Atomic Structure and Planetary Motion: Key Concepts

Atomic Structure

Elements are a type of atom. Atoms are made up of protons, neutrons, and electrons. The nucleus is the center of an atom.

• Proton: +1 charge
• Electron: -1 charge
• Neutron: 0 charge (neutral)

Rutherford (1911) conducted the gold foil experiment.

Different elements have different numbers of protons in the nucleus.

• Atomic number (Z): Number of protons
• Mass number (A): Number of protons + Number of neutrons

Z

Chemical Symbol

A

Neutron number = Mass number - Atomic number

The number of neutrons can change (protons stay the same) in a given atom, making them isotopes.

Atomic mass is the total mass of elements and all its isotopes. U represents atomic mass units.

A radioactive isotope consists of nuclides whose nuclei undergo spontaneous decay.

Three Types of Radioactive Decay

1. Alpha decay (He nucleus)
2. Beta decay (electron)
3. Gamma decay

• Alpha decay: ₂⁴HE (mass # -4, atomic # -2)
• Beta decay: ₁⁰e (mass # ±0, atomic # -1)
• Gamma decay: ɣ (mass # ±0, atomic # ±0)

Celestial Mechanics

Aristotle believed all celestial motion must be circular and that celestial bodies were spheres.

The Geocentric Model (GM) of planetary motion (PM) required all motion to be perfect circles. It was a Sun-centered model (Earth was not the center).

Epicycles: A planet moved in a circle around a point on the main circle around the Earth. This explained the brightness variations of planets and retrograde motion.

The Copernican Revolution marked the end of the GM. De Revolutionibus (Nicolaus) asserted that Earth rotated on its axis daily and traveled around the Sun yearly.

Retrograde motion of a planet occurs when the Earth passes the planet.

Planets do not move in circles but in ellipses. The wider the ellipse, the more eccentric it is.

Kepler's Laws of Planetary Motion

1. Kepler's 1^st law: Orbital paths are elliptical, with the Sun at one focus.
2. Kepler's 2^nd law: An imaginary line connecting the Sun to any planet sweeps out equal areas of the ellipse in equal intervals of time.
3. Kepler's 3^rd law: The square of a planet's orbital period is proportional to the cube of its semi-major axis.

Orbital Period (P): P² (Earth years) = a³ (in astronomical units)

Galileo built a better version of the telescope in 1609.

Galileo's Observations

• The Sun and Moon are not perfect.
• Venus goes through phases.
• Jupiter has moons.
• The Milky Way has millions of stars.