I. Atom and Ion theory

A. Atomic history

Historic scientist	Key contribution / model
Dalton	• Proposed an early modern atomic theory: matter is composed of atoms. • Treated atoms of each element as characteristic particles that combine in whole-number ratios.
Thomson	• Discovered the electron using cathode-ray tube experiments. • Proposed the plum-pudding model: electrons embedded in a diffuse positively charged sphere.
Millikan	• Measured the elementary charge of the electron using the oil-drop experiment. • Combined with Thomson’s e/m value, this allowed the electron’s mass to be determined.
Rutherford	• Discovered the small, dense, positively charged nucleus using the gold-foil (alpha-particle scattering) experiment. • Concluded that most of the atom is empty space.
Bohr	• Proposed quantized electron energy levels (orbits) for the hydrogen atom. • Successfully explained the line spectrum of hydrogen.

B. Atomic structure

1. Atom

• An atom is the smallest unit of an element that retains the chemical properties of that element.

• Chemical reactions involve the rearrangement of atoms; atoms are not created or destroyed in ordinary chemical reactions.

• An atom consists of a nucleus (protons + neutrons) surrounded by electrons.

Fig. 1 Structure of atom

2. Classification of subatomic particles

Particle	Key features
Proton	• Charge: +1. • Number of protons determines the atomic number (Z) and therefore the identity of the element. • Located in the nucleus.
Electron	• Charge: −1. • Located outside the nucleus. • Ions form when atoms gain or lose electrons.
Neutron	• Charge: 0. • Located in the nucleus. • Different numbers of neutrons produce isotopes and change the mass number.

C. Ion

An ion is an atom or group of atoms that has gained or lost one or more electrons and therefore has a net charge.

Cation and anion

• Cation (+): an atom or ion formed by losing one or more electrons.

• Anion (−): an atom or ion formed by gaining one or more electrons.

Fig. 2 Cation and anion

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Isoelectronic species

• Isoelectronic species are different atoms or ions that have the same number of electrons.

• Examples: K⁺ and Ar are isoelectronic; F⁻ and Mg²⁺ are isoelectronic.

3. Isotopes

• Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, so they have different mass numbers.

• Isotopes of an element have very similar chemical properties but may differ in physical and nuclear properties.

• Radioactive isotopes can be used for radiometric dating of ancient materials and fossil remains.

• Example: hydrogen isotopes—protium (¹H), deuterium (²H), and tritium (³H).

D. Atomic number and mass number

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Classification	Equations and features
Neutral atom	• Atomic number: Z = number of protons. • For a neutral atom: number of electrons = number of protons = Z. • Mass number: A = number of protons + number of neutrons. • Example: 9 protons, 9 electrons, 10 neutrons → Z = 9 and A = 19.
Ion	• Atomic number: Z = number of protons; ion formation does not change Z. • Cation with charge n+: electrons = Z − n. • Anion with charge n−: electrons = Z + n. • Mass number: A = number of protons + number of neutrons. • Example: F⁻ has 9 protons and 10 electrons; with 10 neutrons, A = 19.

E. Nuclear reactions

• Many nuclides are radioactive and spontaneously undergo nuclear decay.

• In an electric field, α particles are deflected toward the negative electrode, β⁻ particles toward the positive electrode, and γ rays are undeflected.

• In a balanced nuclear equation, the sum of the mass numbers (A) is conserved.

• Example: ²¹⁰Po → ²⁰⁶Pb + ⁴He

Mass numbers: 210 = 206 + 4

• The sum of the atomic numbers (Z, proton numbers) is also conserved.

Atomic numbers: 84 = 82 + 2

Fig. 3 Radiation in an electric field

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Radioactive decay	Key features
Alpha (α) decay	• Emits an α particle, ⁴₂He (2 protons + 2 neutrons). • Daughter mass number decreases by 4; atomic number decreases by 2. • α particles are positively charged and are deflected toward the negative electrode. • Example: ²¹⁰₈₄Po → ²⁰⁶₈₂Pb + ⁴₂He.
Beta-minus (β⁻) decay	• A neutron is converted into a proton while an electron (β⁻) and an antineutrino are emitted. • Mass number is unchanged; atomic number increases by 1. • β⁻ particles are negatively charged and are deflected toward the positive electrode. • Example: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e + ν̅ₑ.
Gamma (γ) decay	• Emits a high-energy photon from an excited nucleus. • No change in mass number or atomic number. • γ radiation is uncharged and is not deflected by an electric field. • Example: ⁹⁹ᵐ₄₃Tc → ⁹⁹₄₃Tc + γ.