Enter any element by symbol (C), name (carbon), or atomic number (6), and this free atom calculator returns its protons, neutrons, electrons, ground-state electron configuration, period and group placement, and full isotope notation. Optionally specify a mass number for isotope-specific math, or a charge for cations and anions. Element data from IUPAC's 2021 atomic weights table — accurate for all 118 elements, from hydrogen to oganesson.
An atom is the smallest unit of matter that retains the chemical identity of an element. Every atom has a dense central nucleus containing positively-charged protons and electrically-neutral neutrons, surrounded by negatively-charged electrons arranged in shells and subshells. Three numbers describe any atom: the atomic number (Z) counts protons and defines the element; the mass number (A) counts protons plus neutrons and identifies the isotope; and the charge (q) describes any electron imbalance. From hydrogen (Z = 1) to oganesson (Z = 118), every known atom can be described by these three numbers. The calculator above gives you all of these, plus the ground-state electron configuration that determines how the atom bonds and behaves chemically.
Before diving into isotopes, ions, and electron configurations, make sure these four core ideas are clear. Most atomic-properties problems come back to these.
The number of protons (atomic number Z) is what makes an element what it is. Add or remove a proton and you have a different element entirely. All carbon atoms have exactly 6 protons; remove one and you've made boron (Z = 5).
Atoms of the same element can have different neutron counts. These are called isotopes. Carbon-12 and carbon-14 are both carbon (6 protons each), but C-12 has 6 neutrons while C-14 has 8. Isotopes share chemical behavior but differ in mass and nuclear stability.
In a neutral atom, the electron count equals the proton count. Losing electrons gives a positive ion (cation); gaining them gives a negative ion (anion). Electron count doesn't change the element — sodium and the sodium ion Na⁺ are both sodium, just with different electron counts.
The electron configuration — how electrons distribute across shells and subshells — determines an atom's chemistry. Atoms with full outer shells (noble gases) are unreactive. Atoms with one electron 'too many' (alkali metals) readily give it up; atoms one short (halogens) readily grab one.
The first 20 elements cover hydrogen through calcium and account for the vast majority of high-school and introductory chemistry problems. Note: neutron count uses the most abundant isotope of each element.
| Z | Symbol | Name | Protons | Neutrons | Electrons | Config |
|---|---|---|---|---|---|---|
| 1 | H | Hydrogen | 1 | 0 | 1 | 1s¹ |
| 2 | He | Helium | 2 | 2 | 2 | 1s² |
| 3 | Li | Lithium | 3 | 4 | 3 | [He] 2s¹ |
| 4 | Be | Beryllium | 4 | 5 | 4 | [He] 2s² |
| 5 | B | Boron | 5 | 6 | 5 | [He] 2s² 2p¹ |
| 6 | C | Carbon | 6 | 6 | 6 | [He] 2s² 2p² |
| 7 | N | Nitrogen | 7 | 7 | 7 | [He] 2s² 2p³ |
| 8 | O | Oxygen | 8 | 8 | 8 | [He] 2s² 2p⁴ |
| 9 | F | Fluorine | 9 | 10 | 9 | [He] 2s² 2p⁵ |
| 10 | Ne | Neon | 10 | 10 | 10 | [He] 2s² 2p⁶ |
| 11 | Na | Sodium | 11 | 12 | 11 | [Ne] 3s¹ |
| 12 | Mg | Magnesium | 12 | 12 | 12 | [Ne] 3s² |
| 13 | Al | Aluminum | 13 | 14 | 13 | [Ne] 3s² 3p¹ |
| 14 | Si | Silicon | 14 | 14 | 14 | [Ne] 3s² 3p² |
| 15 | P | Phosphorus | 15 | 16 | 15 | [Ne] 3s² 3p³ |
| 16 | S | Sulfur | 16 | 16 | 16 | [Ne] 3s² 3p⁴ |
| 17 | Cl | Chlorine | 17 | 18 | 17 | [Ne] 3s² 3p⁵ |
| 18 | Ar | Argon | 18 | 22 | 18 | [Ne] 3s² 3p⁶ |
| 19 | K | Potassium | 19 | 20 | 19 | [Ar] 4s¹ |
| 20 | Ca | Calcium | 20 | 20 | 20 | [Ar] 4s² |
Many isotopes have practical importance in medicine, dating, nuclear power, and research. Knowing the differences in neutron count explains why they behave so differently.
