Atoms and Elements

What are Atoms?

An atom is the smallest identifiable unit of an element, retaining the chemical properties of that element. There are approximately 91 naturally occurring elements, with about 20 more that have been synthesized in laboratories. Each element is composed of atoms unique to that element.

History of Atomic Theory

The concept of the atom dates back to the Greek philosopher Democritus (460–370 BC), who proposed that matter is composed of small, indivisible particles called "atomos." This idea laid the foundation for later scientific developments in atomic theory.

Dalton's Model of the Atom

John Dalton (early 19th century) formalized atomic theory with several key postulates:

• All matter is made up of tiny particles called atoms or molecules.

• Molecules can be broken down into atoms by chemical processes, but atoms cannot be broken down further by chemical means.

• All atoms of a given element have the same mass and properties that distinguish them from atoms of other elements.

• Atoms combine in simple, whole-number ratios to form compounds.

Dalton's model was supported by the law of conservation of mass and the law of definite composition, which states that compounds always contain the same proportion of elements by mass.

Thomson's Model of the Atom

J.J. Thomson (1856–1940) discovered the electron, a negatively charged subatomic particle much smaller and lighter than atoms. He proposed the "plum pudding model," where electrons are embedded in a sphere of positive charge, maintaining overall electrical neutrality.

Rutherford's Model of the Atom

Ernest Rutherford (1871–1937) conducted the gold foil experiment, firing alpha particles at a thin sheet of gold. Most particles passed through, but some were deflected, indicating a dense, positively charged nucleus at the center of the atom, with electrons distributed around it. This disproved the plum pudding model.

Discovery of the Neutron

Rutherford predicted the existence of neutral particles in the nucleus, later discovered as neutrons by James Chadwick. Neutrons have a mass similar to protons but no charge.

Subatomic Particles

Atoms are composed of three main subatomic particles:

Atomic Notation

Atomic number (Z) is the number of protons in the nucleus and identifies the element. Mass number (A) is the sum of protons and neutrons. Atomic notation expresses the composition of an atomic nucleus:

\[ ^A_Z \text{Element Symbol} \]

For example, sodium (Na) has Z = 11 and A = 23, so it has 11 protons and 12 neutrons (23 – 11 = 12).

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons, resulting in different mass numbers. Isotopes are written as the element name followed by the mass number (e.g., Carbon-12, Carbon-14).

• Hydrogen Isotopes: Protium (1 proton, 0 neutrons), Deuterium (1 proton, 1 neutron), Tritium (1 proton, 2 neutrons; radioactive).

• Carbon Isotopes: Carbon-12 (6 protons, 6 neutrons), Carbon-13 (6 protons, 7 neutrons), Carbon-14 (6 protons, 8 neutrons; radioactive).

Atomic Mass

The atomic mass of an element is the weighted average mass of all naturally occurring isotopes, measured in atomic mass units (amu):

\[ \text{Atomic mass} = (\text{fraction of isotope 1} \times \text{mass of isotope 1}) + (\text{fraction of isotope 2} \times \text{mass of isotope 2}) + \ldots \]

Example: Chlorine has two main isotopes, Cl-35 (75.77%, 34.97 amu) and Cl-37 (24.23%, 36.97 amu):

\[ \text{Atomic mass} = (0.7577 \times 34.97) + (0.2423 \times 36.97) = 35.45 \text{ amu} \]

The Periodic Table

Mendeleev’s Periodic Table

Dmitri Mendeleev arranged elements in order of increasing atomic mass, grouping elements with similar properties together. He left gaps for undiscovered elements and predicted their properties.

Modern Periodic Table

The modern periodic table arranges elements by increasing atomic number (Z). Elements with similar properties are grouped in columns called groups, and rows are called periods. There are 18 groups and 7 periods.

Classification of Elements

• Metals: Solids at room temperature (except Hg), shiny, good conductors of heat and electricity, malleable, ductile, tend to lose electrons and form cations. Located on the lower left of the table.

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• Nonmetals: Found in all three states, poor conductors, brittle solids, tend to gain electrons and form anions. Located on the upper right of the table (except H).

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• Metalloids (Semimetals): Exhibit properties of both metals and nonmetals, often semiconductors. Example: Silicon is shiny, conducts electricity, but is brittle and does not conduct heat well.

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Groups and Periods

Groups are vertical columns (numbered 1–18); elements in the same group have similar chemical properties. Periods are horizontal rows (numbered 1–7).

Group A Elements (Representative Elements)

• Group 1A: Alkali metals (very reactive)

• Group 2A: Alkaline earth metals (fairly reactive)

• Group 5A: Pnictogens

• Group 6A: Chalcogens

• Group 7A: Halogens (very reactive)

• Group 8A: Noble gases (inert)

Group B Elements (Transition and Inner Transition Elements)

• Transition elements: Middle of the periodic table, properties are less predictable.

• Inner transition elements: Lanthanides (rare earths) and actinides (radioactive), found below the main table.

Ions and Ionic Charge

Formation of Ions

Metals tend to lose electrons to achieve a stable noble gas configuration, forming positively charged ions (cations). Nonmetals tend to gain electrons, forming negatively charged ions (anions).

• Group 1A metals lose one electron to form +1 ions:

• Group 6A nonmetals gain two electrons to form –2 ions:

Element Names and Symbols

Each element has a unique name and symbol, usually derived from Latin, Greek, or the name of a place or scientist. Symbols are one or two letters; the first is always capitalized, the second (if present) is lowercase (e.g., C for carbon, Cd for cadmium).

Summary Table: Subatomic Particles

Additional info: The notes above include expanded academic context and examples to ensure completeness and clarity for introductory chemistry students.