I. Atom and Ion theory

Key focus of this chapter: atomic structure

This chapter focuses on atomic structure and summarizes ions, atomic number, mass number, and nuclear reactions.

Historical scientists	Features
Dalton	• In Dalton's historical model, atoms were indivisible solid spheres. • Elements are composed of atoms.
Thomson	• Discovered the electron through cathode-ray experiments and determined its charge-to-mass ratio. • In Thomson's historical model, electrons were embedded in a sphere of positive charge.
Millikan	• Determined the elementary electric charge using the oil-drop experiment.
Rutherford	• Discovered the small, dense, positively charged atomic nucleus through the gold-foil alpha-particle scattering experiment. • In Rutherford's historical model, electrons surrounded the nucleus.
Bohr	• Proposed quantized electron energy levels and successfully explained the hydrogen atom's line spectrum. • In Bohr's historical model, electrons occupied allowed orbits.

An element consists of atoms with the same number of protons. Atoms contain subatomic particles.
Chemical reactions involve the rearrangement of atoms and their electrons.
An atom consists of a nucleus containing protons and, except for ordinary hydrogen-1, neutrons, surrounded by electrons.

Fig. 1 Structure of an atom

Classification	Features
Proton	• Positive charge. • Number of protons determines atomic number. • Distinguishes one element from another.
Electron	• Negative charge. • Determines the charge of an ion when electrons are gained or lost.
Neutron	• No charge. • Changes the mass number and distinguishes isotopes.

An ion is an atom or group of atoms that has gained or lost electrons and carries a net electric charge.

Fig. 2 Cation and anion

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Isotopes generally have similar chemical properties but different masses and some different physical properties.
Radiometric dating requires an appropriate radioactive isotope for the material and age range being studied.

Atomic number (Z) = number of protons.
Mass number (A) = number of protons + number of neutrons.
Number of neutrons = A − Z.
Neutral atom: electrons = Z.
Cation with charge +n: electrons = Z − n.
Anion with charge −n: electrons = Z + n.
Mass number is a unitless integer; atomic mass is the actual mass of an atom, expressed in u.

Many nuclides are radioactive.
In an electric field, alpha (α) radiation is deflected toward the negative electrode, beta-minus (β⁻) radiation toward the positive electrode, and gamma (γ) radiation is not deflected.
The sum of the reactant mass numbers is equal to the sum of the product mass numbers.

Total electric charge is conserved in nuclear reactions, including the charges of emitted or absorbed particles.

Fig. 3 Radiation emitted from the shielded box