A Particle Has 15 Protons 16 Neutrons and 18 Electrons: Which Change Will Cause the Particle to Have More Positively Charged Particles Than Negatively Charged Particles?

The particle becomes net positive only if it loses electrons (for example, losing 3 or more electrons) or gains protons. With 15 protons and 18 electrons, it currently has 3 more negative charges than positive ones. Removing 3 electrons makes charges equal at +15 and −15; removing a fourth makes it positive. Adding protons has the same effect without touching the electron count.

This atom is phosphorus-31 (15 protons, 16 neutrons). Its neutral form has 15 electrons, so the given particle is a phosphide-type ion with a 3− charge. The question is really about charge balance: count protons, count electrons, compare. Neutrons never enter that comparison because they carry no charge. That single fact resolves most confusion around this problem.

Charge balance is a proton-versus-electron count

Electric charge is quantized. A proton carries +1 elementary charge, an electron carries −1, and a neutron carries 0. The net charge of any isolated particle is therefore (number of protons) − (number of electrons), measured in elementary charge units. For this particle: 15 − 18 = −3. It is an anion with a 3− charge, written P³⁻ when the element is phosphorus.

The mechanism matters more than the arithmetic. Protons sit in the nucleus and are held there by the strong nuclear force, which is roughly 100 times stronger than the electromagnetic repulsion pushing those protons apart. Electrons occupy orbitals far outside the nucleus and are held by electrostatic attraction. Because the two populations live in different energy regimes, they change by different processes: electrons move between atoms during chemical reactions, while protons change only through nuclear processes. That asymmetry is why "lose electrons" is the practical answer and "gain protons" is the physical but impractical one.

A neutral phosphorus atom has 15 electrons. The particle in the question has 3 extra, which is why it is negative. Each added electron also adds repulsion among electrons, so highly negative ions are harder to form than mildly negative ones. This is one reason triply charged anions like P³⁻ appear mainly in solid compounds such as metal phosphides rather than as free-floating ions in solution.

In plain terms: the particle is negative because it has three more electrons than protons, and it flips to positive only when that gap closes.

Quantifying the 15/16/18 particle

The numbers define a specific nuclide and a specific ion. Fifteen protons fixes the element as phosphorus (atomic number 15). Sixteen neutrons give a mass number of 31, so the nucleus is phosphorus-31, the only stable phosphorus isotope and about 100 percent of natural phosphorus. Eighteen electrons give a charge of −3.

The mass sits almost entirely in the nucleus. A proton's mass is about 1,836 times an electron's, so 31 nucleons account for roughly 99.98 percent of the particle's mass. Losing or gaining a few electrons changes charge dramatically but changes mass negligibly. That is why chemistry cares about electrons and nuclear physics cares about nucleons.

Charge magnitude scales directly with the count difference. The elementary charge is about 1.602 × 10⁻¹⁹ coulombs, so a −3 ion carries roughly −4.8 × 10⁻¹⁹ C. Three electrons' worth of charge is tiny in absolute terms but decisive in chemical behavior because electric forces between particles at atomic distances are enormous relative to gravity.

The particle is also an ion of a specific isotope. None of these labels change the answer, but they matter for precision. A particle with 15 protons is always phosphorus regardless of neutron or electron count. Neutron count changes isotope, not element. Electron count changes ion charge, not element.

The electron-loss threshold, step by step

The table below walks the electron count down from 18 and shows the net charge at each step. The crossover to "more positive than negative" happens at 14 electrons.

The threshold is exact and unforgiving: 15 electrons is parity, and 14 is the first positive count. Removing exactly 3 electrons does not produce a positive particle; it produces a neutral one. This is the most common error in student answers, and it mirrors the same logic as asking how many electrons a neutral atom needs.

A caveat from professional practice: real ionization is sequential, not simultaneous. Removing one electron costs energy, and removing each subsequent electron costs more because the remaining electrons feel a stronger effective nuclear charge. Phosphorus has five valence electrons, so the first few removals are comparatively cheap and later ones are expensive. In mass spectrometry, for example, you typically see +1 ions far more often than +3 ions for a given element, because the energy budget rarely allows deep ionization. The arithmetic threshold stays at 14 electrons, but achieving it in a laboratory is not trivial.

