USYD CHEM1011| CHEM 1011 Finals Study Guide: Key ○ Behavior in Container: Fills the bottom of the
Concepts in Chemistry (Fundamentals of Chemistry 1A) container and takes its shape.
University of Sydney
○ Interactions: Weaker interactions than in solids, but
still significant.
○ Sketch: Imagine a container with particles loosely
arranged, filling the lower portion, with a distinct
surface.
LECTURE 1 - HISTORY ● Gas:
○ Particle Arrangement: Particles are far apart, highly
disordered (random), and move wildly. They are not
in close contact.
○ Behavior in Container: Fills all available space in the
container.
LECTURE 2 - ATOMIC STRUCTURE
○ Interactions: Very weak interactions between
particles.
This study guide covers fundamental concepts in chemistry, including
○ Sketch: Imagine a container with particles scattered
the nature of matter, physical and chemical properties and changes,
throughout, moving freely and randomly.
atomic structure, and the mole concept. Mastering this content is crucial
for understanding all subsequent chemistry topics.
I. Introduction to Chemistry
Chemistry is the study of matter and its properties, and how matter
changes. It's a dynamic field constantly evolving through human
investigation and discovery.
○ Interactions: Strong interactions between particles.
○ Sketch: Imagine a container with atoms neatly
A. Chemical Equations: Representing Change
arranged in a grid at the bottom.
● Definition: Chemical equations are symbolic representations ● Liquid:
of chemical reactions, showing how one substance transforms ○ Particle Arrangement: Particles are less ordered
than in a solid but are still in close contact. They can
into another.
move past each other.
● Example: Methane (CH₄) reacting with oxygen (O₂) to produce
carbon dioxide (CO₂) and water (H₂O).
○ Word Equation: methane + oxygen → carbon
dioxide + water
○ Chemical Formula Equation: CH₄ + 2 O₂ → CO₂ + 2
H₂O
○ Visual Representation (Ball-and-Stick Models):
■ Methane (CH₄): One central grey carbon
atom bonded to four white hydrogen
atoms.
■ Oxygen (O₂): Two red oxygen atoms
bonded together.
■ Carbon Dioxide (CO₂): One central grey
carbon atom bonded to two red oxygen
atoms.
■ Water (H₂O): One central red oxygen atom
bonded to two white hydrogen atoms.
● Significance: Chemical equations help us understand how to
create desired compounds and in what quantities.
II. States of Matter and Changes
Matter exists in different states, primarily solid, liquid, and gas, which
are distinguished by how their particles are arranged and interact.
A. Characteristics of States of Matter
● Solid:
○ Particle Arrangement: Particles are very well
ordered and closely packed in a fixed, rigid structure.
○ Behavior in Container: Sits at the bottom of the
container; does not move or flow.
,B. Physical Changes: Changes of State
● Definition: A physical change involves a change in the physical
form or state of a substance, but not its chemical composition.
No new chemical bonds are formed or broken.
● Examples:
○ Melting: Solid to Liquid (e.g., ice to water).
○ Boiling/Evaporation: Liquid to Gas (e.g., water to
steam).
○ Condensation: Gas to Liquid (e.g., steam to water
droplets).
○ Freezing: Liquid to Solid (e.g., water to ice).
○ Sublimation: Solid to Gas (e.g., dry ice (solid CO₂) to
CO₂ gas). This is less common.
○ Deposition: Gas to Solid (e.g., frost formation). This
is the opposite of sublimation and also less common.
● Demonstration (Dry Ice): Solid CO₂ (dry ice) sublimates
directly into CO₂ gas. This gas is denser than air, so it fills a
container from the bottom up, potentially extinguishing
flames by displacing oxygen. This is a physical change because
the CO₂ molecules themselves remain unchanged, only their
state changes.
C. Allotropes: Different Forms of an Element
● Definition: Allotropes are different structural arrangements or
bonding arrangements of atoms within the same element.
They are different physical forms of the same element.
