Question: How many protons and neutrons is the atomic nucleus of helium-4 composed of?
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).
  "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. 
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. 
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. 
Although all heavier helium isotopes decay with a half-life of less than one second, researchers have created new isotopes through particle accelerator collisions to create unusual atomic nuclei for elements such as helium, lithium and nitrogen.
The unusual nuclear structures of such isotopes may offer insight into the isolated properties of neutrons.
The shortest-lived isotope is helium-5 with a half-life of 7.6 second.
Helium-6 decays by emitting a beta particle and has a half-life of 0.8 second.
Helium-7 also emits a beta particle as well as a gamma ray.
The most widely-studied heavy helium isotope is helium-8.
This isotope, as well as helium-6, are thought to consist of a normal helium-4 nucleus surrounded by a neutron "halo" (containing two neutrons in and four neutrons in ).
Halo nuclei have become an area of intense research.
Isotopes up to helium-10, with two protons and eight neutrons, have been confirmed.
Helium-7 and helium-8 are hyperfragments that are created in certain nuclear reactions.
, despite being a doubly magic isotope, has a very short half-life.