what is chemistry

Course logistics and week prep

• Mastering Chemistry registration: about 75% of class already set up; please register if you haven’t.

• Homework due this Friday at 4:59 PM: HW0 and Intro to Mastering.

• Honors Contract through Barrett: email the instructor by the end of this week for more instructions later.

What is Chemistry?

• Chemistry is the science concerned with the substances matter is composed of, their properties and reactions, and the use of such reactions to form new substances (historically also alchemy).

• It spans applied subfields: biological, inorganic, nuclear, organic, physical, polymer chemistry, etc.

• Earliest English usage around 1605.

• Chemistry also involves modeling to connect submicroscopic structure to macroscopic phenomena.

• In biochemistry, submicroscopic structures/interactions explain properties of living organisms.

What is (bio)chemistry?

• A chemist learns to think intuitively at the scale of chemistry; goal of the course is to cultivate this way of thinking.

• Biochemistry is an experimental science: natural events follow cause-and-effect patterns that can be understood by observation, measurement, experiment, and modeling.

• National Research Council (2003) Beyond the Molecular Frontier defines four core practices of bio-chemists: analysis, modeling, synthesis, and transformation.

Three core concepts underpin chemistry

• Chemistry happens across orders of magnitude and scales; it includes all elements of the periodic table.

• Core idea: atomic-level structures are linked to function and reactivity.

• Guiding questions: e.g., Why is iron in blood? How can we detect whether a cell is cancerous?

• Foundational principles:

  • Forces govern organization.

  • Structure governs properties.

  • Energy governs stability.

• Contextual examples: Magnesium, macroscopic objects, and modeling at the atomic level; monoatomic species as a simple model.

Structure governs properties: carbon as an example

• Do graphite and diamond share similar properties? No, their properties differ due to submicroscopic structure.

• Graphite: black, soft, used as a lubricant; Diamond: shiny, hard, used to cut very hard materials.

• Structure explains properties: different arrangements of carbon atoms lead to different macroscopic behaviors.

• Fun fact: Graphite is more thermodynamically stable than diamond (as a form of carbon).

• Visual cue: submicroscopic structures differ despite both being forms of carbon.

Atomic structure and properties

• Matter is often described as having mass and occupying space.

• The particulate model: substances consist of many small, identical particles in constant motion.

• Assumption 1: A macroscopic sample contains a large number of very small, identical particles.

• The smallest particle of an element is an atom.

• Learning outcome: Explain atomic properties using models of atomic structure that show locations/energies of subatomic particles.

• For now, model particles as very small, rigid objects; deeper internal structure to be learned later.

• Size scale: a particle (atom) is about $1~ ext{nm} = 1 imes 10^{-9}~ ext{m}$, though exact values depend on particle type (submicroscopic scale).

An element is a substance that cannot be broken down

• An element is represented as a unique type of particle; breaking the ball into smaller pieces means it is no longer the same element.

• Analogy: the atom is the fundamental unit for identifying an element.

How do we classify different forms of matter?

• Elements are the basic building blocks.

• Compounds are combinations of elements in fixed ratios and are held together by chemical bonds.

• Mixtures are combinations of pure substances.

• If a mixture is uniform throughout, it is homogeneous; if not, it is heterogeneous.

Molecules: how should we describe them?

• Molecules can be described as:

  • Mixtures of two or more pure substances (not correct for a molecule).

  • Mixtures of two or more elements with a specific ratio (not correct for a molecule).

  • Two or more atoms chemically joined together (correct description of a molecule).

  • Heterogeneous mixtures (not a molecule).

Homogeneous mixtures

• Which of the following is a homogeneous mixture? Vodka (most classic example of a homogeneous solution).

• Other examples like chocolate chip cookies or cereal with milk are heterogeneous mixtures.

The Periodic Table of the Elements

• The periodic table catalogs all known elements; its organization is both a catalog of information and a predictive tool.

• Each box corresponds to an element; the center contains the chemical abbreviation (symbol), and the full name is shown at the bottom.

• Chemistry uses the periodic table to understand and predict properties of matter by studying the structures of the particles that comprise it.

Thinking about elements: the case of Lithium

• Lithium is important for Li-ion batteries; two samples show mass differences:

  • Natural lithium mineral: average atomic mass ≈ $6.94~ ext{Da}$

  • Recycled lithium from a military source: average atomic mass ≈ $6.979~ ext{Da}$

• Why heavier? All lithium atoms have the same number of protons, so mass differences arise from neutrons (isotopes).