| Isotope | Protons | Neutrons | Half-life / status | Used for |
|---|---|---|---|---|
| ¹H (protium) | 1 | 0 | Stable | 99.985% of natural hydrogen |
| ²H (deuterium) | 1 | 1 | Stable | Heavy water, NMR spectroscopy |
| ³H (tritium) | 1 | 2 | 12.3 years | Self-luminous signs, fusion fuel |
| ¹²C | 6 | 6 | Stable (98.9%) | The standard for atomic mass unit |
| ¹⁴C | 6 | 8 | 5,730 years | Radiocarbon dating organic remains |
| ¹⁶O | 8 | 8 | Stable (99.76%) | The 'O' in almost every chemical formula |
| ¹³¹I | 53 | 78 | 8.0 days | Thyroid cancer treatment |
| ¹³⁷Cs | 55 | 82 | 30.2 years | Radiation therapy, food irradiation |
| ²³⁵U | 92 | 143 | 703 million years | Nuclear fission (power plants, weapons) |
| ²³⁸U | 92 | 146 | 4.47 billion years | Fertile material in breeder reactors |
| ²³⁹Pu | 94 | 145 | 24,100 years | Nuclear weapons, RTGs in spacecraft |
Almost every 'how many protons/neutrons/electrons' problem reduces to these three simple equations. Memorize them and the rest is just arithmetic.
The number of protons always equals the atomic number, regardless of isotope or charge. Atomic number is on every periodic table — usually the small integer above each element symbol. This number is the element's identity card.
Iron has Z = 26 on the periodic table, so every iron atom — regardless of isotope or ion state — has exactly 26 protons. There's no scenario where iron has 25 or 27 protons; that would make it a different element.
Mass number A is the total count of protons + neutrons. Subtract Z to isolate neutrons. The mass number is given in isotope notation (¹²C, ²³⁵U) or can be estimated by rounding the standard atomic weight to the nearest whole number when only the average is known.
Uranium-235 (²³⁵U) has mass number 235 and Z = 92, so it has 235 − 92 = 143 neutrons. Uranium-238 has 238 − 92 = 146 neutrons — three more, making it more nuclear-stable but less fissile.
For a neutral atom (charge 0), electrons equal protons. For ions, subtract the signed charge. Lost electrons mean positive charge, so subtract a positive number to get fewer electrons. Gained electrons mean negative charge, so subtracting a negative adds.
A sodium cation Na⁺ has Z = 11 and charge +1, so 11 − (+1) = 10 electrons. An oxide ion O²⁻ has Z = 8 and charge −2, so 8 − (−2) = 10 electrons. Both have the same electron count as neutral neon — they're 'isoelectronic.'
These three terms get used loosely but mean very specific things. Mixing them up is the most common conceptual error in introductory chemistry.
The smallest electrically neutral unit of an element. One nucleus (protons + neutrons), surrounded by an equal number of electrons.
Molecules are groups of atoms bonded together (H₂O, CO₂); ions are atoms or molecules with a net charge (Na⁺, SO₄²⁻).
| Property | Atom | Molecule | Ion |
|---|---|---|---|
| Number of atoms | 1 | 2 or more | 1 or more |
| Net electric charge | 0 (neutral) | Usually 0 | Non-zero |
| Held together by | Nuclear forces | Chemical bonds | Bonds + charge |
| Examples | Fe, He, C | H₂O, CO₂, N₂ | Na⁺, Cl⁻, SO₄²⁻ |
| This calculator | Yes (input element) | No (use mole calc) | Yes (set charge) |
These three particles make up every atom, but they have wildly different masses, charges, and locations. Understanding their differences explains most of chemistry.
Heavy particles that live in the dense nucleus. Together they account for over 99.9% of an atom's mass.
