UPCAT Chemistry — Atomic Theory & StructureCheat Sheet
Cheat sheet for UPCAT Chemistry — Atomic Theory & Structure. Compact, printable, and organised around the concepts University of the Philippines tests most frequently in the UPCAT 2026. Perfect for the week before exam day.
Exam context
On the UPCAT 2026, the Chemistry subtest carries a "Core" weight in University of the Philippines's pattern. Atomic Theory & Structure lands at position 2nd out of 7 in the standard review order. Target score is UPG ≤ 2.2 typical, and roughly 20 items come from Chemistry on a typical UPCAT paper.
Atomic Theory & Structure - Cheat sheet
Your last-minute revision companion for atomic theory, electron configuration, and atomic models — master the fundamentals before your UPCAT exam
Sections
Formulas
Formula
Atomic Mass = Protons + Neutrons
Meaning
A = sum of protons and neutrons in nucleus
Watch Out
Don't include electrons in atomic mass calculation
When To Use
When calculating mass number or finding neutron count
Formula
Number of Electrons = Number of Protons (neutral atom)
Meaning
In neutral atoms, positive and negative charges balance
Watch Out
For ions, electrons ≠ protons
When To Use
For neutral atoms only, not ions
Formula
c = λν
Meaning
c = speed of light, λ = wavelength, ν = frequency
Watch Out
Use correct units: c = 3.00 × 10⁸ m/s
When To Use
When dealing with electromagnetic radiation
Common Values
Value
1.602 × 10⁻¹⁹ C
Symbol
e
Quantity
Electron charge
Value
1.66054 × 10⁻²⁴ g
Symbol
amu
Quantity
Atomic mass unit
Section Title
Atomic Structure Basics
Important Facts
- Protons: positive charge, located in nucleus, mass ≈ 1 amu
- Neutrons: no charge, located in nucleus, mass ≈ 1 amu
- Electrons: negative charge, orbit nucleus, mass ≈ 1/1836 amu
- Nucleus contains 99.9% of atom's mass but only 1/100,000 of its volume
- Atom diameter ≈ 10⁻⁸ cm, nucleus diameter ≈ 10⁻¹³ cm
Key Definitions
Term
Atom
Example
Carbon atom with 6 protons, 6 neutrons, 6 electrons
Definition
Smallest unit of an element that retains its chemical properties
Term
Atomic Number (Z)
Example
Oxygen has Z = 8
Definition
Number of protons in the nucleus of an atom
Term
Mass Number (A)
Example
Carbon-12 has A = 12
Definition
Sum of protons and neutrons in an atom's nucleus
Term
Isotopes
Example
Carbon-12, Carbon-13, Carbon-14
Definition
Atoms of same element with different numbers of neutrons
Diagrams To Know
- Basic atomic structure showing nucleus and electron cloud
- Notation for atomic symbols (A/Z X format)
- Relative sizes of subatomic particles
Section Title
Atomic Models Evolution
Important Facts
- Dalton: atoms are indivisible (later disproven by subatomic particles)
- Thomson: discovered electron through cathode ray experiments
- Rutherford: gold foil experiment proved existence of nucleus
- Bohr: explained hydrogen spectrum with quantized energy levels
- Quantum model: electrons exist in probability clouds called orbitals
Key Definitions
Term
Dalton's Atomic Theory
Example
All carbon atoms are identical
Definition
Matter consists of indivisible atoms; atoms combine in whole number ratios
Term
Thomson's Plum Pudding Model
Example
Uniform positive sphere with negative electrons scattered
Definition
Electrons embedded in sphere of positive charge like plums in pudding
Term
Rutherford's Nuclear Model
Example
Solar system model with nucleus as sun
Definition
Dense positive nucleus with electrons orbiting around it
Term
Bohr's Model
Example
Electrons in n=1, n=2, n=3 energy levels
Definition
Electrons occupy fixed energy levels or shells around nucleus
Diagrams To Know
- Thomson's plum pudding model diagram
- Rutherford's gold foil experiment setup
- Bohr's energy level diagram
- Modern orbital shapes (s, p, d, f)
Formulas
Formula
Maximum electrons in shell = 2n²
Meaning
n = principal quantum number (shell number)
Watch Out
This is maximum capacity, not actual electrons present
When To Use
To find maximum electron capacity of any shell
Formula
Maximum electrons in subshell = 2(2l + 1)
Meaning
l = angular momentum quantum number
Watch Out
s=2, p=6, d=10, f=14 electrons maximum
When To Use
To find electron capacity of s, p, d, f subshells
Common Values
Value
2 electrons
Symbol
s²
Quantity
s subshell capacity
Value
6 electrons
Symbol
p⁶
Quantity
p subshell capacity
Value
10 electrons
Symbol
d¹⁰
Quantity
d subshell capacity
Section Title
Quantum Numbers & Electron Configuration
Important Facts
- Aufbau Principle: electrons fill lowest energy orbitals first
- Pauli Exclusion Principle: no two electrons can have same 4 quantum numbers
- Hund's Rule: electrons occupy orbitals singly before pairing
- Orbital filling order: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p...
- s orbitals hold 2e⁻, p holds 6e⁻, d holds 10e⁻, f holds 14e⁻
Key Definitions
Term
Principal Quantum Number (n)
Example
n = 1, 2, 3, 4...
