Atoms and Elements: Foundations of Chemistry

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Atoms and Elements

Experiencing Atoms

Atoms are the fundamental building blocks of matter, composing everything in the universe. The structure of an atom determines the properties of the element it forms.

Atoms are incredibly small and are the smallest units of elements.
The arrangement and type of subatomic particles within an atom define the chemical behavior of the element.
Atoms combine to form molecules, which make up all substances.
Example: Water is composed of hydrogen and oxygen atoms bonded together.

Elements: Diversity and Uniqueness

Elements are pure substances consisting of only one kind of atom. Each element has unique properties due to differences in atomic structure.

There are about 91 elements found in nature.
Over 20 elements have been synthesized in laboratories.
Each element has a unique atomic structure, leading to distinct physical and chemical properties.
Example: Carbon atoms form graphite or diamond, depending on their arrangement.

Atomic Theory: Historical Development

Key Contributors to Atomic Theory

The concept of atoms has evolved over centuries, with significant contributions from various scientists.

Democritus (460–370 B.C.E.): First to propose that matter is composed of indivisible units called atoms.
John Dalton (1766–1844): Formalized atomic theory, stating that atoms of each element are identical and combine in fixed ratios.
J.J. Thomson (1856–1940): Discovered the electron, a negatively charged subatomic particle.
Ernest Rutherford (1871–1937): Discovered alpha particles and proposed the nuclear structure of the atom.
Niels Bohr (1885–1962): Developed the Bohr model, describing electrons orbiting the nucleus in defined energy levels.

Dalton’s Atomic Theory

Dalton’s theory laid the foundation for modern chemistry by describing the nature and behavior of atoms.

Matter is composed of very small particles called atoms.
Atoms of a given element are identical in mass and properties (with the exception of isotopes).
Atoms cannot be created, destroyed, or subdivided in chemical reactions.
Chemical reactions involve the rearrangement of atoms to form new substances.
Atoms combine in simple, whole-number ratios to form compounds.

Consequences of Atomic Theory

Law of Conservation of Mass: Atoms are neither created nor destroyed in chemical reactions; mass is conserved.
Law of Multiple Proportions: Elements combine in simple, whole-number ratios.
Law of Constant Composition: A given compound always contains the same kinds and numbers of atoms.

Structure of the Atom

Subatomic Particles

Atoms are composed of three main subatomic particles: 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	9.109 × 10-31	0.00055	-1

Protons and neutrons are found in the nucleus.
Electrons orbit the nucleus in energy levels.
The nucleus is very small compared to the overall size of the atom.

Bohr Model of the Atom

The Bohr model describes electrons orbiting the nucleus in specific energy levels.

Electrons occupy orbits with set sizes and energies.
The lowest energy is found in the smallest orbit.
Electrons absorb or emit energy when moving between orbits.

Modern Atomic Theory

Atoms of the same element can have different numbers of neutrons (isotopes).
Atoms are divisible into subatomic particles.
Atomic mass is measured in atomic mass units (amu):

The Periodic Table

Organization and Properties

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

Each element has a unique name and symbol (one or two letters).
Atomic number (Z): Number of protons in the nucleus.
Atomic mass number (A): Sum of protons and neutrons.
Elements are arranged in rows (periods) and columns (groups).
Elements in the same group have similar chemical properties.

Main Group and Transition Elements

Main group elements: Properties are predictable based on position in the table.
Transition elements: Properties are less predictable.

Properties of Metals and Nonmetals

Property	Metals	Nonmetals
Location	Left side of table	Upper right side
Conductivity	Good conductors	Poor conductors
Malleability	Can be pounded flat	Varied
Ductility	Can be drawn into wires	Varied
Luster	Shiny	Dull or varied
Electron Behavior	Tend to lose electrons	Tend to gain electrons

Ions and Isotopes

Formation of Ions

Atoms can gain or lose electrons to form ions, which carry a net charge.

Cations: Positively charged ions (loss of electrons).
Anions: Negatively charged ions (gain of electrons).
Example: Sodium atom (Na) loses one electron to form Na+ cation.

Isotopes

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

Isotopes have the same atomic number but different mass numbers.
Example: Carbon-12 and Carbon-14 are isotopes of carbon.

Classical View: Matter and Energy

Definitions

Matter: Has mass and occupies space.
Energy: Does not have mass or volume; can travel in waves.

The Nature of Light

Wave Properties of Light

Light is a form of electromagnetic radiation, which travels as waves.

Key characteristics: speed, amplitude, wavelength, frequency.
Speed of light in vacuum:
Wavelength (): Distance between two crests or troughs.
Frequency (): Number of wave cycles per second (Hz).
Relationship:

Energy of Light

Energy is proportional to amplitude and frequency.
Energy of a photon:
Planck’s constant:

Electromagnetic Spectrum

Light can be separated into a spectrum of colors, each with a different wavelength and energy.

Visible light ranges from red (long wavelength, low energy) to violet (short wavelength, high energy).
Order of electromagnetic radiation by wavelength (short to long): Gamma rays < Ultraviolet < Green < Red < Microwaves.
Order by frequency (low to high): Microwaves < Red < Green < Ultraviolet < Gamma rays.
Order by energy (least to most): Microwaves < Red < Green < Ultraviolet < Gamma rays.

Particles of Light: Photons

Light also behaves as particles called photons, each carrying a specific amount of energy.

High-frequency (short wavelength) photons have more energy.
Low-frequency (long wavelength) photons have less energy.
High-energy electromagnetic radiation can damage biological molecules (ionizing radiation).

Light and Atomic Spectra

Emission and Absorption Spectra

Atoms emit or absorb energy in the form of light at specific wavelengths, producing unique spectra.

Emission spectrum: Shows wavelengths emitted by an atom.
Absorption spectrum: Shows wavelengths absorbed by an atom.
Each element has a characteristic spectrum, useful for identification.
Example: Hydrogen emits light at specific wavelengths (e.g., 656.3 nm, 486.1 nm).