Atom Calculator

Common atom profiles

Atom Calculator: Complete Guide

Every atom is defined by three numbers: the atomic number (Z, the number of protons), the number of neutrons (N), and the number of electrons. The mass number (A = Z + N) gives the total nucleon count. The LazyTools atom calculator takes any combination of element, atomic number, neutron count and ion charge and returns the complete atomic profile: protons, neutrons, electrons, mass number and ion type.

How to find protons, neutrons and electrons

Protons: equal to the atomic number Z. For chlorine (Z=17): 17 protons. Neutrons: mass number minus atomic number. For Cl-35: A=35, N=35-17=18 neutrons. For Cl-37: N=37-17=20 neutrons. Electrons in a neutral atom: equal to Z. For a Cl- ion (charge -1): electrons = 17+1 = 18. For a Ca2+ ion: electrons = 20-2 = 18. The mass number A is always a whole number (the sum of protons and neutrons). The atomic mass on the periodic table is a weighted average of all natural isotopes and is not a whole number.

Isotopes: same element, different neutrons

Isotopes of an element share the same Z but differ in N. Carbon has three natural isotopes: C-12 (Z=6, N=6), C-13 (Z=6, N=7), C-14 (Z=6, N=8). Hydrogen has three: H-1 (protium, Z=1, N=0), H-2 (deuterium, Z=1, N=1), H-3 (tritium, Z=1, N=2). The atom calculator lets you specify the exact neutron number to identify the isotope. Common exam questions: how many neutrons in Fe-56? N = 56-26 = 30. How many electrons in Fe3+? Electrons = 26-3 = 23.

Ions: atoms with gained or lost electrons

An ion forms when an atom gains or loses electrons. Cations (positive charge) lose electrons: Na+ has 11 protons and 10 electrons. Mg2+ has 12 protons and 10 electrons. Al3+ has 13 protons and 10 electrons. Anions (negative charge) gain electrons: Cl- has 17 protons and 18 electrons. O2- has 8 protons and 10 electrons. N3- has 7 protons and 10 electrons. Isoelectronic species have the same electron count: Na+, Mg2+, Al3+, Ne, O2-, N3-, F- all have 10 electrons.

Mass number vs atomic mass: what is the difference?

The mass number A is an integer -- the total count of protons and neutrons in one specific isotope. The atomic mass (or standard atomic weight) printed on the periodic table is a weighted average of all naturally occurring isotopes of that element. Chlorine's atomic mass is 35.45 because it is approximately 75% Cl-35 (mass 34.969) and 25% Cl-37 (mass 36.966). The weighted average: (0.7577 x 34.969) + (0.2423 x 36.966) = 35.45. The atom calculator uses mass number (integer) for isotope identification; for weighted atomic mass calculations use the Average Atomic Mass Calculator.

Nuclear notation and shorthand

Nuclear notation writes the element symbol with mass number as superscript and atomic number as subscript. For chlorine-35: the symbol is written with 35 above left and 17 below left of Cl. Shorthand: Cl-35, or just the mass number when the element is specified. In nuclear reactions, the mass numbers and atomic numbers must balance on both sides of the equation. For the beta decay of C-14: C-14 becomes N-14 plus an electron (beta particle). The atom calculator supports any Z from 1 (hydrogen) to 118 (oganesson), the complete periodic table.

Common atom calculation exam questions

How many neutrons in uranium-238? N = 238 - 92 = 146. How many electrons in Cu2+? Electrons = 29 - 2 = 27. How many protons in an atom with 18 neutrons and mass number 35? Z = 35 - 18 = 17 (chlorine). What is the mass number of an iron atom with 30 neutrons? A = 26 + 30 = 56 (Fe-56, the most abundant iron isotope). These four calculation types cover the majority of atomic structure questions in GCSE, A-level, AP Chemistry and university general chemistry courses worldwide.

How to use this chemistry calculator effectively

All LazyTools chemistry calculators run entirely in your browser -- no data is sent to a server, no account is required, and results appear instantly. The tools are designed for chemistry students, educators and professionals who need quick, accurate answers during study sessions, lab work or problem-set solving. Each calculator shows the underlying formula alongside the result so you can verify the calculation method and understand the chemistry, not just get a number. Copy the result to your clipboard for use in lab reports, assignments or messages with one click.

Related chemistry topics and further study

Chemistry calculators are most useful when paired with a solid understanding of the underlying concepts. This calculator addresses one specific quantitative aspect of chemistry -- the formula, units and typical values are described in the article above. For a deeper understanding, pair this tool with a standard university-level general chemistry textbook such as Atkins, Zumdahl, or Chang and Overby. The LazyTools chemistry suite covers all major quantitative topics in general, physical, organic and biochemistry -- see the related tools section below for the calculators most commonly used alongside this one.

Atomic structure across the periodic table

Atomic structure varies systematically across the periodic table in ways that explain chemical behaviour. Moving left to right across a period, the atomic number increases (more protons) while the number of electron shells stays the same, so the nuclear charge pulls electrons closer and atomic radius decreases. Moving down a group, each new period adds a shell of electrons, increasing atomic radius and shielding the outer electrons from the nucleus. The atom calculator shows the proton count (Z) and electron configuration context for any element, making it a quick reference for periodic trends in atomic structure problems.

Subatomic particles and their properties

The three subatomic particles define atomic structure: the proton (charge +1, mass 1.007276 amu, located in nucleus), the neutron (charge 0, mass 1.008665 amu, located in nucleus), and the electron (charge -1, mass 0.000549 amu, located in shells/orbitals). The nucleus contains protons and neutrons (collectively called nucleons) and is approximately 10^-15 m in diameter -- about 100,000 times smaller than the atom itself. The electron cloud occupies approximately 10^-10 m. Most of the mass of an atom is in the nucleus (protons + neutrons), while most of the volume is empty space occupied by fast-moving electrons. This nuclear model was established by Rutherford's gold foil experiment in 1911, which showed that the atom is mostly empty with a tiny dense positive centre.

Worked GCSE and A-level atom calculation examples

Example 1: A calcium ion Ca2+ has atomic number 20. Protons = 20; electrons = 20 - 2 = 18; if it is Ca-40, neutrons = 40 - 20 = 20; mass number = 40. Example 2: An atom has 17 protons, 18 neutrons and 17 electrons. Element: Z=17 = chlorine. Mass number = 35. Ion: neutral (electrons = protons). Example 3: An anion has 16 protons, 16 neutrons and 18 electrons. Z=16 = sulfur; charge = 16-18 = -2 (S2- ion); mass number = 32 (S-32). These are the three standard question types for atomic structure at GCSE and A-level. The LazyTools atom calculator handles all three when you specify Z, N, and charge correctly. These calculation patterns are equally applicable at university level in general chemistry courses, where atomic structure forms the foundation for understanding chemical bonding, periodic trends, oxidation states and spectroscopy. Mastering the proton-neutron-electron model is the entry point to all of quantitative chemistry and the LazyTools atom calculator provides a fast, reliable reference for any atom or ion encountered during study. The LazyTools chemistry suite covers all quantitative topics from atomic structure through thermodynamics and electrochemistry.

The LazyTools chemistry calculator suite is free, browser-based and covers all major quantitative chemistry topics taught at GCSE, A-level and undergraduate level.

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