Atomic structure and nuclide notation

Key ideas

An atom is made of a tiny, dense nucleus surrounded by electrons in mostly empty space. The nucleus contains two types of particle, together called nucleons:

Particle	Location	Relative mass	Relative charge
Proton	nucleus	1	$+1$
Neutron	nucleus	1	0
Electron	orbiting the nucleus	$\dfrac{1}{1836}$ (negligible)	$-1$

• The number of protons fixes which element the atom is — this is the atomic number.
• A neutral atom has equal numbers of protons and electrons, so its charges balance.
• Almost all the atom's mass is in the nucleus, because electrons are so much lighter than protons and neutrons.

Nuclide notation

Every nucleus (a nuclide) is written with two numbers next to its element symbol:

$^{A}_{Z}X$

• $X$ — the chemical symbol of the element
• $Z$ — atomic number, the number of protons
• $A$ — mass number, the total number of protons + neutrons

The number of neutrons is found from these two numbers:

$N = A - Z$

• $N$ — number of neutrons
• Subtract the atomic number from the mass number to get the neutron count.

Isotopes

• Isotopes are atoms of the same element (same $Z$, same number of protons) with different numbers of neutrons (different $A$).
• Isotopes of an element have identical chemical properties but different masses, and often different nuclear stability.
  • Example: carbon-12 ($^{12}_{6}\text{C}$) has 6 neutrons; carbon-14 ($^{14}_{6}\text{C}$) has 8 neutrons. Both have 6 protons.

Nuclear stability

• A nucleus is held together by the strong nuclear force, which acts between protons and neutrons over very short range and overcomes the electrical repulsion between protons.
• Whether a nucleus is stable or unstable (radioactive) depends on its proton-to-neutron ratio.
  • Small nuclei are most stable with roughly equal numbers of protons and neutrons.
  • Larger nuclei need more neutrons than protons to stay stable, because the extra neutrons add strong-force attraction without adding electrical repulsion.
  • A nucleus with too many or too few neutrons for its size is unstable and will undergo radioactive decay to move toward a more stable arrangement.

Practice question

An atom of uranium is written $^{238}_{92}\text{U}$. State the number of protons, neutrons and electrons in a neutral atom of this isotope.

Worked solution: protons $= Z = 92$. Neutrons $= A - Z = 238 - 92 = 146$. A neutral atom has equal protons and electrons, so electrons $= 92$.