Light particles in fuzzy quantum-mechanical clouds around the nucleus. They occupy almost all of an atom's volume but contribute almost none of its mass.
| Property | Proton | Neutron | Electron |
|---|---|---|---|
| Symbol | p (or p⁺) | n (or n⁰) | e (or e⁻) |
| Charge (e units) | +1 | 0 | −1 |
| Mass (kg) | 1.673×10⁻²⁷ | 1.675×10⁻²⁷ | 9.109×10⁻³¹ |
| Mass (atomic units) | 1.0073 u | 1.0087 u | 0.000549 u |
| Location in atom | Nucleus | Nucleus | Orbital cloud |
| Quark composition | uud | udd | None (fundamental) |
| Stability (free) | Stable | ~10 min half-life | Stable |
Atomic structure isn't just textbook chemistry — it's behind technologies and natural phenomena you encounter every day.
Nuclear medicine uses radioactive isotopes (radionuclides) for both imaging and treatment. Iodine-131 treats thyroid cancer because the thyroid concentrates iodine. Technetium-99m, the most-used medical isotope, traces blood flow in heart and bone scans. PET scans use fluorine-18 attached to glucose to spotlight active tissue. Each isotope is chosen for its specific half-life and decay properties.
Geologists date rocks using uranium-235/238 (millions to billions of years), potassium-40 (volcanic rocks, ~1.25 billion year half-life), and carbon-14 (organic remains under 50,000 years). The math is simple: measure the ratio of parent to daughter isotope, then use the known decay rate to compute elapsed time. This is how we know the Earth is 4.54 billion years old.
Power reactors split uranium-235 (or plutonium-239) atoms to release energy. A typical nuclear plant uses fuel enriched from 0.7% U-235 (natural) to about 3-5% U-235. Weapons require ~90% enrichment. Why the difference? Because the chain reaction needs enough U-235 neutrons to sustain rapid fission. The neutron-to-proton ratio in heavy isotopes is what makes some splittable and others not.
Every chemical bond, drug interaction, and biological process comes down to electron configuration. Sodium gives up one electron to chlorine to form table salt. Carbon's 4 valence electrons let it form complex molecules — the basis of all known life. Noble gases (full outer shells) don't bond with anything, which is why helium balloons stay helium-only. The configuration tells you what an element will and won't do.
Before counting anything, isolate three numbers from the problem: atomic number Z (defines the element), mass number A (defines the isotope), and charge q (defines the ion state). Once you have these three, the formulas p = Z, n = A − Z, e = Z − q give every answer. Most errors come from skipping this step and trying to count directly from a confusing problem statement.
If a problem gives you 'chlorine atomic mass = 35.45' without specifying an isotope, round to 35 to get the dominant isotope's mass number. So natural chlorine is typically ³⁵Cl (17 protons, 18 neutrons). Don't try to use 35.45 directly as a mass number — that gives a non-integer neutron count, which is impossible for a single atom.
Two species are isoelectronic when they have the same electron count. Na⁺, Mg²⁺, Al³⁺, F⁻, O²⁻, and N³⁻ all have 10 electrons (same as neutral Ne). They have similar electron behavior but different nuclei. Problems often ask you to compare these — the ones with more protons are smaller (pull electrons in tighter). Knowing that all of these have the neon configuration helps.
Standard Aufbau order works for ~93 of the first 100 elements, but chromium and copper have famous exceptions: Cr is [Ar] 3d⁵ 4s¹ (not 3d⁴ 4s²) and Cu is [Ar] 3d¹⁰ 4s¹ (not 3d⁹ 4s²). Half-filled and fully-filled d subshells have extra stability. Similar exceptions occur for Mo, Ag, and a few others. If you write the 'predicted' configuration for one of these and your answer doesn't match, this is probably why.
Writing iron as 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s² is correct but unwieldy. The noble-gas form [Ar] 3d⁶ 4s² is equivalent and faster to write/read because everyone knows what 'argon's configuration' means. Higher-Z elements get this benefit even more — uranium written out is a long line; [Rn] 5f³ 6d¹ 7s² is much cleaner. Both forms are acceptable on exams unless specified otherwise.
Atomic number Z (protons, defines element) and mass number A (protons + neutrons, defines isotope) are different. A periodic table shows Z above each symbol; mass number is part of isotope notation (¹²C). Putting the average atomic weight where the mass number belongs gives non-integer neutron counts, which is nonsensical for a single atom.
