The Bohr model is still taught today in quantum mechanics despite its shortcomings. This is because it is simple to understand and still makes good approximations. The Bohr model approximation will likely be sufficient for future discussion on my research.

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One thought on “Introduction to Research: Atoms

  1. Pingback: Introduction to Research: Light | Material World

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