Atoms and Elements: Foundations of Matter and the Periodic Table

Atoms and Elements

Experiencing Atoms in the Sea and Mountains

Atoms are the fundamental building blocks of all matter, forming the basis of our physical sensations and the substances around us.

• Atoms are the foundation of our sensations and the materials we encounter daily.

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

• Seaside air contains nitrogen and oxygen molecules.

• Substances called amines (e.g., triethylamine) may be present in seaside air, contributing to characteristic odors such as the fishy smell from decaying fish.

Small Size and Large Number of Atoms

Atoms are extremely small and numerous, even in tiny objects.

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

Atoms and Elements: Definitions

Atoms and elements are central concepts in chemistry, defining the composition and properties of matter.

• 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 by chemical means.

• There are about 91 naturally occurring elements, each with unique atoms; about 20 additional elements are synthetic.

The Development of Atomic Theory

Democritus and Leucippus: Matter is Made of Particles

Early Greek philosophers proposed that matter is composed of tiny, indivisible particles called atoms.

• Democritus and Leucippus theorized that matter is made of atomos ("indivisible" particles).

• They suggested that dividing matter repeatedly would eventually yield these fundamental particles.

John Dalton's Atomic Theory

Dalton formalized the atomic theory in the early 19th century, providing a scientific foundation for the concept of atoms.

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

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

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

Modern Evidence for the Atomic Theory

Technological advances, such as the scanning tunneling microscope (STM), allow scientists to manipulate and visualize individual atoms, providing direct evidence for their existence.

• IBM scientists used STM to move atoms and create images, including the world's smallest movie, A Boy and His Atom.

Structure of the Atom

Discovery of Electrons: J. J. Thomson

J. J. Thomson discovered the electron, a fundamental subatomic particle, and proposed the "plum-pudding" model of the atom.

• Electrons are negatively charged, much smaller and lighter than atoms, and present in all substances.

• Thomson's model suggested electrons are embedded in a sphere of positive charge.

Rutherford’s Gold Foil Experiment

Ernest Rutherford's experiment with alpha particles and gold foil led to the discovery of the atomic nucleus.

• Most alpha particles passed through the foil, but some were deflected at sharp angles, contradicting the plum-pudding model.

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

Rutherford: Nuclear Theory of the Atom

Rutherford's nuclear model describes the atom as mostly empty space with a dense nucleus at the center.

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

• Electrons occupy the surrounding space, and their number equals the number of protons, making the atom electrically neutral.

• The nucleus is extremely dense; if matter were composed only of nuclei, it would be incredibly heavy (e.g., a grain of sand would weigh 5 million kg).

Subatomic Particles

Protons, Neutrons, and Electrons: Relative Mass

The three main subatomic particles differ greatly in mass and charge.

• The proton is nearly 2000 times as massive as the electron.

• Protons and neutrons have similar masses; electrons have negligible mass.

Protons, Neutrons, and Electrons: Electrical Charge

Electrical charge is a fundamental property of protons and electrons.

• Protons have a charge of +1, electrons have a charge of -1, and neutrons are neutral.

• Opposite charges attract; like charges repel.

• When protons and electrons combine, their charges cancel, resulting in a neutral atom.

Table: Subatomic Particles

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–

Elements and the Periodic Table

Elements: Defined by Their Number of Protons

Each element is uniquely identified by the number of protons in its nucleus, known as the atomic number (Z).

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

The Periodic Table of the Elements

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

• Each element is represented by a unique chemical symbol, often derived from its English, Latin, or Greek name.

Table: Names and Symbols of Selected Elements

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

Origins of Element Names

Element names often reflect their properties, countries, or scientists.

• Argon (Greek: argos, "inactive")

• Bromine (Greek: bromos, "stench")

• Some elements are named after countries (e.g., Polonium, Francium, Americium) or scientists (e.g., Curium).