The Atom

Definition — What is an atom?
An atom is the minutest entity that can exist and be identified as a chemical element or isotope.

A Simplified Diagram of An Atom

Basic structure of an atom
In brief, an atom has two major components — a central nucleus and orbital electrons.
The nucleus is not a distinct particle in itself but is a conglomeration of protons and neutrons. The components of the nucleus are called nucleons. The protons and neutrons are tightly bound and hence the nucleus is extremely compact and dense.
The electrons orbit the nucleus at comparatively great distances. The orbits vary according to the energy (momentum) of the electron.
The electron is the lightest of the three particles that compose an atom. The proton is about 2000 times heavier than the electron, and the neutron is slightly heavier and larger than the proton.
As far as size is concerned, the electron is so minute that its actual size cannot be measured by current state-of-the-art measuring technology. The proton is millions of times larger than the electron; the neutron is just a bit larger than the proton.

Table 1
S.No Entity Charge Mass Size
 1.  Electron   -1   1   too small to measure 
 2.  Proton   +1   1,836   2.5 x 10-15
 3.  Neutron   0   1,839   2.5 x 10-15
Comparison Of Electrons, Protons, and Neutrons.
Note: Mass is expressed relative to the mass of an electron.
Data source: Wikipedia

The gross volume of the atom is largely vacant. The gross volume (or size) of the atom is due to the electrons that revolve around the nucleus at comparatively great distances. Even so, the average radius of an atom is (very loosely) about 105 fm — [fm stands for femtometre(/er) and that is 10-15 m, or 0.000 000 000 000 001 m!] — and that translates to 0.000 000 000 1 m or 1 billionth of a metre. The radius of the nucleus is about 5 fm (very loosely, again) and that is about 5 quadrillionths of a metre.

Trivia: The unit femtometre (fm) has an alias — the fermi, in honour of the physicist Enrico Fermi.

Table 2
 Entity   Radius 
 Atom   100000 fm 
 Nucleus   5 fm 

Electrical Charge of the Components
The electron carries a negative charge. The proton is positively charged, whereas the neutron has no charge or, in other words, it is neutrally charged. The nucleus has a net positive charge due to the presence of protons in it.
The number of electrons is equal to the number of protons in the nucleus. Hence the atom as a whole is electrically neutral. There are occasions when the number of electrons and protons are unbalanced — this leads to a net negative or positive charge on the atom; such a charged atom is referred to as an ion.
The electrons move about the nucleus due to the force of attraction between the positively-charged nucleus and the negatively-charged electrons.
Within the nucleus the protons tend to repel each other. However, a nuclear binding force applies to the nucleons in a fashion very similar to that of the molecular force that acts on molecules. The nuclear binding force is strongly attractive and this produces a tightly bound bunch of nucleons. However, when the particles cross a certain limit, the same repulsive nature of the nuclear force forces them apart. Thus there is a constant balancing act between the attractive and repulsive modes of the nuclear force that gives the nucleus its size (and shape).
The Classical Model
The description given so far of the atomic structure is considered to be the classical description of the atom and its components. This is how the atom and its structure is described in most high school textbooks. The classical model does not posit any particles other than the electron, proton, and neutron. Everything we see (and experience) in the physical world is built up from atoms and atoms are composed of just electrons, protons and neutrons with a healthy dose of energy.
Other Models
Modern technology and methodology has thrown up discrepancies and anomalies in the observations regarding the behaviour of atoms and their components. These discrepancies cannot be explained under the classical model. This has forced scientists to develop other models that can better explain the observed behaviour.
From the early part of the 20th century scientists (especially physicists) have stressed on defining and validating scientific phenomena and theories using mathematical models. Today, mathematical models are considered de rigeur. It is the formulation of such mathematical models that has led to the verification of such major hypotheses as the Big Bang Hypothesis and the CDM hypothesis among others. Using mathematical models to validate scientific phenomena (that are either unobservable with modern technology or cannot be explained by classical physics) is a big enhancement over scientists presenting personal explanations as validation.
Observational evidence has weakened the classical model of the atom, and in some cases the classical model stands invalidated. In fact, we find that the atom is a Janus portal — on the one side, we see the universe as we know it, the macro universe; and on the other side we see an entire universe, the sub-micro-universe that is beyond normal comprehension! This strange universe is populated with sub-atomic entities that are immeasurably minute. Physicists call these immeasurably minute entities quanta thus giving birth to quantum mechanics, quantum physics, quantum electrodynamics (QED), quantum chromodynamics (QCD), and quantum … ad nauseam.