• Related context: Lithium cobalt oxide (LiCoO₂) in Li-ion batteries; notable researcher: John B. Goodenough.

Atoms and subatomic particles: The nucleus

• Most of an atom is empty space; the nucleus contains protons and neutrons (collectively, nucleons).

• Nuclear radius is dramatically smaller than atomic radius:

  • Atomic radius ≈ $r_ ext{atomic} \,\approx\, 100~\text{pm}$

  • Nuclear radius ≈ $r_ ext{nuclear} \,\approx\, 5\times 10^{-3}~\text{pm}$ (about 1/20{,}000 of the atomic radius)

• The identity of an atom (which element it is) is defined by the number of protons, the atomic number $Z$.

The Hydrogen atom

• Periodic table organization by atomic number: Hydrogen has atomic number 1.

• Therefore, every hydrogen atom has exactly one proton in the nucleus: $Z=1$.

• The presence of precisely one proton defines an atom as hydrogen.

Atomic-number quiz samples

• What is the atomic number of Phosphorus? Options include 15, 30.97, 5, 31, 46. Correct answer: $Z=15$.

• How many protons are in the nucleus of Iron (Fe)? Options include 15, 26, 30, 56. Correct answer: $Z=26$.

Circling back to Lithium: mass differences and isotopes

• Revisit mass difference question: same number of protons across lithium isotopes; mass variation arises from differing neutron counts (isotopes).

• Subatomic particle types (mass, charge, location): a quick overview of protons, neutrons, and electrons.

The three subatomic particles

• Protons: located in the nucleus; positive charge.

• Neutrons: located in the nucleus; neutral charge; similar mass to protons.

• Electrons: distributed around the nucleus in orbitals; much smaller mass; carry negative charge; negligible mass contribution to the atom relative to protons/neutrons.

• Electron mass: $m_e \approx 9.10936\times 10^{-28}~\text{g}$

• Proton charge: $q_p = +1.6022\times 10^{-19}~\text{C}$

• Electron charge: $q_e = -1.6022\times 10^{-19}~\text{C}$

• Atoms have equal numbers of protons and electrons in neutral state, so the net charge is zero.

Electrical charge and interactions

• Coulomb’s law describes electrostatic forces between charges: $F = k\,\frac{q<em>1 q</em>2}{r^2}$

• The electric field around a charged particle defines a region where electrostatic forces act.

• Opposite charges attract; like charges repel.

Isotopes and atomic structure recap

• Atoms of the same element have the same number of protons (atomic number $Z$) but can have different numbers of neutrons.

• Isotopes: atoms with the same $Z$ but different neutron count (different atomic masses).

• Lithium example: natural lithium vs recycled lithium illustrating isotopic variation.

Connections to broader themes

• Linking micro (subatomic) to macro (properties of matter) is central to chemistry.

• The four core practices of chemistry emphasized in biochemistry: analysis, modeling, synthesis, transformation.

• Real-world relevance: batteries and energy storage (Li-ion), material properties (carbon forms), and isotopic mass implications in materials.

Practical and ethical implications

• Experimental design relies on measurable cause-and-effect relationships; modeling aids prediction and understanding.

• Advances (e.g., battery tech) depend on understanding atomic structure, isotopes, and materials science.

• Honours contracts and curricular options imply opportunities for advanced scholarly work and interdisciplinary applications.

Quick reference formulas and constants

• Atomic scale and length units:

  • $1~\mathrm{nm} = 1\times 10^{-9}~\mathrm{m}$

  • $1~\mathrm{pm} = 1\times 10^{-12}~\mathrm{m}$

• Atomic and nuclear sizes:

  • $r_ ext{atomic} \approx 100~\mathrm{pm}$

  • $r_ ext{nuclear} \approx 5\times 10^{-3}~\mathrm{pm}$

• Subatomic particle properties:

  • Proton charge: $q_p = +1.6022\times 10^{-19}~\mathrm{C}$

  • Electron charge: $q_e = -1.6022\times 10^{-19}~\mathrm{C}$

  • Electron mass: $m_e \approx 9.10936\times 10^{-28}~\mathrm{g}$

• Coulomb’s law: $F = k\, \frac{q<em>1 q</em>2}{r^2}$

• Isotope concept: same $Z$ (protons), different neutrons (N) -> different mass number $A = Z + N$