The formula e = Z − q is signed. A +1 charge means LOST one electron, so subtract a positive number → fewer electrons. A −1 charge means GAINED one, so subtract a negative → more electrons. Common error: Cl⁻ has 17 − 1 = 16 electrons (WRONG). It's 17 − (−1) = 18 electrons. Watch the signs.
Iron has Z = 26, but iron(III) ion Fe³⁺ has 26 − 3 = 23 electrons, not 26. The element gives you the proton count (constant) and the maximum neutral electron count. The actual electron count depends on the ion state. If a problem says 'Fe³⁺,' the +3 is information, not decoration — use it.
Carbon-12 and carbon-14 are both carbon — same chemistry, same place on the periodic table, same Z. They differ only in neutron count, which affects nuclear stability and mass but not chemical behavior. Problems sometimes set traps by asking 'which has more electrons, C-12 or C-14?' Answer: same — both are carbon, so both have 6 electrons when neutral.
Predicting Cr as [Ar] 3d⁴ 4s² is wrong — the correct ground state is [Ar] 3d⁵ 4s¹ because the half-filled d shell is more stable. Cu is similar: not [Ar] 3d⁹ 4s², but [Ar] 3d¹⁰ 4s¹. These exceptions trip up exam writers and students equally. Memorize Cr and Cu at minimum; the others (Mo, Ag, etc.) follow the same pattern.
An atom is a single unit of an element (one nucleus with its electrons). A molecule is two or more atoms held together by chemical bonds. Hydrogen gas H₂ is a molecule made of two hydrogen atoms; water H₂O is a molecule of two hydrogens and one oxygen. Some elements exist naturally as single atoms (noble gases: He, Ne, Ar) and others as molecules (O₂, N₂, Cl₂, etc.). 'Atom' is always one nucleus; 'molecule' is multiple atoms bonded.
Atoms gain charge by gaining or losing electrons during chemical interactions. The proton count in the nucleus is fixed — it requires nuclear-physics-level energy to change. But electrons sit in the outer regions and can be relatively easily added or removed in chemical bonding. Metals tend to lose electrons (form cations); nonmetals tend to gain them (form anions). The drive comes from getting closer to a noble-gas electron configuration, which is energetically favorable.
Protons and neutrons are bound together by the strong nuclear force, which acts only over very short distances (a few femtometers). It's much stronger than electromagnetic repulsion at those distances, so it overcomes the electrical repulsion between positive protons. Electrons are bound to the nucleus by electromagnetic attraction, which is much weaker, so electrons orbit at distances roughly 100,000 times the nuclear diameter. The atom is mostly empty space — if a nucleus were a marble, the nearest electrons would be over a kilometer away.
Yes — that's a cation, a positively-charged ion. Sodium loses one electron to become Na⁺ (11 protons, 10 electrons). Calcium loses two to become Ca²⁺ (20 protons, 18 electrons). The maximum positive charge an atom can have is limited by how much energy it takes to rip off each successive electron — eventually you'd need more energy than any chemical process supplies. In stars, atoms can be stripped of most or all electrons (becoming highly-charged ions or even bare nuclei).
Right now, oganesson (Og, Z = 118) is the heaviest element with a confirmed name. It's a synthetic noble gas, made one atom at a time in particle accelerators by smashing calcium-48 into californium-249. Each atom of oganesson exists for less than a millisecond before decaying. Elements 119 and 120 are actively being searched for; their discovery would extend the periodic table to a new row. There's a theoretical 'island of stability' around Z = 114–120 where some isotopes might be much longer-lived than the synthetic elements made so far.
An orbital is a 3D region around the nucleus where an electron is likely to be found — typically the volume containing about 90% of the probability of finding the electron. Orbitals have shapes: s orbitals are spherical, p orbitals are dumbbell-shaped, d orbitals have four-lobed cloverleaf shapes, and f orbitals are even more complex. Each orbital can hold up to 2 electrons (with opposite spins, per the Pauli exclusion principle). The number and type of filled orbitals in an atom IS its electron configuration.