The Atomic Portal

Quanta behave quite differently from the particles we are used to in the "normal" universe. The Standard Model is based on quantum mechanics to describe particle physics.
The Standard Model (and quantum mechanics) is the ultimate answer to everything!
Appealing as that may sound, that is nowhere near the truth. It will be better to 'pronounce' the Standard Model (SM) (and Quantum Mechanics (QM)) to be the answer to almost everything!
Before we get too enthusiastic about the Standard Model, let's hear what Wikipedia has to say about the Standard Model:

 Because of its success in explaining a wide variety of experimental results, the Standard Model is sometimes regarded as a "theory of almost everything".
The Standard Model falls short of being a complete theory of fundamental interactions. It does not incorporate the full theory of gravitation as described by general relativity, or account for the accelerating expansion of the universe (as possibly described by dark energy). The model does not contain any viable dark matter particle that possesses all of the required properties deduced from observational cosmology. It also does not incorporate neutrino oscillations (and their non-zero masses). Although the Standard Model is believed to be theoretically self-consistent and has demonstrated huge and continued successes in providing experimental predictions, it does leave some phenomena unexplained (emphasis added by HC).

Well, there you are … but to be fair, all the theories and hypotheses that we have regarding this wonderful universe of ours are presented in good faith and with the utmost allegiance to the "scientific method". The good thing about the scientific method is that it debars personal opinion and beliefs from adulterating the presentation of knowledge. "Do you have any empirical proof for what you say? No? Then, sorry, we're not interested! Yeah, yeah, we agree with you, it is a fact, but the word is: no proof, no science! Why don't you call it religion? Religion requires no proof, just belief!"

The Electron's Orbital (Standard Model):
According to the Standard Model the electron describes a wave as it moves about the nucleus.

The electron's position and momentum in the vicinity of the nucleus is described by a mathematical "wave function". This wave function is central to the understanding of physics according to the Standard Model. Hence we'll take a deeper look at the wave function.

The Wave Function (also known as 'wavefunction' and 'state function'):
The really fundamental components of all things physical are best described as wave-particle dualities. Mathematical models of quanta have helped physicists to define specific wave functions for specific particles in specific contexts. The wave function concept for validating the existence (and the associated properties) of particles has been proven with the known particles. Suffice it to say, that the wave function concept stands validated. With the confidence of this validation under their collective belts, physicists are able to define wave functions for heretofore undiscovered particles; validation of new particle discoveries are based on whether the "discovery" adheres to the wave function that has been determined for it.

Check out these links for more about the wave function —
Hyperphysics Site
Encyclopaedia Britannica
Carnegie Mellon University
University of Texas
and (but of course!)...
Wikipedia, the free encyclopedia

Back to "The Electron's Orbital (Standard Model)":
We were talking about the electron's orbital as it revolves around the nucleus.
Please take a really DEEP breath as we casually inform you that the electron does not revolve around the nucleus!
Nope, nada, nyet!
Instead, the Standard Model informs us that electrons have no definite location at all! Worse still, there is just no way we can actually (directly) locate an electron in the vicinity of the nucleus. The electron is almost massless, and this makes it very susceptible to interference by external forces — just the act of flashing a beam of light on it is enough to throw it out of position! Inexplicably, any attempt to measure the momentum or position of an electron causes the wave function to collapse. So the position and momentum of a specific electron is determined indirectly by extremely complex mathematics — the wave function referred to earlier — the symbol for the wave function defining the position of the electron is Ψ (the Greek letter psi). The complex wave function involves an imaginary component as well as a real component. The output of the wave function indicates the interaction between the real and the imaginary components producing a number of possible values (eigenvalues) indicating the probable positions of the electron. Again, to reiterate an earlier point, the scientific community is confident of the validity of the wave function as a descriptor for an electron's position.
So the wave function Ψ defines the area (volume, actually) of probability for the position of an electron that is bound to the nucleus of an atom. In the very first shell (helium/hydrogen atom class) this area of probability is spherical, but in higher shells the area is more like a squashed sphere or lobe.
Below, we have the "classical" model of the electron orbiting the nucleus in well-defined shells. This is the model that's been taught to us through school, and even in some colleges.

