Particles in the Atom and Atomic Radius

Full Notes

Atomic Structure and the Location of Subatomic Particles

Atoms are composed of a central nucleus and surrounding electrons.

The nucleus is tiny but contains almost all the atom’s mass. It consists of:

• Protons – positively charged particles (+1) with a relative mass of 1.
• Neutrons – neutral particles with a relative mass of 1.
• Electrons – negatively charged (–1) with negligible mass (~1/1836) that orbit the nucleus in shells.

Even though the nucleus is dense, it occupies only a very small part of the atom's volume – the rest is empty space where electrons move in shells.

2. Key Terms: Atomic and Mass Numbers

• Atomic number (Z): the number of protons in an atom. It defines the element.
• Mass number (A): the total number of protons and neutrons.
• Nucleon number: another name for the mass number.
• Atoms are neutral because they have equal numbers of protons and electrons.
• Ions are formed when electrons are gained or lost:
  • Positive ions (cations) form by losing electrons.
  • Negative ions (anions) form by gaining electrons.

Example: If an atom has an atomic number of 11 and mass number 23:
Protons = 11  •  Neutrons = 12  •  Electrons = 11 (if neutral)

3. Distribution of Mass and Charge in an Atom

• Almost all the mass is in the nucleus (protons and neutrons).
• Charge is:
  • +1 per proton (in the nucleus).
  • –1 per electron (orbiting the nucleus).
  • Neutrons have no charge.
• Thus, atoms are electrically neutral overall but have a small, dense positively charged nucleus.

4. Behaviour of Subatomic Particles in an Electric Field

If a beam of protons, neutrons, and electrons travel at the same speed through an electric field:

• Protons (positive) are deflected towards the negative plate.
• Electrons (negative) are deflected towards the positive plate – more sharply, because they are much lighter.
• Neutrons (neutral) are not deflected at all.

This demonstrates that charge and mass affect how particles behave in electric fields.

5. Determining Numbers of Subatomic Particles in Ions and Atoms

To determine the number of each particle:

• Protons = Atomic number (Z)
• Neutrons = Mass number (A) – Atomic number (Z)
• Electrons:
  • Neutral atom: same as protons.
  • Positive ion: fewer electrons than protons.
  • Negative ion: more electrons than protons.

For Ions: Positive ions lose electrons (e.g. Na⁺ has 11 protons and 10 electrons). Negative ions gain electrons (e.g. O²⁻ has 8 protons and 10 electrons).

6. Trends in Atomic and Ionic Radius

These trends are crucial for understanding the structure of the periodic table:

Across a period (left to right):

• Atomic radius decreases.
• More protons = greater nuclear charge.
• Electrons are added to the same shell.
• Stronger attraction pulls electrons closer.

Down a group:

• Atomic radius increases.
• More shells = outer electrons are further from the nucleus.
• Increased shielding from inner electrons weakens nuclear attraction.

Ionic radius:

• Cations (positive ions) are smaller than their atoms – fewer electrons = stronger attraction to the nucleus.
• Anions (negative ions) are larger than their atoms – extra electrons = more repulsion and weaker nuclear attraction per electron.

Summary

• Atoms have a tiny, dense nucleus (protons and neutrons) with electrons in surrounding shells; most of the atom is empty space.
• Atomic number Z = protons; mass number A = protons + neutrons; ions form by electron loss/gain.
• In an electric field: protons and electrons deflect in opposite directions; electrons deflect more; neutrons are undeflected.
• Atomic radius decreases across a period and increases down a group; cations shrink vs atoms, anions expand.