Atoms and Elements: Foundations of Matter

Atoms and Elements

Experiencing Atoms in Everyday Life

Atoms are the fundamental building blocks of matter and are present in all substances around us, from rocks to air. Their properties determine the characteristics of the materials we encounter daily.

• Atoms are the foundation of our sensations and experiences.

• Seaside rocks are composed of silicates, which are compounds of silicon and oxygen atoms.

• Seaside air contains nitrogen and oxygen molecules, and may also contain amines such as triethylamine, which is emitted by decaying fish and contributes to the characteristic fishy smell.

Scale and Number of Atoms

Atoms are incredibly small, yet their numbers in even tiny objects are vast.

• A single pebble contains more atoms than can be counted, far exceeding the number of pebbles in large areas like San Francisco Bay.

• If every atom in a pebble were the size of the pebble itself, the resulting object would be larger than Mount Everest.

Definition of Atoms and Elements

Understanding the distinction between atoms and elements is crucial in chemistry.

• Atoms compose all matter.

• The properties of atoms determine the properties of matter.

• An atom is the smallest identifiable unit of an element.

• An element is a substance that cannot be broken down into simpler substances.

• There are about 91 naturally occurring elements, each with unique atoms.

• Scientists have created about 20 synthetic elements not found in nature.

Atomic Theory

Democritus and the Indivisibility of Atoms

The concept of atoms dates back to ancient Greece, where philosophers first theorized about the smallest units of matter.

• Democritus (460–370 B.C.E.) and Leucippus proposed that matter is composed of tiny, indestructible particles called atomos (atoms), meaning "indivisible."

Dalton's Atomic Theory

John Dalton formalized atomic theory in 1808, providing a scientific foundation for the concept of atoms.

1. Each element is composed of tiny, indestructible particles called atoms.

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

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

Modern Evidence for Atomic Theory

Technological advances have allowed scientists to observe and manipulate individual atoms.

• Scientists at IBM used a scanning tunneling microscope (STM) to move xenon atoms and spell out letters, demonstrating the physical reality of atoms.

Structure of the Atom

Discovery of Subatomic Particles

Atoms are not indivisible; they are composed of smaller particles.

• J. J. Thomson discovered the electron, a negatively charged particle much smaller and lighter than atoms.

• Electrons are present in all substances, and atoms must contain positive charge to balance the negative charge of electrons.

Plum Pudding Model

Thomson proposed the "plum pudding" model, where electrons are embedded in a sphere of positive charge.

Rutherford's Gold Foil Experiment

Rutherford's experiment led to a new model of the atom.

• Alpha particles were directed at gold foil; most passed through, but some were deflected sharply.

• This indicated that atoms have a small, dense, positively charged nucleus.

Nuclear Theory of the Atom

Rutherford's nuclear theory describes the structure of the atom:

1. Most of the atom's mass and all positive charge are in the nucleus.

2. Most of the atom's volume is empty space, with electrons dispersed throughout.

3. The number of electrons equals the number of protons, making the atom electrically neutral.

Distribution of Mass in the Atom

The nucleus contains nearly all the mass of the atom but occupies a tiny fraction of its volume.

• Electrons are distributed in a much larger region but have negligible mass.

• If matter were composed only of nuclei, it would be extremely dense, as seen in neutron stars and black holes.

Relative Size of Subatomic Particles

• A proton is nearly 2000 times as massive as an electron.

• If a proton had the mass of a baseball, an electron would have the mass of a rice grain.

Properties of Subatomic Particles

Electrical Charge

Electrical charge is a fundamental property of protons and electrons.

• Positive and negative charges attract; like charges repel.

• When paired, positive and negative charges cancel, resulting in charge neutrality.

Table: Properties of Protons, Neutrons, and Electrons

Particle	Mass (kg)	Mass (amu)	Charge
proton	1.67262×10−27	1.0073	1+
neutron	1.67493×10−27	1.0087	0
electron	0.00091×10−27	0.00055	1−

Charge in Matter

Normally, matter is charge-neutral, but disturbances such as electrical storms can disrupt this balance, resulting in phenomena like lightning.

Elements and the Periodic Table

Atomic Number and Element Identity

Elements are defined by the number of protons in their nuclei, known as the atomic number (Z).

• If the number of protons changes, the element changes.

Periodic Table Organization

The periodic table arranges elements by atomic number and groups elements with similar properties into columns.

Origins of Element Names and Symbols

Element symbols are often derived from English, Latin, or Greek names.

Element	Symbol	Origin
lead	Pb	plumbum
mercury	Hg	hydrargyrum
iron	Fe	ferrum
silver	Ag	argentum
tin	Sn	stannum
copper	Cu	cuprum

Example

The symbol for potassium is K (from kalium), and sodium is Na (from natrium).

Additional info: These notes are based on textbook slides and provide foundational knowledge for Chapter 4: Atoms and Elements in an introductory chemistry course.