● Examples:
○ Carbon: Diamond, graphite, buckminsterfullerene
(buckyballs), carbon nanotubes. All are pure carbon
but have different atomic arrangements, leading to
vastly different properties.
■ Stability: Graphite is thermodynamically
more stable than diamond under ambient
conditions, though the conversion of
diamond to graphite is extremely slow.
○ Oxygen: O₂ (dioxygen) and O₃ (ozone).
● Significance: Allotropes demonstrate that an element can exist
in multiple forms with distinct physical properties, even
though their chemical composition is identical.
D. Physical Properties
, ● Definition: Properties of a substance that can be observed or form or break. (e.g., burning H₂ with O₂, rusting iron, digestion
measured without changing its chemical composition or of food).
interacting with another substance.
● Examples:
III. Atomic Structure
○ Color: The visual appearance of a substance (e.g.,
bromine is reddish-brown).
The concept of atoms has evolved significantly over time. Initially
○ Melting Point: The temperature at which a solid
thought to be indivisible, we now know atoms are composed of even
turns into a liquid.
smaller subatomic particles.
○ Conductivity: The ability to conduct heat or
electricity.
A. Historical Context (Non-Assessable for Exam, but Important for
○ Density: Mass per unit volume (e.g., 1 liter of
Understanding)
mercury is much heavier than 1 liter of bromine).
○ Surface Tension: The cohesive forces between liquid
● 1808: John Dalton's Atomic Theory:
molecules at the surface, causing it to behave like an
○ All matter consists of atoms, which are tiny,
elastic film (e.g., water droplets on leaves).
indivisible particles of an element.
○ Atoms cannot be created or destroyed.
E. Chemical Properties ○ Atoms of one element cannot be converted into
atoms of another element.
● Definition: Properties of a substance that describe how it
○ Atoms of an element are identical and different from
reacts with other substances, resulting in the formation of
atoms of any other element.
new substances. These properties are only observed during a
○ Key takeaway: Atoms were considered indivisible.
chemical change.
● 1897: J.J. Thomson's Discovery of Electrons:
● Examples: ○ Experiment: Studied cathode rays, which are
○ Reactivity: How readily a substance undergoes
streams of particles emitted from a cathode
chemical reactions.
(negative electrode) when a high voltage is applied.
○ Flammability: The ability of a substance to burn or
○ Observations:
ignite, causing fire or combustion (a reaction with
■ All metals produced the same type of
oxygen). negatively charged particles.
○ Corrosiveness: The ability of a substance to cause ■ These particles were about 1000 times
damage to other materials through chemical lighter than a hydrogen atom (the lightest
reaction (e.g., acids corroding metals). known atom).
○ Reactivity with Acid/Base: How a substance ○ Conclusion: Atoms are divisible and contain smaller,
interacts chemically with acids or bases. negatively charged particles.
○ Naming: These cathode rays were later named
F. Chemical Changes electrons. This discovery fundamentally changed the
understanding of atomic structure.
● Definition: A process that involves the rearrangement of the
atomic structure of a substance, leading to the formation of
B. Subatomic Particles
new substances with different chemical compositions.
Chemical bonds are formed or broken.
Atoms are composed of three main subatomic particles: protons,
● Examples: neutrons, and electrons.
○ Burning Gas: Methane reacting with oxygen to
produce carbon dioxide and water (e.g., a candle ● Schematic of an Atom:
burning). ○ Nucleus: A tiny, dense central core containing
○ Rusting of Iron: Iron reacting with oxygen and water protons and neutrons.
to form iron oxides (rust). ○ Electrons: Orbit the nucleus in a "cloud" of
○ Digestion of Food: Complex food molecules are probability.
broken down into simpler molecules by chemical ○ Size: The atom's overall size is approximately 10⁻¹⁰
reactions in the body.
meters. The nucleus is much smaller, about 10⁻¹⁴
○ Reacting Zinc in Hydrochloric Acid: Zinc metal reacts meters (four orders of magnitude smaller). This
with hydrochloric acid to produce hydrogen gas and means an atom is mostly empty space.
