Imagine a neutral hydrogen atom. It has one electron. If enough energy is supplied to separate that electron, does the hydrogen disappear? Does it become another chemical element? This matters for stars: in their hot material, many electrons are no longer bound to individual nuclei.
An atom has a small positively charged nucleus and electrons bound to it by electrical interaction. The number of protons in the nucleus identifies the element. Losing an electron changes the charge, but not that number: what remains is a hydrogen ion. Understanding matter on Earth and in stars means keeping separate counts of protons, neutrons, and electrons. OpenStax: atomic structure and charge.
The small nucleus holds almost all the mass
The nucleus contains protons, which have positive charge, and, in most varieties of atom, neutrons, which have no net electrical charge. One exception is useful from the start: the nucleus of hydrogen-1 is a single proton, with no neutrons. Neutral hydrogen-1 also has one electron.
An electron has negative charge equal in magnitude to a proton’s positive charge. Protons and neutrons have similar masses, while electrons are much lighter. Almost all an atom’s mass is therefore concentrated in its nucleus, although its electron distribution extends over a much larger region of space. This is not a hard ball inside a shell; we will return to how electrons are described. DOE: nuclei.
A neutral atom has equal numbers of protons and electrons. Their charges cancel in the total; the charged particles themselves do not disappear. A neutron contributes neither positive nor negative net charge.
Branch · A closer lookOptional · you can skip this
Why the positive protons do not fly apart
Like charges repel, so protons repel one another electrically. In nuclei with several protons, the strong nuclear interaction counteracts that repulsion, binding protons and neutrons at very short distances. It has a short range: it does not replace electrical attraction as the interaction binding electrons to a nucleus. OpenStax: forces within a nucleus.
Neutrons contribute to nuclear binding without adding electrical repulsion. But “add more neutrons to make a nucleus more stable” is not a general rule: stability depends on nuclear composition, and some isotopes are radioactive. The counts below help us identify a particle; they do not, by themselves, predict its stability. DOE: isotopes.
Element, isotope, and ion answer different questions
Start by counting protons. One means hydrogen, two means helium, and six means carbon. This count is called the atomic number. Hydrogen remains hydrogen when it becomes part of water or loses an electron: its nucleus still contains one proton. Changing the proton count changes the element.
Now compare nuclei with the same number of protons but different numbers of neutrons. These are isotopes of the same element. Hydrogen-1 has no neutrons; hydrogen-2 has one. Hydrogen-2 is also called deuterium. Both are hydrogen, but their nuclear compositions and masses differ. The number after the name is the total number of protons and neutrons, called the mass number. DOE: what defines an isotope.
Finally, compare the electron and proton counts. If an atom loses an electron, its positive charges are no longer fully balanced: it becomes a positive ion. Gaining an extra electron can produce a negative ion. Neither the element nor the isotope changes, because the nucleus keeps the same composition. Ions can also be molecular; here we consider only ions of individual atoms.
In the figure, all three lower examples are compared with the same neutral deuterium atom at the top. Just one count differs in each comparison. Notice that increasing the proton count while keeping the electron count fixed changes both the element and the charge.
Change one count in your mind
Compare each of the three lower examples with the neutral hydrogen-2 atom above.
Starting example
Hydrogen-2 · deuterium
- Protons
- 1
- Neutrons
- 1
- Electrons
- 1
- Mass number A
- 2
- Charge q/e
- 0
A neutral atom: one positive and one negative charge.
One fewer neutron
Hydrogen-1
- Protons
- 1
- Neutrons
- 0
- Electrons
- 1
- Mass number A
- 1
- Charge q/e
- 0
Same element, different isotope. The charge is unchanged.
One more proton
Helium-3 ion
- Protons
- 2
- Neutrons
- 1
- Electrons
- 1
- Mass number A
- 3
- Charge q/e
- +1
A different element. With the electron count fixed, the charge also becomes +1.
One fewer electron
Hydrogen-2 ion
- Protons
- 1
- Neutrons
- 1
- Electrons
- 0
- Mass number A
- 2
- Charge q/e
- +1
Same element and isotope. The charge becomes +1.
Branch · With formulasOptional · you can skip this
Read three counts without guessing
Let Z be the number of protons, N the number of neutrons, and N_(e) the number of electrons. The mass number A counts the particles in the nucleus:
A = Z + N.
Let e be the positive elementary charge. The total charge q follows from the difference between two counts:
(q)/(e) = Z − N_(e).
The numbers Z, N, N_(e), and A are dimensionless. So is q/e: a value of + 1 means a charge of + e, not one coulomb.
For the helium-3 ion in the figure:
Z = 2,; N = 1,; N_(e) = 1.
Therefore:
A = 2 + 1 = 3,; q/e = 2 − 1 = + 1.
Neutral helium-3 would have two electrons, making its charge zero while A remained 3. Mass number is a count, not an atom’s exact mass: it does not account for the mass difference between protons and neutrons, electrons, or binding energy. OpenStax: mass number and atomic mass.
An electron cloud is not a drawn orbit
Familiar atom symbols put electrons on circles or ellipses. That is an easy way to show “something around a center,” but an atom is not a miniature classical solar system. In the modern quantum description, an electron does not follow a specified planetary trajectory around the nucleus.
An atomic orbital is a spatial quantum description of an electron’s state, from which probabilities of detecting the electron in particular regions can be calculated. An “electron cloud” picture represents that probability distribution. Darker areas may mean higher probability density, if that is the convention chosen for the image. It is neither a solid cloud of material nor a photograph of one electron’s path. OpenStax: the quantum description of electrons.
An atom therefore has no single hard outer surface like a metal ball. Giving its size requires a chosen definition. We need no such definition for our comparison: the tokens simply count particles, without locating them or showing distances between them.
Removing an electron and changing a nucleus are different events
In hot gas, collisions and radiation can separate electrons from atoms. Under suitable conditions this produces plasma: a medium with free electrons and ions whose behavior depends significantly on electrical and magnetic interactions. Plasma can be partially ionized, so neutral atoms and charged particles coexist. Conditions inside a star and across the layers of its atmosphere differ; we should not imagine the whole star in one uniform state. Princeton Plasma Physics Laboratory: plasma.
Once hydrogen-1 has lost its only electron, its positive ion is simply an isolated proton. But it is still a hydrogen nucleus, not helium. Ionization does not create a new element.
Ordinary chemical reactions change electronic bonds and how atoms combine, while the proton and neutron counts in their nuclei remain unchanged. Hydrogen can thus become part of a new molecule. Nuclear reactions change the nuclei themselves; if a nucleus’s proton count changes, so does its element. OpenStax: chemical and nuclear changes.
Nuclear transformations, rather than the mere loss of electrons, allow the Sun to turn hydrogen into helium and release energy. This involves a sequence of reactions with nuclei and other particles; “add an electron to get helium” cannot represent that process. OpenStax: nuclear reactions in the Sun.
Try the reasoning: a nucleus contains two protons and two neutrons, with just one electron bound to it. What is it? Two protons identify helium. Four particles in the nucleus give helium-4. The electron balances only one of the two positive charges: this is a helium-4 ion with charge + e. The answer does not depend on where a schematic picture places the electron.
Atomic structure gives us three distinct guides: protons identify the element; protons together with neutrons identify the isotope; the balance of protons and electrons determines charge. These help us understand both water molecules and stellar plasma without confusing chemical and nuclear change. The next course question is where the chemical elements themselves came from.