Build an Atom: Atomic Structure

Choose the protons, neutrons and electrons of any atom or ion from hydrogen to calcium. Protons fix the element, A = Z + N gives the mass number, and charge = Z - electrons. Natural abundances and atomic masses come from the NIST isotope tables.

proton neutron electron Not to scale: a real nucleus is tens of thousands of times smaller than its atom.
Natural abundance
In nature: 98.93 % of carbon atoms
Protons fix the element; neutrons fix the isotope; electrons fix the charge.
Protons (Z)6
Neutrons (N)6
Electrons6
Element
Carbon (C)
Z = 6 · period 2 · group 14
Mass number  A = Z + N
12
carbon-12 · A = 6 + 6
Net charge  Z - electrons
0
neutral atom
0 C
Electron shells
2, 4
outer shell holds 4
Atomic mass12.000000 u
neutral-atom mass (NIST)
Mass in the nucleus99.97 %
of the particle mass is in the nucleus
Nuclear radius2.75 fm
nuclear radius, about 1.2 × A1/3 fm
Particle masses (CODATA): proton 1.00727647 u · neutron 1.00866492 u · electron 0.000548580 u · e = 1.602176634 × 10-19 C · isotope masses and abundances: NIST.
The 2, 8, 8, 2 shell pattern only works up to calcium: after it the 3d subshell fills. Shells are a counting model; the quantum picture uses orbitals. The mass share uses free-particle masses and ignores binding energy, which makes a real atom slightly lighter than its parts.