zinc chloride. ● Properties of Subatomic Particles:
● Distinction from Physical Changes: In a chemical change, the
identity of the substance changes. In a physical change, only Particle Symbol Relative Mass Relative Charge Location in Atom Proton p
the form or state changes, but the substance remains or p⁺ 1 +1 Nucleus Neutron n or n⁰ 1 0 (neutral) Nucleus Electron e or e⁻
chemically the same. 1/1836 -1 Outside nucleus
Summary of Changes: ● Nucleons: Protons and neutrons are collectively called
nucleons because they reside in the nucleus.
● Physical Change: Alters physical form, not composition. No
chemical bonds form or break. (e.g., boiling water, freezing
C. Defining an Element: Atomic Number (Z)
wax, distillation of alcohol).
● Chemical Change: Converts substance(s) into new
● Atomic Number (Z): The number of protons in the nucleus of
substance(s). Compound/allotrope changes, chemical bonds
an atom.
, ○ Unique Identifier: The atomic number uniquely ○ $^{14}_{6}$C: 6 protons, 8 neutrons (Mass Number
defines an element. Every atom of a given element = 14)
has the same number of protons. ○ Both are carbon because they both have 6 protons,
○ Periodic Table: Elements are arranged in the but they differ in their neutron count.
periodic table in increasing order of their atomic ● Example: Uranium Isotopes:
number (Z). ○ $^{235}_{92}$U: 92 protons, 143 neutrons (Mass
○ Nuclear Charge: The atomic number is also equal to Number = 235)
the positive charge of the nucleus. ○ $^{238}_{92}$U: 92 protons, 146 neutrons (Mass
● Examples: Number = 238)
○ Z = 2: Helium (He) ● Key Point: The number of protons determines the element;
○ Z = 8: Oxygen (O) the number of neutrons can vary within an element, creating
○ Z = 26: Iron (Fe) isotopes.
○ Z = 92: Uranium (U)
H. Changes to Atomic Composition and Their Effects
D. Mass Number (A)
Let's consider a starting atom, Carbon-12 ($^{12}_{6}$C), which has 6
● Definition: The total number of protons and neutrons in the protons, 6 neutrons, and 6 electrons (if neutral).
nucleus of an atom.
○ Formula: Mass Number (A) = Number of Protons (Z) Change Effect on Mass Effect on Charge New Particle Formed (if element
+ Number of Neutrons (n⁰) changes) Add a proton +1 +1 $^{13}_{7}$N⁺ (Nitrogen ion) Add a
○ Note: Electrons contribute negligible mass to the neutron +1 No change $^{13}_{6}$C (Carbon-13 isotope) Remove an
atom. electron No change +1 $^{12}_{6}$C⁺ (Carbon cation) Add an electron
No change -1 $^{12}_{6}$C⁻ (Carbon anion) Add a proton & a neutron +2
E. Atomic Symbol Notation +1 $^{14}_{7}$N⁺ (Nitrogen ion) Remove a proton & an electron -1 No
change $^{11}_{5}$B (Boron) Add a neutron & an electron +1 -1
● Format: $^{A}_{Z}$X $^{13}_{6}$C⁻ (Carbon-13 anion)
○ X: Chemical symbol of the element (e.g., C for
Carbon, Li for Lithium). ● Which changes alter the element? Any change in the number
○ A: Mass number (superscript). of protons changes the element.
○ Z: Atomic number (subscript). ○ Adding a proton (changes C to N).
● Example: $^{12}_{6}$C ○ Removing a proton (changes C to B).
○ X = C (Carbon)
○ Z = 6 (6 protons) IV. The Mole Concept
○ A = 12 (6 protons + 6 neutrons)
The mole is a fundamental unit in chemistry used to quantify large
F. Neutral Atoms and Ions numbers of atoms or molecules, making it practical to work with
substances in the lab.
● Neutral Atom: An atom with no overall electrical charge.
