Atoms & Elements

The Atom

The atom is the smallest unit of an element that retains the chemical properties of that element. It consists of subatomic particles: protons, neutrons, and electrons.

• Protons: Positively charged particles found in the nucleus.

• Neutrons: Neutral particles also located in the nucleus.

• Electrons: Negatively charged particles that orbit the nucleus.

Example: Protons and electrons have charges of the same magnitude but opposite signs. The number of protons equals the number of electrons in a neutral atom.

The Nucleus

The nucleus contains protons and neutrons, held together by nuclear forces.

• Nuclear Force: Holds protons and neutrons together.

• Electrostatic Force: Acts between protons and electrons.

Example: If the nuclear force is greater than the electrostatic force, the nucleus remains intact.

Subatomic Particles

Subatomic particles differ in mass and charge. Their properties are summarized below:

Example: Osmium has an actual mass of 190.23 grams. To find the number of atoms, divide by the atomic mass unit.

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons.

• Atomic Number (Z): Number of protons; defines the element.

• Mass Number (A): Number of protons plus neutrons.

• Number of Electrons: Equals number of protons in a neutral atom.

Example: Calcium-43 has 20 protons, 23 neutrons, and 20 electrons.

Ions

Ions are formed when atoms gain or lose electrons.

• Cation: Positively charged ion (loss of electrons).

• Anion: Negatively charged ion (gain of electrons).

• Isoelectronic: Species with the same number of electrons.

Example: The cation Al3+ has 13 protons, 10 electrons, and 14 neutrons.

Atomic Mass

Atomic mass is the weighted average mass of all isotopes of an element.

• Atomic mass can be found using the periodic table or calculated from isotopic masses and abundances.

Atomic Mass Formula:

Example: Calculate the atomic mass of an element given the masses and percent abundances of its isotopes.

Periodic Table Classifications

The periodic table classifies elements as metals, nonmetals, and metalloids.

• Metals: Largest classification; good conductors, malleable, shiny.

• Nonmetals: Poor conductors, brittle, dull.

• Metalloids: Properties intermediate between metals and nonmetals.

Example: Arsenic is a metalloid; barium is a metal.

Periodic Table: Group Names

Elements are organized into periods (rows) and groups (columns).

• Groups share similar chemical properties.

• Metals are found on the left; nonmetals on the right.

Example: Sodium is a metal in the 4th period.

Representative Elements & Transition Metals

Groups can be divided into representative elements (Groups 1A-8A) and transition metals (Groups 3B-12B).

• Transition Metals: Found in the center of the table; variable oxidation states.

• Representative Elements: Found in Groups 1A-8A; predictable properties.

Example: Copper is a transition metal; sodium is a representative element.

Element Symbols

Each element is represented by a unique symbol, often derived from its Latin name.

• Periodic Law: Properties of elements repeat periodically with atomic number.

Example: F is the symbol for fluorine; Na for sodium.

Elemental Forms

Elements exist in different forms in nature:

• Monatomic Elements: Stable as single atoms (e.g., noble gases).

• Diatomic Elements: Stable as molecules of two atoms (e.g., H2, O2).

• Polyatomic Elements: Stable as molecules of more than two atoms (e.g., S8).

Example: Oxygen exists as O2 in nature.

Phases of Elements

At standard temperature and pressure, elements can exist as solids, liquids, or gases.

• Solid: Definite shape and volume.

• Liquid: Definite volume, no definite shape.

• Gas: No definite shape or volume.

Example: Argon is a gas at room temperature.

Periodic Table: Charges

Elements form ions to achieve stable electron configurations.

• Metals: Lose electrons to form cations.

• Nonmetals: Gain electrons to form anions.

Example: Gallium typically forms a +3 ion.

Calculating Molar Mass

Molar mass is the mass of one mole of a substance, expressed in grams per mole.

Molar Mass Formula:

Example: Calculate the molar mass of (NH4)2SO4 by summing the atomic masses of all atoms in the formula.

Mole Concept

The mole is a counting unit in chemistry, representing 6.022 × 1023 particles (Avogadro's number).

• Used to convert between mass, number of particles, and volume.

Example: How many moles of chlorine gas are in 8.13 × 1024 molecules?

Law of Conservation of Mass

Mass is conserved in chemical reactions; the total mass of reactants equals the total mass of products.

Example: 2 H2 + O2 → 2 H2O

Law of Definite Proportions

A chemical compound always contains the same proportion of elements by mass.

Example: CO2 always contains carbon and oxygen in a fixed mass ratio.

Atomic Theory

John Dalton's Atomic Theory (1803) laid the foundation for modern chemistry.

• All matter is composed of atoms.

• Atoms of the same element are identical.

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

• Atoms are rearranged in chemical reactions.

Example: Nitrogen and phosphorus have the same mass; all lead atoms are identical.

Law of Multiple Proportions

When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in ratios of small whole numbers.

Example: NO and NO2 show the law of multiple proportions.

Millikan Oil Drop Experiment

Robert Millikan's experiment determined the charge of the electron.

• Measured the charge-to-mass ratio of the electron.

Example: The fundamental charge of an electron is C.

Rutherford Gold Foil Experiment

Ernest Rutherford's experiment led to the discovery of the atomic nucleus.

• Alpha particles were scattered by a thin gold foil.

• Most particles passed through; some were deflected, indicating a dense nucleus.

Example: Rutherford's experiment disproved Thomson's Plum Pudding Model.

*Additional info: Some tables and diagrams have been described in text for clarity. All equations are provided in LaTeX format as required.*