The number of protons in an atom always equals its atomic number (Z). Carbon has atomic number 6, so every carbon atom has exactly 6 protons. The atomic number is what defines the element — change the proton count and you have a different element. You can find Z on any periodic table; it's the small integer above each element symbol.
Subtract the atomic number (Z) from the mass number (A): neutrons = A − Z. For example, carbon-14 has mass number 14 and atomic number 6, so it has 14 − 6 = 8 neutrons. The mass number is the integer total of protons + neutrons, not the standard atomic weight (which is a weighted average across all natural isotopes). If your problem gives you a standard atomic weight, round it to the nearest whole number to get the typical mass number.
A neutral atom has the same number of electrons as protons — equal to the atomic number. Carbon (Z = 6) has 6 electrons when neutral. For ions, subtract the charge from the proton count: a sodium ion Na⁺ has 11 − 1 = 10 electrons (it lost one electron, giving it a net +1 charge). A chloride ion Cl⁻ has 17 − (−1) = 18 electrons (it gained one).
Electron configuration describes how electrons are distributed across atomic orbitals (shells and subshells), following the Aufbau principle. For carbon: 1s² 2s² 2p². The notation reads as: 2 electrons in the 1s subshell, 2 in 2s, and 2 in 2p — totaling 6 electrons. For heavier elements, we abbreviate inner shells using the previous noble gas, e.g., [Ar] 4s² for calcium means 'all of argon's electrons plus 2 in 4s.' Knowing the configuration tells you about chemical behavior — outer-shell electrons determine bonding.
Atomic number (Z) is the count of protons, and it defines the element. Mass number (A) is the count of protons + neutrons, and it identifies which isotope you have. A specific atom is written ᴬZ-X where X is the element symbol. For example, ¹²₆C is carbon-12 (Z = 6, A = 12, so 6 neutrons), while ¹⁴₆C is carbon-14 (Z = 6, A = 14, so 8 neutrons). Both are carbon, but the second has 2 extra neutrons.
The atomic mass listed on the periodic table is a weighted average of all naturally occurring isotopes of that element, weighted by their natural abundance. Chlorine's atomic mass is 35.45 because natural chlorine is roughly 76% chlorine-35 and 24% chlorine-37. The mass NUMBER of any specific atom is always a whole number — it just counts protons + neutrons — but the average across all isotopes in a typical sample isn't whole. For most quick problems, round the atomic mass to the nearest whole number to get the dominant isotope's mass number.
An ion is an atom (or group of atoms) with an unequal number of protons and electrons, giving it a net electric charge. Cations are positive (more protons than electrons — lost electrons), like Na⁺ or Ca²⁺. Anions are negative (more electrons than protons — gained electrons), like Cl⁻ or O²⁻. The number of protons doesn't change when you form an ion — only the electron count does. This is how atoms bond: most ionic compounds form when one atom gives electrons to another (like sodium giving an electron to chlorine to form NaCl).
Atomic numbers and proton counts are exact integers — there's no approximation possible. Standard atomic weights come from IUPAC's 2021 published table and are accurate to the precision shown (typically 3–5 significant figures). Electron configurations use the ground-state Aufbau order with the well-known exceptions (Cr is [Ar] 3d⁵ 4s¹, not 3d⁴ 4s²; Cu is [Ar] 3d¹⁰ 4s¹). For the heaviest synthetic elements (Z ≥ 100), some properties are predicted rather than measured; we mark these with their longest-lived isotope mass numbers.
All standard atomic weights in this calculator come from IUPAC's 2021 Table of Standard Atomic Weights, the international standard maintained by the IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW). Mass numbers for default isotopes use the most abundant naturally occurring isotope where one dominates, or the longest-lived isotope for synthetic elements (Z ≥ 84 generally). Electron configurations follow the standard ground-state Aufbau order with the well-known exceptions (Cr, Cu, Mo, Pd, Ag, Au, Pt, La, Ce, Gd, Lu, Ac, Th, Pa, U, Np, Cm). Period and group placements match the modern 18-column IUPAC layout; lanthanides and actinides are shown as 'no group' since they don't fit cleanly into the main 1–18 numbering. Particle masses are CODATA 2018 recommended values.