○ Condition: Number of protons = Number of A. Atomic Mass Units (amu)
electrons.
○ Therefore: For a neutral atom, the atomic number ● Need for a Reference: To quantify the incredibly small mass of
(Z) also equals the number of electrons. an atom, a reference point is needed.
● Ions: Atoms or molecules that have gained or lost one or more ● Historical Reference (Dalton): Hydrogen atom was initially
electrons, resulting in a net electrical charge. assigned a mass of 1.
○ Cation: A positively charged ion. ● Modern Reference: The carbon-12 ($^{12}$C) isotope is the
■ Condition: Fewer electrons than protons. current standard.
(Lost electrons) ○ A $^{12}$C atom is defined as having a mass of
■ Example: Na⁺ (Sodium ion) has 11 protons exactly 12 atomic mass units (amu).
and 10 electrons. ○ 1 amu = 1/12 of the mass of one atom of $^{12}$C.
○ Anion: A negatively charged ion. ○ Value: 1 amu = 1.66054 × 10⁻²⁴ grams.
■ Condition: More electrons than protons. ● Problem: Working with individual atoms or amu in the lab is
(Gained electrons) impractical due to their extremely small size and mass.
■ Example: Cl⁻ (Chloride ion) has 17 protons Chemical reactions involve vast numbers of atoms.
and 18 electrons.
B. Avogadro's Number (N_A) and the Mole
G. Isotopes
● The Solution: Instead of individual atoms, chemists use a large
● Definition: Atoms of the same element (same number of collection of atoms called a mole.
protons, Z) that have different numbers of neutrons, and ● Definition of a Mole: A mole is a unit of quantity, similar to a
therefore different mass numbers (A). "dozen." It represents a specific number of particles.
● Example: Carbon Isotopes: ● Avogadro's Number (N_A): The number of particles (atoms,
○ $^{12}_{6}$C: 6 protons, 6 neutrons (Mass Number molecules, ions, etc.) in one mole of any substance.
= 12) ○ Value: N_A = 6.022 × 10²³ particles/mol.
Concepts in Chemistry (Fundamentals of Chemistry 1A) container and takes its shape.
University of Sydney
○ Interactions: Weaker interactions than in solids, but
still significant.
○ Sketch: Imagine a container with particles loosely
arranged, filling the lower portion, with a distinct
surface.
LECTURE 1 - HISTORY ● Gas:
○ Particle Arrangement: Particles are far apart, highly
disordered (random), and move wildly. They are not
in close contact.
○ Behavior in Container: Fills all available space in the
container.
LECTURE 2 - ATOMIC STRUCTURE
○ Interactions: Very weak interactions between
particles.
This study guide covers fundamental concepts in chemistry, including
○ Sketch: Imagine a container with particles scattered
the nature of matter, physical and chemical properties and changes,
throughout, moving freely and randomly.
atomic structure, and the mole concept. Mastering this content is crucial
for understanding all subsequent chemistry topics.
I. Introduction to Chemistry
Chemistry is the study of matter and its properties, and how matter
changes. It's a dynamic field constantly evolving through human
investigation and discovery.
○ Interactions: Strong interactions between particles.
○ Sketch: Imagine a container with atoms neatly
A. Chemical Equations: Representing Change
arranged in a grid at the bottom.
● Definition: Chemical equations are symbolic representations ● Liquid:
of chemical reactions, showing how one substance transforms ○ Particle Arrangement: Particles are less ordered
than in a solid but are still in close contact. They can
into another.
move past each other.
● Example: Methane (CH₄) reacting with oxygen (O₂) to produce
carbon dioxide (CO₂) and water (H₂O).
○ Word Equation: methane + oxygen → carbon
dioxide + water
○ Chemical Formula Equation: CH₄ + 2 O₂ → CO₂ + 2
H₂O
○ Visual Representation (Ball-and-Stick Models):
■ Methane (CH₄): One central grey carbon
atom bonded to four white hydrogen
atoms.
