The Anatomy of an Atom

If matter is composed of atoms, what are atoms composed of? Are they the smallest particles, or is there something smaller? 

Discovering the Inside of an Atom

In the late 1800s, scientists began to investigate the particles that exist inside of an atom. The first particle to be discovered within atoms was the electron, an extremely tiny particle with a negative charge (charge = -1). Experimental evidence for these particles helped scientist develop models of the inside of the atom. One prescient model for the atom was proposed in 1903 by Hantaro Nagaoka, who postulated a Saturn-like atom, consisting of a positively charged sphere surrounded by a halo of electrons.

A photograph of the planet Saturn, which has rings. To the right, an atom model is a sphere of positively charged matter encircled by a ring of negatively charged electrons.
Nagaoka proposed that atoms resembled the planet Saturn, with a ring of electrons surrounding a positive “planet.” (credit a: modification of work by “Man vyi”/Wikimedia Commons; credit b: modification of work by “NASA”/Wikimedia Commons)

In the following years, experiments by Ernest Rutherford confirmed that a small, relatively heavy, positively charged body must be at the center of each atom. This positively charged body was named the nucleus of the atom. In Rutherford’s model of the atom, the small, positively charged nucleus contains most of the mass of the atom, and is surrounded by the negatively charged electrons. The negatively charged electrons orbit around the nucleus because they are attracted to the positive charge in the nucleus (opposite charges attract). They also balance out the charge of the nucleus so that the atom is electrically neutral (total charge equals zero). 

Further experiments revealed that the nucleus contains two types of particles: protons and neutrons.  A proton is a positively charged particle (charge = +1) with a mass of 1 atomic mass unit (amu). (The atomic mass unit (amu) is an extremely small unit of mass used to keep track of the mass of protons and neutrons in atoms.) Protons are incredibly small, however, their mass is about 2,000 times greater than the mass of an electron. A neutron is an uncharged (neutral, charge = 0) particle with a mass approximately the same as that of a proton (1 amu). 

The nucleus contains the majority of an atom’s mass because protons and neutrons are much heavier than electrons, whereas electrons occupy almost all of an atom’s volume. For a perspective about the relative sizes of an atom and its nucleus, consider this: If the nucleus were the size of a blueberry, the atom would be about the size of a football stadium! Most of the volume within an atom consists of empty space!

The diagram on the left shows a picture of an atom that is 10 to the negative tenth power meters in diameter. The nucleus is labeled at the center of the atom and is 10 to the negative fifteenth power meters. The central figure shows a photograph of an American football stadium. The figure on the right shows a photograph of a person with a handful of blueberries.
If an atom could be expanded to the size of a football stadium, the nucleus would be the size of a single blueberry. (credit middle: modification of work by “babyknight”/Wikimedia Commons; credit right: modification of work by Paxson Woelber)

The “planetary” model described above is essentially the same model that we use today to describe atoms but with one important modification. The planetary model suggests that electrons occupy certain specific, circular orbits about the nucleus. We know now that this model is overly simplistic. A better description is that electrons form fuzzy clouds around nuclei. The figure below shows a more modern version of our understanding of atomic structure.

(a) A model of an atom showing a dark orange nucleus at the center surrounded by a fuzzy area where the darker the orange color, the higher probability of finding an electron. (b) This image superimposes orange dots over the fuzzy are to show the likelihood of finding an electron at different distances from the nucleus. Electrons are less likely to be found as the distance from the nucleus increases.
A more modern understanding of atoms, reflected in these representations of the electron in a hydrogen atom, is that electrons occupy regions of space about the nucleus; they are not in discrete orbits like planets around the sun. (a) The darker the color, the higher the probability that an electron will be at that point. (b) In a two-dimensional cross section of the electron in a hydrogen atom, the more crowded the dots, the higher the probability that an electron will be at that point. In both (a) and (b), the nucleus is in the center of the diagram.

Summary of the Three Subatomic Particles: Electron, Proton, and Neutron

We now understand that all atoms consist of three subatomic particles: protons, neutrons, and electrons. The table below lists some of their important characteristics:

Name Location Charge Mass (amu)
electron outside nucleus -1 ~0 (much smaller than 1)
proton nucleus +1 1
neutron nucleus 0 1

You can build your own atoms and investigate their properties using this interactive simulation! Build an Atom


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