Definition
Indicates electron shell number and energy level
Term
Angular Momentum Quantum Number (l)
Example
l = 0(s), 1(p), 2(d), 3(f)
Definition
Indicates subshell type and orbital shape
Term
Magnetic Quantum Number (ml)
Example
For p subshell: ml = -1, 0, +1
Definition
Indicates orbital orientation in space
Term
Spin Quantum Number (ms)
Example
ms = +½ or -½
Definition
Indicates electron spin direction
Diagrams To Know
- Orbital shapes (s spherical, p dumbbell, d complex)
- Electron configuration notation
- Orbital filling diagram (arrows in boxes)
- Aufbau diagonal rule diagram
Common Values
Value
4.0
Symbol
χ
Quantity
Fluorine electronegativity
Section Title
Periodic Trends
Important Facts
- Across period (left to right): radius decreases, IE increases, EN increases
- Down group: radius increases, IE decreases, EN decreases
- Noble gases have highest ionization energies
- Metals have low IE and EN, nonmetals have high IE and EN
- Fluorine is most electronegative element (4.0 on Pauling scale)
Key Definitions
Term
Atomic Radius
Example
Li > Be > B > C (decreases across period)
Definition
Half the distance between nuclei of two bonded identical atoms
Term
Ionization Energy
Example
He > Ne > F > O (increases across period)
Definition
Energy required to remove an electron from gaseous atom
Term
Electronegativity
Example
F > O > N > Cl (F is most electronegative)
Definition
Ability of atom to attract electrons in chemical bond
Term
Electron Affinity
Example
Cl > F > Br > I (generally increases across period)
Definition
Energy change when electron is added to gaseous atom
Diagrams To Know
- Periodic table with trend arrows
- Atomic radius comparison diagrams
- Ionization energy graph across periods
Formulas
Formula
t₁/₂ = 0.693/λ
Meaning
Half-life equation where λ is decay constant
Watch Out
Different isotopes have different half-lives
When To Use
For radioactive decay calculations
Section Title
Ions and Nuclear Chemistry
Important Facts
- Metals form cations by losing valence electrons
- Nonmetals form anions by gaining electrons to complete octet
- Nuclear reactions involve changes in nucleus, not electrons
- Alpha particles: +2 charge, mass = 4 amu, low penetration
- Beta particles: -1 charge, negligible mass, moderate penetration
- Gamma rays: no charge, no mass, high penetration
Key Definitions
Term
Cation
Example
Na⁺ (sodium lost 1 electron)
Definition
Positively charged ion formed by losing electrons
Term
Anion
Example
Cl⁻ (chlorine gained 1 electron)
Definition
Negatively charged ion formed by gaining electrons
Term
Alpha Decay
Example
²³⁸U → ²³⁴Th + ⁴He
Definition
Emission of alpha particle (helium nucleus, ²He⁴)
Term
Beta Decay
Example
²³⁴Th → ²³⁴Pa + ⁰e⁻¹
Definition
Emission of beta particle (electron or positron)
Diagrams To Know
- Nuclear decay equations with proper notation
- Types of radiation and their penetrating power
- Ion formation diagrams showing electron transfer
Reactions Or Equations
Note
Mass number decreases by 4, atomic number by 2
Equation
²³⁸U → ²³⁴Th + ⁴α
Conditions
Alpha decay of uranium-238
Note
Mass number same, atomic number increases by 1
Equation
¹⁴C → ¹⁴N + ⁰β⁻
Conditions
Beta decay of carbon-14
Must Remember
- Atomic number = number of protons = number of electrons (neutral atom)
- Mass number = protons + neutrons
- Isotopes have same protons, different neutrons
- Electron configuration order: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p...
- Maximum electrons per shell: 2n² (2, 8, 18, 32)
- Aufbau: fill lowest energy first; Pauli: max 2e⁻ per orbital; Hund: single before pair
- Across period: radius decreases, ionization energy increases
- Down group: radius increases, ionization energy decreases
- Metals form cations (+), nonmetals form anions (-)
- Alpha decay: mass -4, atomic number -2; Beta decay: mass same, atomic number +1
Last Minute Tips
- For electron config questions, always use the aufbau diagram - don't memorize the order
- Remember exceptions: Cr and Cu have unusual electron configurations due to stability
- When identifying isotopes, focus on neutron count difference, not mass
- For periodic trends, draw arrows on your exam: → for across period, ↓ for down group
- Nuclear equations must balance: sum of mass numbers equal, sum of atomic numbers equal
Comparison Tables
Rows
Values
- Indivisible atoms
- No subatomic particles
Property
Dalton
Values
- Electrons in positive sphere
- No nucleus concept
Property
Thomson
Values
- Dense nucleus, orbiting electrons
- Electrons should spiral into nucleus
Property
Rutherford
Values
- Fixed energy levels
- Only works for hydrogen
Property
Bohr
Values
- Probability orbitals
- Complex mathematics
Property
Quantum
Columns
- Model
- Key Features
- Limitations
Table Title
Atomic Models Comparison
Rows
Values
- +1
- 1.007
- Nucleus
Property
Proton
Values
- 0
- 1.009
- Nucleus
Property
Neutron
Values
- -1
- 0.0005
- Electron cloud
Property
Electron
Columns
- Particle
- Charge
- Mass (amu)
- Location
Table Title
Subatomic Particles
Rows
Values
- Principal
- 1,2,3,4...
- Shell/energy level
Property
n
Values
- Angular
- 0 to n-1
- Subshell type
Property
l
Values
- Magnetic
- -l to +l
- Orbital orientation
Property
ml
Values
- Spin
- +½, -½
- Electron spin
Property
ms
Columns
- Symbol
- Name
- Values
- Describes
Table Title
Quantum Numbers Summary
Previous chapter
Matter, Mixtures & Measurement
Next chapter
Periodic Table, Bonding & Chemical Language
Ready to practise for the UPCAT 2026?
Super Tutor's AI review plan adapts to your weak areas and builds a weekly practice schedule around your target UPCAT exam date.