■ Oxygen (O₂): Two red oxygen atoms
bonded together.
■ Carbon Dioxide (CO₂): One central grey
carbon atom bonded to two red oxygen
atoms.
■ Water (H₂O): One central red oxygen atom
bonded to two white hydrogen atoms.
● Significance: Chemical equations help us understand how to
create desired compounds and in what quantities.
II. States of Matter and Changes
Matter exists in different states, primarily solid, liquid, and gas, which
are distinguished by how their particles are arranged and interact.
A. Characteristics of States of Matter
● Solid:
○ Particle Arrangement: Particles are very well
ordered and closely packed in a fixed, rigid structure.
○ Behavior in Container: Sits at the bottom of the
container; does not move or flow.
,B. Physical Changes: Changes of State
● Definition: A physical change involves a change in the physical
form or state of a substance, but not its chemical composition.
No new chemical bonds are formed or broken.
● Examples:
○ Melting: Solid to Liquid (e.g., ice to water).
○ Boiling/Evaporation: Liquid to Gas (e.g., water to
steam).
○ Condensation: Gas to Liquid (e.g., steam to water
droplets).
○ Freezing: Liquid to Solid (e.g., water to ice).
○ Sublimation: Solid to Gas (e.g., dry ice (solid CO₂) to
CO₂ gas). This is less common.
○ Deposition: Gas to Solid (e.g., frost formation). This
is the opposite of sublimation and also less common.
● Demonstration (Dry Ice): Solid CO₂ (dry ice) sublimates
directly into CO₂ gas. This gas is denser than air, so it fills a
container from the bottom up, potentially extinguishing
flames by displacing oxygen. This is a physical change because
the CO₂ molecules themselves remain unchanged, only their
state changes.
C. Allotropes: Different Forms of an Element
● Definition: Allotropes are different structural arrangements or
bonding arrangements of atoms within the same element.
They are different physical forms of the same element.
● Examples:
○ Carbon: Diamond, graphite, buckminsterfullerene
(buckyballs), carbon nanotubes. All are pure carbon
but have different atomic arrangements, leading to
vastly different properties.
■ Stability: Graphite is thermodynamically
more stable than diamond under ambient
conditions, though the conversion of
diamond to graphite is extremely slow.
○ Oxygen: O₂ (dioxygen) and O₃ (ozone).
● Significance: Allotropes demonstrate that an element can exist
in multiple forms with distinct physical properties, even
though their chemical composition is identical.
D. Physical Properties
, ● Definition: Properties of a substance that can be observed or form or break. (e.g., burning H₂ with O₂, rusting iron, digestion
measured without changing its chemical composition or of food).
interacting with another substance.
● Examples:
III. Atomic Structure
○ Color: The visual appearance of a substance (e.g.,
bromine is reddish-brown).
The concept of atoms has evolved significantly over time. Initially
○ Melting Point: The temperature at which a solid
thought to be indivisible, we now know atoms are composed of even
turns into a liquid.
smaller subatomic particles.
○ Conductivity: The ability to conduct heat or
electricity.
A. Historical Context (Non-Assessable for Exam, but Important for
○ Density: Mass per unit volume (e.g., 1 liter of
Understanding)
mercury is much heavier than 1 liter of bromine).
○ Surface Tension: The cohesive forces between liquid
● 1808: John Dalton's Atomic Theory:
molecules at the surface, causing it to behave like an
○ All matter consists of atoms, which are tiny,
elastic film (e.g., water droplets on leaves).
indivisible particles of an element.
○ Atoms cannot be created or destroyed.
E. Chemical Properties ○ Atoms of one element cannot be converted into
atoms of another element.
● Definition: Properties of a substance that describe how it
○ Atoms of an element are identical and different from
reacts with other substances, resulting in the formation of
atoms of any other element.
new substances. These properties are only observed during a
○ Key takeaway: Atoms were considered indivisible.
chemical change.
