The real picture of Atomic Structure

School level

Atom has electrons, protons and neutrons
  1. Nucleus has protons and neutrons
  2. Electrons revolve around nucleus
College level (Quantum physics)

Electrons are not particles. They are described as waves with a probability distribution. We have a mathematical function for each electron which tells us the probability of finding an electron at a given point in space. Electron has higher probability of being found around - what we earlier thought to be the traditional orbit of revolution. However, there is a non-zero probability of finding an electron at other points too. It’s just that such probability decreases as we move radially away from the “traditional orbit”.

When we plot the probability distribution around the nucleus, we get a picture with dark pixels around the traditional orbit and the density of pixels reduces as we move away. It looks like a “cloud”.

So, unlike the traditional picture of orbits, as taught in school, with particle electrons revolving in that path, the quantum mechanical picture of electron is that of an “electron cloud” spread across the space. (Image source)

Coming to Protons and Neutrons, we thought of them in school as indivisible particles. But it turns out that they aren’t indivisible. Protons and Neutrons are made up of quarks.

Advanced (Quantum Field Theory and Quantum Chromodynamics)

Quantum Field Theory (QFT) has a different picture of particles and forces.

In QFT, fundamental particles are described as vibrations (excited states) of a particle field. For instance, there exists a “quark field” all through the space. “All” “quarks” are just vibrations of the “same quark field” but at different points of space.

Forces in QFT is described as an exchange of force particles between the bodies. For instance, the electromagnetic force between two bodies is due to exchange of “electromagnetic force carriers” (photons) between them.

As per this, the picture of atom is like this:

Electrons are localized vibrations of “the” electron field.

Protons and Neutrons are made of “quarks” (quark fields!), which come in several “flavours”. Quantum Chromodynamics.

Further, Protons are held together in nucleus by “strong force” (otherwise they would repel each other and get out of nucleus). Remember that, in Quantum Field Theory (QFT), force is exerted by exchanging force particles. In QFT terms, strong force is exerted due to exchange of “gluons” between the protons.

The mass of proton is not a simple addition of mass of quarks that it is made up of. Sum of the masses of quarks in protons is around 1/2000 of total mass of the proton. It’s because the “strong field” inside proton has energy. The field energy has mass because of Mass-Energy Equivalence, a concept from Theory of Relativity (Remember E=mc^2?).

In nuclear decay, protons get converted to neutrons by exchanging W and Z bosons (weak force). “During beta minus decay, a down quark within a neutron is changed into an up quark, thus converting the neutron to a proton and resulting in the emission of an electron and an electron antineutrino”.