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 molecules such as nitrogen and oxygen.

• Substances called amines (e.g., triethylamine) may be present in the air, contributing to characteristic smells like that of 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.

Atomic Theory and the Nature of Matter

Atoms and Elements: Definitions

Atoms and elements are the core 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.

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

• About 20 synthetic elements have been created by scientists.

• The exact number of naturally occurring elements is debated due to trace discoveries.

Historical Development: Democritus, Leucippus, and Dalton

The concept of atoms originated in ancient Greece and was formalized in the 19th century.

• Democritus and Leucippus theorized that matter is made of tiny, indestructible particles called atomos ("atoms").

• John Dalton (1808) developed the first widely accepted atomic theory:

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

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

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

Modern Evidence for Atomic Theory

Technological advances have allowed direct manipulation and observation of atoms.

• The scanning tunneling microscope (STM) enables scientists to move and visualize individual atoms, as demonstrated by IBM's creation of 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.

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

• Thomson proposed that atoms must also contain positive charge to balance the negative electrons.

Thomson's Plum-Pudding Model

Thomson's model depicted electrons embedded in a sphere of positive charge, like plums in a pudding.

• Electrons (negative) are distributed within a positively charged sphere.

Rutherford’s Gold Foil Experiment

Ernest Rutherford's experiment revolutionized the understanding of atomic structure.

• Alpha particles were directed at thin gold foil.

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

Rutherford: Nuclear Theory of the Atom

Rutherford proposed a new atomic model based on his experimental results.

1. Most of the atom's mass and all its positive charge are in a small core called 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.

• The nucleus is extremely dense, containing over 99.9% of the atom's mass but occupying a tiny fraction of its volume.

• Electrons occupy a much larger region but contribute little mass.

• Black holes and neutron stars are examples of matter with nuclear-level density.

Subatomic Particles

Protons, Neutrons, and Electrons: Relative Mass

The three main subatomic particles differ greatly in mass.

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

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

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

Protons, Neutrons, and Electrons: Electrical Charge

Subatomic particles carry fundamental electrical charges.

• Protons: positive charge (+1)

• Electrons: negative charge (−1)

• Neutrons: no charge (0)

• Opposite charges attract; like charges repel.

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

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−

Evidence of Charge in Matter

Electrical storms demonstrate the presence and movement of electrical charges in matter.

• Normally, matter is charge-neutral.

• During storms, negative charge builds up on clouds and positive charge on the ground, leading to lightning as charges rebalance.

The Periodic Table and Elements

Elements: Defined by Their Number of Protons

Each element is uniquely identified by the number of protons in its nucleus.

• The atomic number (Z) is the number of protons in an atom.

• Changing the number of protons changes the element.

The Periodic Table of the Elements

The periodic table organizes all known elements by increasing atomic number.

• Elements are listed in order of atomic number, which reflects the number of protons.

Names and Symbols of the Elements

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

• Examples: C for carbon, Br for bromine, K for potassium (from kalium), Na for sodium (from natrium).

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 properties, countries, or scientists.

• Argon: from Greek argos, meaning "inactive"

• Bromine: from Greek bromos, meaning "stench"

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