● 1897: J.J. Thomson's Discovery of Electrons:
● Examples: ○ Experiment: Studied cathode rays, which are
○ Reactivity: How readily a substance undergoes
streams of particles emitted from a cathode
chemical reactions.
(negative electrode) when a high voltage is applied.
○ Flammability: The ability of a substance to burn or
○ Observations:
ignite, causing fire or combustion (a reaction with
■ All metals produced the same type of
oxygen). negatively charged particles.
○ Corrosiveness: The ability of a substance to cause ■ These particles were about 1000 times
damage to other materials through chemical lighter than a hydrogen atom (the lightest
reaction (e.g., acids corroding metals). known atom).
○ Reactivity with Acid/Base: How a substance ○ Conclusion: Atoms are divisible and contain smaller,
interacts chemically with acids or bases. negatively charged particles.
○ Naming: These cathode rays were later named
F. Chemical Changes electrons. This discovery fundamentally changed the
understanding of atomic structure.
● Definition: A process that involves the rearrangement of the
atomic structure of a substance, leading to the formation of
B. Subatomic Particles
new substances with different chemical compositions.
Chemical bonds are formed or broken.
Atoms are composed of three main subatomic particles: protons,
● Examples: neutrons, and electrons.
○ Burning Gas: Methane reacting with oxygen to
produce carbon dioxide and water (e.g., a candle ● Schematic of an Atom:
burning). ○ Nucleus: A tiny, dense central core containing
○ Rusting of Iron: Iron reacting with oxygen and water protons and neutrons.
to form iron oxides (rust). ○ Electrons: Orbit the nucleus in a "cloud" of
○ Digestion of Food: Complex food molecules are probability.
broken down into simpler molecules by chemical ○ Size: The atom's overall size is approximately 10⁻¹⁰
reactions in the body.
meters. The nucleus is much smaller, about 10⁻¹⁴
○ Reacting Zinc in Hydrochloric Acid: Zinc metal reacts meters (four orders of magnitude smaller). This
with hydrochloric acid to produce hydrogen gas and means an atom is mostly empty space.
zinc chloride. ● Properties of Subatomic Particles:
● Distinction from Physical Changes: In a chemical change, the
identity of the substance changes. In a physical change, only Particle Symbol Relative Mass Relative Charge Location in Atom Proton p
the form or state changes, but the substance remains or p⁺ 1 +1 Nucleus Neutron n or n⁰ 1 0 (neutral) Nucleus Electron e or e⁻
chemically the same. 1/1836 -1 Outside nucleus
Summary of Changes: ● Nucleons: Protons and neutrons are collectively called
nucleons because they reside in the nucleus.
● Physical Change: Alters physical form, not composition. No
chemical bonds form or break. (e.g., boiling water, freezing
C. Defining an Element: Atomic Number (Z)
wax, distillation of alcohol).
● Chemical Change: Converts substance(s) into new
● Atomic Number (Z): The number of protons in the nucleus of
substance(s). Compound/allotrope changes, chemical bonds
an atom.
, ○ Unique Identifier: The atomic number uniquely ○ $^{14}_{6}$C: 6 protons, 8 neutrons (Mass Number
defines an element. Every atom of a given element = 14)
has the same number of protons. ○ Both are carbon because they both have 6 protons,
○ Periodic Table: Elements are arranged in the but they differ in their neutron count.
periodic table in increasing order of their atomic ● Example: Uranium Isotopes:
number (Z). ○ $^{235}_{92}$U: 92 protons, 143 neutrons (Mass
○ Nuclear Charge: The atomic number is also equal to Number = 235)
the positive charge of the nucleus. ○ $^{238}_{92}$U: 92 protons, 146 neutrons (Mass
● Examples: Number = 238)
○ Z = 2: Helium (He) ● Key Point: The number of protons determines the element;
○ Z = 8: Oxygen (O) the number of neutrons can vary within an element, creating
○ Z = 26: Iron (Fe) isotopes.
