Atomic Structure and Nuclear Chemistry

Who Thought of the Atom?

The concept of the atom has evolved over centuries, beginning with philosophical ideas and advancing through scientific experimentation. Early theories laid the groundwork for our modern understanding of atomic structure.

• Ancient Greek Philosophers: Proposed that all matter was composed of four elements: air, earth, fire, and water.

• Democritus (~460–370 B.C.): Suggested that matter could be divided until reaching an indivisible particle, which he called the atom (from Greek atomos, meaning "uncuttable").

• Antoine Lavoisier (1743–1794): Discovered the law of conservation of mass, stating that mass is neither created nor destroyed in chemical reactions.

• Joseph Proust (1754–1826): Demonstrated the law of constant composition (also known as the law of definite proportions), which states that a chemical compound always contains the same proportion of elements by mass.

Example: Water (H2O) always contains hydrogen and oxygen in a mass ratio of approximately 1:8.

Dalton's Atomic Theory

In 1808, John Dalton formulated the first modern atomic theory, building on the ideas of his predecessors. Dalton's postulates provided a scientific framework for understanding chemical reactions and the nature of matter.

• All matter consists of solid and indivisible atoms.

• Atoms are indestructible and retain their identity in chemical reactions.

• All atoms of a given chemical element are identical in mass and other properties.

• Atoms of different elements differ in mass and other properties.

• Compounds are formed by the combination of atoms of different elements in small whole-number ratios.

Additional info: Modern atomic theory has modified Dalton's postulates, recognizing that atoms can be divided (into subatomic particles) and that isotopes exist (atoms of the same element with different masses).

What's in an Atom?

Atoms are composed of three fundamental subatomic particles: protons, neutrons, and electrons. The arrangement and properties of these particles determine the behavior and identity of each atom.

• Proton: Positively charged particle located in the nucleus. Mass ≈ kg; Charge = C.

• Neutron: Neutral particle located in the nucleus. Mass ≈ kg; Charge = 0 C.

• Electron: Negatively charged particle found outside the nucleus. Mass ≈ kg; Charge = C.

• Most of an atom's volume is empty space; the nucleus is extremely small compared to the overall size of the atom.

• The nucleus contains most of the atom's mass and is positively charged due to protons.

• Electrons move around the nucleus, balancing the overall charge of the atom.

• Atomic charge formula:

Example: A neutral carbon atom has 6 protons and 6 electrons, so its charge is zero.

Defining an Element

Elements are defined by their atomic number, which determines their identity. Atoms of the same element can have different mass numbers due to varying numbers of neutrons.

• Atomic number (Z): Number of protons in the nucleus.

• Mass number (A): Total number of nucleons (protons + neutrons) in the nucleus.

• Isotopes: Atoms of the same element with different mass numbers (different numbers of neutrons).

Example: Carbon-12 and Carbon-14 are both carbon atoms (Z = 6), but have different mass numbers due to different numbers of neutrons.

Additional info: The term nucleon refers to either a proton or a neutron.