Question: The atomic nucleus of helium-4 is composed of how many protons and how many neutrons?
About the article "Subatomic particle", section "By composition".

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The elementary particles of the Standard Model include:
Six "flavors" of quarks: up, down, bottom, top, strange, and charm;
Six types of leptons: electron, electron neutrino, muon, muon neutrino, tau, tau neutrino;
Twelve gauge bosons (force carriers): the photon of electromagnetism, the three W and Z bosons of the weak force, and the eight gluons of the strong force;
The Higgs boson.
Various extensions of the Standard Model predict the existence of an elementary graviton particle and many other elementary particles.
Composite subatomic particles (such as protons or atomic nuclei) are bound states of two or more elementary particles.
For example, a proton is made of two up quarks and one down quark, while the atomic nucleus of helium-4 is composed of two protons and two neutrons.
Composite particles include all hadrons: these include baryons (such as protons and neutrons) and mesons (such as pions and kaons).
Before the 1970s, physicists were uncertain about the binding mechanism of the atomic nucleus.
It was known that the nucleus was composed of protons and neutrons and that protons possessed positive electric charge while neutrons were electrically neutral.
However, these facts seemed to contradict one another.
By physical understanding at that time, positive charges would repel one another and the nucleus should therefore fly apart.
However, this was never observed.
New physics was needed to explain this phenomenon.
A stronger attractive force was postulated to explain how the atomic nucleus was bound together despite the protons' mutual electromagnetic repulsion.
This hypothesized force was called the "strong force", which was believed to be a fundamental force that acted on the nucleons (the protons and neutrons that make up the nucleus).
It was later discovered that protons and neutrons were not fundamental particles, but were made up of constituent particles called quarks.
The strong attraction between nucleons was the side-effect of a more fundamental force that bound the quarks together in the protons and neutrons.
The theory of quantum chromodynamics explains that quarks carry what is called a color charge, although it has no relation to visible color.
Quarks with unlike color charge attract one another as a result of the strong interaction, which is mediated by particles called gluons. 
  "This article is about the physics of atomic helium.
For other properties of helium, see helium."
A helium atom is an atom of the chemical element helium.
Helium is composed of two electrons bound by the electromagnetic force to a nucleus containing two protons along with either one or two neutrons, depending on the isotope, held together by the strong force.
Unlike for hydrogen, a closed-form solution to the Schrödinger equation for the helium atom has not been found.
However, various approximations, such as the Hartree–Fock method, can be used to estimate the ground state energy and wavefunction of the atom. 
The nucleus of the helium-4 atom is identical with an alpha particle.
High-energy electron-scattering experiments show its charge to decrease exponentially from a maximum at a central point, exactly as does the charge density of helium's own electron cloud.
This symmetry reflects similar underlying physics: the pair of neutrons and the pair of protons in helium's nucleus obey the same quantum mechanical rules as do helium's pair of electrons (although the nuclear particles are subject to a different nuclear binding potential), so that all these fermions fully occupy 1s orbitals in pairs, none of them possessing orbital angular momentum, and each cancelling the other's intrinsic spin.
Adding another of any of these particles would require angular momentum and would release substantially less energy (in fact, no nucleus with five nucleons is stable).
This arrangement is thus energetically extremely stable for all these particles, and this stability accounts for many crucial facts regarding helium in nature.
For example, the stability and low energy of the electron cloud state in helium accounts for the element's chemical inertness, and also the lack of interaction of helium atoms with each other, producing the lowest melting and boiling points of all the elements.
In a similar way, the particular energetic stability of the helium-4 nucleus, produced by similar effects, accounts for the ease of helium-4 production in atomic reactions involving both heavy-particle emission, and fusion.
Some stable helium-3 is produced in fusion reactions from hydrogen, but it is a very small fraction, compared with the highly favorable helium-4.
The unusual stability of the helium-4 nucleus is also important cosmologically: it explains the fact that in the first few minutes after the Big Bang, as the "soup" of free protons and neutrons which had initially been created in about 6:1 ratio cooled to the point that nuclear binding was possible, almost all first compound atomic nuclei to form were helium-4 nuclei.
So tight was helium-4 binding that helium-4 production consumed nearly all of the free neutrons in a few minutes, before they could beta-decay, and also leaving few to form heavier atoms such as lithium, beryllium, or boron.
Helium-4 nuclear binding per nucleon is stronger than in any of these elements (see nucleogenesis and binding energy) and thus no energetic drive was available, once helium had been formed, to make elements 3, 4 and 5.
It was barely energetically favorable for helium to fuse into the next element with a lower energy per nucleon, carbon.
However, due to lack of intermediate elements, this process requires three helium nuclei striking each other nearly simultaneously (see triple alpha process).
There was thus no time for significant carbon to be formed in the few minutes after the Big Bang, before the early expanding universe cooled to the temperature and pressure point where helium fusion to carbon was no longer possible.
This left the early universe with a very similar ratio of hydrogen/helium as is observed today (3 parts hydrogen to 1 part helium-4 by mass), with nearly all the neutrons in the universe trapped in helium-4.
All heavier elements (including those necessary for rocky planets like the Earth, and for carbon-based or other life) have thus been created since the Big Bang in stars which were hot enough to fuse helium itself.
All elements other than hydrogen and helium today account for only 2% of the mass of atomic matter in the universe.
Helium-4, by contrast, makes up about 23% of the universe's ordinary matter—nearly all the ordinary matter that is not hydrogen. 
The nucleus of an atom consists of protons and neutrons (two types of baryons) bound by the nuclear force (also known as the residual strong force).
These baryons are further composed of subatomic fundamental particles known as quarks bound by the strong interaction.
Which chemical element an atom represents is determined by the number of protons in the nucleus.
Each proton carries a single positive charge, and the total electrical charge of the nucleus is spread fairly uniformly throughout its body, with a fall-off at the edge.
Major exceptions to this rule are the light elements hydrogen and helium, where the charge is concentrated most highly at the single central point (without a central volume of uniform charge).
This configuration is the same as for 1s electrons in atomic orbitals, and is the expected density distribution for fermions (in this case, protons) in 1s states without orbital angular momentum.
As each proton carries a unit of charge, the charge distribution is indicative of the proton distribution.
The neutron distribution probably is similar.
While protons define the entire charge of a nucleus and, hence, its chemical identity, neutrons are electrically neutral, but contribute to the mass of a nucleus to the same extent.
Neutrons explain the phenomenon of isotopes – varieties of a chemical element which differ in atomic mass.