Classical Model of Electrons Orbiting the Nucleus

The Standard Model says that electrons don't revolve around the nucleus in circular or spherical orbits as we were given to understand all this time — instead the Standard Model says that electrons hover about the nucleus in clouds. A specific wave function describes the shape and extent of the lobes (clouds) for each of the electrons surrounding the nucleus of a particular atom. So this is actually how the electrons are disposed about a nucleus:

Standard Model of Electrons Surrounding the Nucleus
More info …Atomic orbitals
More info …Electron clouds
The following is taken from the University of Illinois' Department of Physics website, "Ask The Van":


Why doesn't the electron get sucked into the nucleus since the nucleus is positive and the electron is negative?


The picture we often have of electrons as small objects circling a nucleus in well defined "orbits" is actually quite wrong. The positions of these electrons at any given time are not well-defined, but we CAN figure out the volume of space where we are likely to find a given electron if we do an experiment to look. For example, the electron in a hydrogen atom likes to occupy a spherical volume surrounding the proton. If you think of the proton as a grain of salt, then the electron is about equally likely to be found anywhere inside a ten foot radius sphere surrounding this grain, kind of like a cloud.
The weird thing about that cloud is that its spread in space is related to the spread of possible momenta (or velocities) of the electron. So here's the key point, which we won't pretend to explain here. The more squashed in the cloud gets, the more spread out the range of momenta has to get. That's called Heisenberg's uncertainty principle. Big momenta mean big kinetic energies. So the cloud can lower its potential energy by squishing in closer to the nucleus, but when it squishes in too far its kinetic energy goes up more than its potential energy goes down. So it settles at a happy medium, and that gives the cloud and thus the atom its size.

Source: Ask The Van 1, Ask The Van 2

In summary, electrons (and other subatomic particles) exhibit a dual nature — (1) particle; (2) wave. At the quantum levels of their existence, these entities can transform from one nature to the other nature. As of now, physicists are still uncertain of the exact or actual behaviour and structure of these particles. As a matter of fact, one of the major principles used in quantum mechanics is called the "Uncertainty Principle"!
Electrons do not move around the nucleus, least of all in spherical (or circular) orbits, that's it.
Electrons do occupy certain "orbitals" around the nucleus, and these orbitals form groups of "shells" and sub-shells (as in the classical model).
A shell is a band of orbitals bound within a range of distinct energy levels. Within a shell the electrons can easily transfer between orbitals. However electrons cannot jump from one band (or shell) to another energy band quite so easily — the electron needs a quantum energy bump to transfer to a higher shell and conversely it needs to lose a quantum packet of energy to get demoted to a lower-energy shell. Such inter-shell electron jumps don't happen without external energy agency — either by natural means or by deliberate excitation by humans.
The orbitals are a factor of the quantum properties of the electron, especially momentum.
An orbital describes the potential energy and the kinetic energy of the electron. An orbital close to the nucleus indicates high kinetic energy and low potential energy. An orbital distant from the nucleus indicates low kinetic energy and high potential energy.

Variation of Potential (a, b) and Kinetic Energy (c, d)
v/s Distance From The Nucleus

Graph of Standing Wave Function
The shaded area represents the volume of the probability of the electron's position about the nucleus.

The wave function defining the probability distribution of an electron relative to its binding nucleus represents a standing wave as shown in the diagram above. Though the graph is certainly not that of a genuine electron wave function it is fairly close enough.

Volume of Containment of Electron
Being Defined By a Specific Standing Wave Function

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