Structure of the Atom (Physics Chap. 2)

Atomic Theory

• The concept of atomic theory dates back to ancient Greece, with Leucippus and Democritus being prominent contributors.
• Democritus hypothesized that all things are made up of tiny, indivisible structures called atoms, which vary in size, shape, and structure.
• He believed that the nature of an object depends on its atoms, with smooth atoms making up sweet things and sharp atoms making up bitter things.

Development of Atomic Theory

• In the early 1800s, John Dalton developed a sound atomic theory based on scientific evidence, proposing that:
  • Elements are composed of tiny, indivisible, and indestructible particles called atoms.
  • Atoms are unique to each element in their size and mass.
  • Compounds are formed by molecules, which are made up of fixed ratios of each type of constituent atom.
  • Chemical reactions involve the rearrangement of atoms.
• Later, Dmitri Mendeleev organized the known elements into the periodic table, which demonstrates that elements have similar chemical properties when arranged in order of increasing atomic mass.

Subatomic Particles

• Joseph John Thomson discovered the electron, which is a negatively charged particle that orbits the nucleus of an atom.
• The plum pudding model describes the atom as a positively charged sphere with negatively charged electrons embedded in it.
• Ernest Rutherford's experiments led to the development of the nuclear model, which proposes that:
  • The atom has a dense, positively charged nucleus at its center.
  • Electrons orbit the nucleus in defined paths.

Atomic Structure

• The atom is composed of three fundamental particles: protons, neutrons, and electrons.
• The nucleus is made up of protons and neutrons, which are collectively called nucleons.
• Protons have a positive electrical charge, neutrons have no charge, and electrons have a negative electrical charge.
• The atomic mass of an atom is the sum of the masses of its protons and neutrons.

Atomic Charge

• The electrical charge of an atom is determined by the number of protons and electrons it has.
• If an atom has an equal number of protons and electrons, it has no net charge.
• If an atom gains or loses electrons, it becomes positively or negatively charged, respectively.

Electron Orbits

• Electrons occupy specific energy levels, or electron shells, which are at different distances from the nucleus.
• Each electron shell has a maximum number of electrons it can hold, which is determined by the formula 2n^2.
• The outermost shells of atoms may not be fully occupied, and the number of electrons in the outermost shell can vary.

Binding Energy

• Binding energy is the energy required to hold the nucleus together.
• Electron-binding energy is the energy required to hold an electron in its orbit.
• The binding energy of an atom determines its ability to interact with other atoms and particles.

Classification and Bonding

• Elements are classified based on the number of protons, neutrons, and electrons in their atoms.

• Isotopes, isotones, isobars, and isomers are classifications of elements based on the number of protons, neutrons, and electrons in their atoms.

• The periodic table is a way of organizing elements based on their atomic number and atomic mass.

• The periodic table is arranged by periods and groups, with elements in each period and group having similar characteristics.### Atomic Bonding

• Atoms bond to form molecules, which are the building blocks of complex matter, including living tissue.

• A molecule is formed when two or more atoms join together chemically.

• A compound is a molecule that contains at least two different elements.

Types of Bonds

• There are two primary ways atoms bond to form molecules: ionic bonds and covalent bonds.

Ionic Bonds

• Ionic bonding is based on the attraction of opposing charges between atoms.
• When an atom gives up an electron, it becomes positively charged (cation), and when it gains an electron, it becomes negatively charged (anion).
• In an ionic bond, one atom gives up an electron, and the other takes on an electron, resulting in a net positive and negative charge, which attracts the two atoms together.

Covalent Bonds

• Covalent bonding is based on two atoms sharing electrons that orbit both nuclei.
• In a covalent bond, an outermost electron from one atom orbits the nucleus of another adjacent atom, creating a figure-eight path.
• This shared electron completes the outermost shell of each atom.

Atomic Theory

• The concept of atomism or atomic theory is attributed to Leucippus, and further developed by Democritus of Abdera.
• John Dalton proposed an atomic theory in the early 1800s, which remains fundamentally sound today.
• The work of Thomson, Rutherford, and Bohr further developed Dalton's atomic theory.

Atomic Structure

• An atom consists of three fundamental particles: protons, neutrons, and electrons.
• Protons have one unit of positive charge, electrons have one unit of negative charge, and neutrons have no charge.
• The atom is held together by a strong nuclear force (nuclear binding energy) and electrostatic attraction (electron-binding energy) between the nucleus and orbiting electrons.