Particles in the atom and atomic radius

Structure of the Atom

The atom is mostly empty space surrounding a very small, dense nucleus containing protons and neutrons, with electrons occupying shells around it.

The nucleus is roughly 10,000 times smaller in diameter than the atom itself. Almost all the atomic mass is concentrated in this central nucleus, and electrons move through the surrounding empty space in fixed shells.

Mass distribution: protons and neutrons in the nucleus carry essentially all the atomic mass. Electrons contribute negligibly because each electron has a mass of about $\frac{1}{1840}$ of a proton.

Charge distribution: the nucleus carries a concentrated positive charge from its protons. The negative charge from electrons is spread across the surrounding shells. A neutral atom has equal numbers of protons and electrons, so the net charge is zero.

Protons, Neutrons and Electrons

The three subatomic particles differ in their relative charges and relative masses, which determines how each behaves inside and around the atom.

Protons and neutrons have almost identical masses, so each is assigned a relative mass of 1. The electron’s mass is so small that it is treated as negligible in mass calculations. The proton and electron carry equal-magnitude but opposite charges, so atoms are electrically neutral.

Proton Number and Nucleon Number

Every atom is described by two key whole numbers, its proton number and nucleon number, which together specify the composition of the nucleus.

• Proton number (atomic number, Z): the number of protons in the nucleus of an atom.
• Nucleon number (mass number, A): the total number of protons and neutrons in the nucleus.

Standard notation places A above Z to the left of the chemical symbol, for example ²³₁₁Na. This sodium atom has 11 protons and 23 nucleons, so it contains 12 neutrons.

Number of neutrons = A − Z. Two atoms of the same element with different nucleon numbers are called isotopes, because they have the same Z but different A.

Behaviour of Particle Beams in an Electric Field

When beams of protons, neutrons and electrons travel at the same velocity through an electric field, each is deflected differently because of its charge and mass.

• Protons (charge +1, relative mass 1): deflected towards the negative plate by a small amount.
• Electrons (charge −1, relative mass $\frac{1}{1840}$): deflected towards the positive plate by a much larger amount, because of their very small mass.
• Neutrons (charge 0): not deflected at all, because the electric field exerts no force on a neutral particle.

The direction of deflection depends on the sign of the charge, while the magnitude of deflection depends on the charge-to-mass ratio. Electrons have a far larger charge-to-mass ratio than protons, so they curve much more sharply.

Working Out Particles in Atoms and Ions

Use the proton number, nucleon number and charge to determine the numbers of protons, neutrons and electrons in any atom or ion.

Rules:

• Number of protons = proton number (Z).
• Number of neutrons = nucleon number − proton number (A − Z).
• Number of electrons in a neutral atom = number of protons.
• For a positive ion, number of electrons = number of protons − charge.
• For a negative ion, number of electrons = number of protons + magnitude of charge.

Adding or removing electrons changes the charge but never changes Z or A. Only nuclear processes alter proton or neutron count.

Trends in Atomic and Ionic Radius

The atomic radius decreases across a period and increases down a group, with ionic radii following related patterns governed by nuclear charge and shielding.

Across a period (left to right):

• Each successive element gains one extra proton, so nuclear charge increases.
• Electrons are added to the same outer shell, so shielding [the screening of outer electrons from nuclear attraction by inner electrons] stays approximately constant.
• The increased nuclear attraction pulls outer electrons closer to the nucleus.
• Therefore atomic radius decreases across the period.

Down a group (top to bottom):

• Each successive element has one additional occupied electron shell.
• The outer electrons sit further from the nucleus.
• Inner shells provide more shielding from the nuclear charge.
• Therefore atomic radius increases down the group.

Ionic radius patterns:

Positive ions (cations) are smaller than their parent atoms. Losing electrons reduces electron–electron repulsion, and often empties the outermost shell entirely. The remaining electrons are pulled closer by the now-greater proton-to-electron ratio.

Negative ions (anions) are larger than their parent atoms. Gaining electrons increases electron–electron repulsion in the outer shell, so the electron cloud expands outward.

For an isoelectronic series [ions and atoms with the same number of electrons], ionic radius decreases as nuclear charge increases. For example, N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺ and Al³⁺ all have 10 electrons, but radius decreases from N³⁻ to Al³⁺.

MisconceptionStudents often say atomic radius decreases across a period because “more electrons are added”. The real cause is increased nuclear charge attracting the same outer shell more strongly, while shielding stays nearly constant. Exam cue: always cite both nuclear charge and shielding when explaining radius trends.