○ Z = 92: Uranium (U)
H. Changes to Atomic Composition and Their Effects
D. Mass Number (A)
Let's consider a starting atom, Carbon-12 ($^{12}_{6}$C), which has 6
● Definition: The total number of protons and neutrons in the protons, 6 neutrons, and 6 electrons (if neutral).
nucleus of an atom.
○ Formula: Mass Number (A) = Number of Protons (Z) Change Effect on Mass Effect on Charge New Particle Formed (if element
+ Number of Neutrons (n⁰) changes) Add a proton +1 +1 $^{13}_{7}$N⁺ (Nitrogen ion) Add a
○ Note: Electrons contribute negligible mass to the neutron +1 No change $^{13}_{6}$C (Carbon-13 isotope) Remove an
atom. electron No change +1 $^{12}_{6}$C⁺ (Carbon cation) Add an electron
No change -1 $^{12}_{6}$C⁻ (Carbon anion) Add a proton & a neutron +2
E. Atomic Symbol Notation +1 $^{14}_{7}$N⁺ (Nitrogen ion) Remove a proton & an electron -1 No
change $^{11}_{5}$B (Boron) Add a neutron & an electron +1 -1
● Format: $^{A}_{Z}$X $^{13}_{6}$C⁻ (Carbon-13 anion)
○ X: Chemical symbol of the element (e.g., C for
Carbon, Li for Lithium). ● Which changes alter the element? Any change in the number
○ A: Mass number (superscript). of protons changes the element.
○ Z: Atomic number (subscript). ○ Adding a proton (changes C to N).
● Example: $^{12}_{6}$C ○ Removing a proton (changes C to B).
○ X = C (Carbon)
○ Z = 6 (6 protons) IV. The Mole Concept
○ A = 12 (6 protons + 6 neutrons)
The mole is a fundamental unit in chemistry used to quantify large
F. Neutral Atoms and Ions numbers of atoms or molecules, making it practical to work with
substances in the lab.
● Neutral Atom: An atom with no overall electrical charge.
○ Condition: Number of protons = Number of A. Atomic Mass Units (amu)
electrons.
○ Therefore: For a neutral atom, the atomic number ● Need for a Reference: To quantify the incredibly small mass of
(Z) also equals the number of electrons. an atom, a reference point is needed.
● Ions: Atoms or molecules that have gained or lost one or more ● Historical Reference (Dalton): Hydrogen atom was initially
electrons, resulting in a net electrical charge. assigned a mass of 1.
○ Cation: A positively charged ion. ● Modern Reference: The carbon-12 ($^{12}$C) isotope is the
■ Condition: Fewer electrons than protons. current standard.
(Lost electrons) ○ A $^{12}$C atom is defined as having a mass of
■ Example: Na⁺ (Sodium ion) has 11 protons exactly 12 atomic mass units (amu).
and 10 electrons. ○ 1 amu = 1/12 of the mass of one atom of $^{12}$C.
○ Anion: A negatively charged ion. ○ Value: 1 amu = 1.66054 × 10⁻²⁴ grams.
■ Condition: More electrons than protons. ● Problem: Working with individual atoms or amu in the lab is
(Gained electrons) impractical due to their extremely small size and mass.
■ Example: Cl⁻ (Chloride ion) has 17 protons Chemical reactions involve vast numbers of atoms.
and 18 electrons.
B. Avogadro's Number (N_A) and the Mole
G. Isotopes
● The Solution: Instead of individual atoms, chemists use a large
● Definition: Atoms of the same element (same number of collection of atoms called a mole.
protons, Z) that have different numbers of neutrons, and ● Definition of a Mole: A mole is a unit of quantity, similar to a
therefore different mass numbers (A). "dozen." It represents a specific number of particles.
● Example: Carbon Isotopes: ● Avogadro's Number (N_A): The number of particles (atoms,
○ $^{12}_{6}$C: 6 protons, 6 neutrons (Mass Number molecules, ions, etc.) in one mole of any substance.
= 12) ○ Value: N_A = 6.022 × 10²³ particles/mol.