Gaining protons versus losing electrons

Both routes reach a positive count, but only one is chemically realistic. Gaining a proton changes the element: 16 protons would make the particle sulfur, not phosphorus. That is nuclear transmutation, requiring energies on the order of millions of electron volts and typically occurring in stars, accelerators, or nuclear reactions. It does not happen in a beaker.

Losing electrons is ordinary chemistry. Ionization energies for removing valence electrons sit in the range of a few to roughly a dozen electron volts per electron for main-group elements, which is within reach of flames, plasmas, and electrical discharge. The energy difference between the two routes spans roughly six orders of magnitude, which is why "lose electrons" is the answer that matters.

A second alternative is to add negative charge to something else, but that changes the comparison partner, not this particle. If the question is strictly about this particle's own counts, only electron loss (or proton gain) works. If the particle is part of a compound, the relevant comparison may be between ions rather than within one particle.

There is a subtle nuclear alternative: beta decay. A neutron can convert to a proton, emitting an electron and an antineutrino. In phosphorus-31 this does not occur because the isotope is stable. In neutron-rich isotopes it does, and it would raise the proton count to 16 while lowering neutrons to 15. That is a genuine way to gain a proton, but it is a nuclear decay event, not a chemical change, and it is not available to this stable nuclide.

Where intuition goes wrong

Three misconceptions dominate this topic, and each has a clean correction.

Neutrons are not neutral players in charge balance. They contribute zero charge, so changing the neutron count changes mass and isotope but never the proton-to-electron comparison. Removing 3 electrons does not make the particle positive. It makes it neutral, because 15 protons and 15 electrons cancel exactly. The first positive count is 14 electrons. The number of protons is fixed for a given element. A phosphorus particle always has 15 protons. If the proton count changes, the element changes, and the question is no longer about the same particle.

A fourth, more professional caveat: charge state and reactivity are not the same thing. A highly charged ion is not automatically more reactive in every context. Solvation, lattice energy, and orbital availability govern what actually happens. A P³⁻ ion in a solid phosphide is stabilized by surrounding metal cations; the same ion in vacuum would be unstable toward electron loss. The arithmetic answer is universal, but the physical accessibility of each charge state depends on environment.

Practical checks and classroom actions

Use a repeatable procedure whenever a particle's charge is in question.

Identify the element from the proton count. Fifteen protons means phosphorus, full stop. Compute the neutral electron count, which equals the proton count for any neutral atom. Here it is 15. Compare actual electrons to the neutral count. Eighteen minus 15 gives 3 extra, so the ion is 3−. To reach positive, subtract enough electrons to drop below the proton count. Below 15 means 14 or fewer, so remove at least 4 electrons from the current 18. Sanity-check by recomputing the net charge after the change: 15 − 14 = +1.

The key threshold to remember is parity. Equal protons and electrons means neutral. More electrons means negative. Fewer electrons means positive. Every charge problem reduces to that comparison, and the 15/16/18 case is just one instance.

For exam-style questions, watch the wording. "Which change will cause the particle to have more positively charged particles than negatively charged particles" asks about the count comparison, not about net charge alone, though here the two coincide. If a question asks how many electrons must be removed to make the particle positive, the answer is 4 from the current state, or 2 from the neutral state.

Limits of the count model

The proton-minus-electron rule is exact for isolated particles and for the bookkeeping the question demands. It does not by itself predict stability, solubility, or reaction pathways. Those require energy calculations and structural context that a simple count cannot supply.

Real ions also rarely exist in isolation. In solution, a P³⁻ ion would be surrounded by solvent and counterions; in a solid, by a lattice. The effective charge experienced by neighbors is screened, sometimes heavily. This is why textbook charge labels and measured behavior can diverge, and why professional chemists treat formal charge as a starting point rather than an endpoint.

Finally, the question assumes integer electron counts and a well-defined particle. In delocalized systems such as metals, charge is shared across many atoms and the single-particle picture breaks down. For an atom or simple ion like this one, the model holds cleanly: 15 protons, 16 neutrons, 18 electrons, net −3, positive only at 14 electrons or fewer.