K326 / K327 / 2027
Radioactivity overview

Topic 1 of 5

Atoms, nuclei and isotopes

An atom has a small positive nucleus, with negatively charged electrons outside it. The nucleus contains protons and usually neutrons.

Proton
A positively charged particle in the nucleus. Its charge is +1 when measured in units of the proton's charge.
Neutron
An uncharged particle in the nucleus. Protons and neutrons are collectively called nucleons.
Electron
A negatively charged particle outside the nucleus. Its charge is -1 in the same units. A neutral atom has equal numbers of protons and electrons.

The nucleus is much smaller than the whole atom. A drawing that makes its particles visible cannot also show their sizes and separations to one realistic scale. Ordinary hydrogen has one proton and no neutron in its nucleus, so a neutron is not present in every atom.

Read a nuclide symbol

A nuclide is a type of nucleus specified by its proton and neutron numbers. Its notation puts the nucleon number A at the upper left and the proton number Z at the lower left of the element symbol.

means sodium-23, with 23 nucleons and 11 protons.

Proton number, Z
The number of protons in a nucleus. It is also called atomic number and identifies the element.
Nucleon number, A
The total number of protons and neutrons. It is also called mass number. It is a count, not the atom's mass in grams.
Number of neutrons = A - ZA and Z have no units. For a neutral atom, the electron number is Z as well.

Particles, nuclide notation and isotope comparisons

A neutral helium-4 atom

The nucleus is greatly enlarged. Positions are schematic; the drawing is not to scale and does not show fixed electron paths.

Two protons, two neutrons and two electrons in a neutral helium-4 atomA greatly enlarged central nucleus contains four separately labelled particles: two p-plus protons and two n neutrons. Two e-minus electrons are outside the nucleus, in a shaded region that does not depict their paths. Protons have charge plus one, neutrons zero, and electrons minus one in proton-charge units. The equal proton and electron numbers make the atom neutral. Lengths, positions and particle sizes are schematic.p+nnp+e-e-NucleusElectrons outside the nucleusp+2 protons; charge +1 eachn2 neutrons; charge 0e-2 electrons; charge -1 each

Read sodium-23's left-hand numbers

Sodium-23: nucleon number 23, proton number 11The element symbol Na has 23 at its upper left and 11 at its lower left. The upper number A counts all protons plus neutrons; the lower number Z counts protons. Na means sodium. Therefore this nucleus contains 11 protons and 23 minus 11 equals 12 neutrons. A neutral sodium-23 atom also has 11 electrons. The upper number is a count, not a mass in grams.23 = protons + neutronsNucleon number A2311NaProton number Z11 protonsElement:sodiumNeutrons = 23 - 11 = 12Neutral atom: 11 electrons

Compare neutral atoms

Same proton number identifies the element; different neutron numbers identify its isotopesThree cards use proper upper-left nucleon numbers and lower-left proton numbers. Sodium-23 has 11 protons, 12 neutrons and 11 electrons. Sodium-24 has 11 protons, 13 neutrons and 11 electrons. Magnesium-24 has 12 protons, 12 neutrons and 12 electrons. Both sodium atoms have proton number 11, but different neutron counts, so they are isotopes. Sodium-24 and magnesium-24 share nucleon number 24 but have different proton numbers, so they are different elements.2311NaSodium-2311 protons12 neutrons11 electrons2411NaSodium-2411 protons13 neutrons11 electrons2412MgMagnesium-2412 protons12 neutrons12 electronsThe sodium pair: same Z, different ANa-24 and Mg-24: same A, different Z
The atom schematic makes the particles visible without using a realistic size scale. In a nuclide symbol, the upper number counts nucleons and the lower number counts protons. Compare the two numbers before deciding whether two atoms are isotopes.

Same element, different neutron number

For sodium-23, the neutron count is 23 - 11 = 12. Sodium-24 still has 11 protons, but has 24 - 11 = 13 neutrons. They are isotopes: atoms of the same element with the same proton number but different neutron numbers.

Magnesium-24 has 12 protons and 12 neutrons. It shares nucleon number 24 with sodium-24, but its different proton number makes it a different element. Equal nucleon numbers alone do not identify isotopes.

Removing or adding electrons changes an atom's charge, producing an ion. It does not change its proton or neutron numbers, so it does not create another isotope. Also, the existence of isotopes does not mean that every isotope of an element is radioactive.

Optional check Three neutral atoms have these nucleon/proton numbers: P has A = 23, Z = 11; Q has A = 24, Z = 11; R has A = 24, Z = 12. Which pair consists of isotopes of the same element?
Three neutral atoms have these nucleon/proton numbers: P has A = 23, Z = 11; Q has A = 24, Z = 11; R has A = 24, Z = 12. Which pair consists of isotopes of the same element?