Atomic Structure and Subatomic Particles

Rutherford's Gold Foil Experiment and Atomic Structure:

• Rutherford used alpha particles, which expose photographic paper (or modern mirror-based detectors), to probe the structure of atoms.

• The experiment involved a very thin material, allowing alpha particles to pass through a few atoms.

• Observations:

  • Most alpha particles passed through undeflected, indicating that atoms are mostly empty space.

  • Some particles were deflected at large angles (disturbing deflections).

  • A significant finding was that $1$ out of every $20,000$ particles actually came all the way back to the emitter.

• Conclusions:

  • This proved that atoms are not a homogeneous, light substance.

  • For an alpha particle (which is $8,000$ times heavier than an electron) to be reflected back, it must hit something significantly heavier than itself.

  • The core of the atom is a very dense, heavy portion called the nucleus.

  • The nucleus contains all of the positive charge, concentrated in a very small area.

  • A direct hit on this positively charged, dense nucleus would cause the alpha particle to reflect straight back.

Components of the Atom:

• Nucleus:

  • The dense core of the atom, containing all protons and neutrons.

  • It is very, very small, often depicted much larger in diagrams for clarity, but in reality, it's a speck at the atom's center.

  • Contains all the positive charge of the atom.

• Protons:

  • Located in the nucleus.

  • Carry a positive charge, represented as $+1$ in relative units.

  • Typically colored red in diagrams.

• Neutrons:

  • Located in the nucleus.

  • Are electrically neutral, represented as $0$ in relative units.

  • Typically colored white in diagrams.

  • Along with protons, they are collectively called nucleons.

• Electron Cloud:

  • The region where electrons exist, orbiting around the nucleus.

  • Most things that do not hit the nucleus should be able to pass through the electron cloud.

  • Electrons are easily accessible and can be pulled out of the atom by an electric field because they orbit around the outside.

Subatomic Particle Properties (Charge and Mass):

• Electron:

  • Relative charge: $-1$

  • Relative mass: $1$ (base unit of mass)

  • Actual mass: $9.11 imes 10^{-31}$ kg

• Proton:

  • Relative charge: $+1$

  • Relative mass: Approximately $2,000$ times heavier than an electron.

• Neutron:

  • Relative charge: $0$

  • Relative mass: Approximately $2,000$ times heavier than an electron.

  • Important Point: The neutron is slightly heavier than the proton (a difference of about $5$ units in relative mass, which has a specific reason for being slightly heavier).

Atomic Notation and Isotopes:

• Atomic Number ($Z$):

  • Defined as the number of protons in an atom.

  • This number uniquely identifies an element and will never change for a given element.

  • It is typically found on the periodic table.

• Mass Number ($A$):

  • Defined as the sum of protons and neutrons in an atom ($A = ext{protons} + ext{neutrons}$).

  • Represents the total number of nucleons.

• Isotope Notation:

  • An element is typically represented as $_Z^A ext{X}$, where (X) is the element symbol.

• Isotopes:

  • Atoms of the same element (meaning they have the same atomic number, $Z$) but have different numbers of neutrons.

  • Since they have different numbers of neutrons, they will have different mass numbers ($A$).

  • Example: Iron can have different isotopes, all with the same number of protons (characteristic of iron) but varying numbers of neutrons.

  • The number of electrons usually balances the number of protons to maintain neutrality in an atom.

Atomic Mass Unit (AMU):

• Defined as $1/12$ the mass of a carbon-$12$ atom.

• This unit provides a standard for measuring the mass of atoms, taking into account the varying weights of different atoms and their isotopes in a sample.