Atoms and Molecules: Comprehensive Study Notes

Key Concepts

Structure of the Atom

Atoms are the smallest particles of an element that can exist and still retain the properties of that element
Every atom consists of three types of subatomic particles:
- Protons: positively charged particles (+1 charge) located in the nucleus
- Neutrons: neutral particles (no charge) located in the nucleus
- Electrons: negatively charged particles (-1 charge) that orbit around the nucleus in electron shells
The nucleus is the dense central core of the atom containing protons and neutrons
The nucleus is extremely small compared to the overall size of the atom
Most of the atom’s mass is concentrated in the nucleus
Electrons move in shells (also called energy levels) around the nucleus at different distances
Atoms are electrically neutral because the number of protons equals the number of electrons
The positive charges and negative charges balance out to give zero overall charge

Relative Masses and Charges

Particle	Relative Mass	Relative Charge	Location
Proton	1	+1	Nucleus
Neutron	1	0	Nucleus
Electron	1/1840 (negligible)	-1	Electron shells

Electrons are about 1840 times lighter than protons and neutrons
For most calculations, we consider the electron mass as negligible

Atomic Number and Mass Number

Atomic number (Z): the number of protons in an atom
The atomic number defines what element an atom is
All atoms of the same element have the same atomic number
For a neutral atom: atomic number = number of protons = number of electrons
Mass number (A): the total number of protons and neutrons in an atom
Mass number = number of protons + number of neutrons
Number of neutrons = mass number - atomic number
Atoms are represented using notation: $$\ce{^{A}_{Z}X}$$ where X is the element symbol
Example: $$\ce{^{23}_{11}Na}$$ represents a sodium atom with mass number 23 and atomic number 11

Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons
Isotopes have the same atomic number but different mass numbers
Isotopes have identical chemical properties because they have the same number of electrons
Isotopes have slightly different physical properties (like density) due to different masses
Examples of isotopes:
- Carbon-12 ($$\ce{^{12}_{6}C}$$): 6 protons, 6 neutrons
- Carbon-14 ($$\ce{^{14}_{6}C}$$): 6 protons, 8 neutrons
- Chlorine-35 ($$\ce{^{35}_{17}Cl}$$): 17 protons, 18 neutrons
- Chlorine-37 ($$\ce{^{37}_{17}Cl}$$): 17 protons, 20 neutrons

Chemical Formulae

A chemical formula shows which elements are present in a substance and how many atoms of each element
The subscript number (written small and low) indicates the number of atoms of that element
If no subscript is written, it means there is only 1 atom of that element
Examples:
- H₂O: 2 hydrogen atoms, 1 oxygen atom
- CO₂: 1 carbon atom, 2 oxygen atoms
- H₂SO₄: 2 hydrogen atoms, 1 sulfur atom, 4 oxygen atoms
- Ca(OH)₂: 1 calcium atom, 2 oxygen atoms, 2 hydrogen atoms
Brackets in formulae: the subscript after the bracket multiplies everything inside the bracket
- Ca(OH)₂ means: 1 Ca, 2 × (OH) = 2 O and 2 H
- Mg(NO₃)₂ means: 1 Mg, 2 × (NO₃) = 2 N and 6 O

Chemical Equations

Word Equations

Word equations show the names of reactants and products in a chemical reaction
Format: Reactant(s) → Product(s)
The arrow (→) means “reacts to form” or “produces”
Reactants are substances that react together (starting materials)
Products are substances formed in the reaction
Multiple reactants or products are separated by plus signs (+)
Example: magnesium + oxygen → magnesium oxide

Symbol Equations

Symbol equations use chemical symbols and formulae instead of words
Symbol equations must be balanced — the number of atoms of each element must be the same on both sides
We balance equations by adding numbers (called coefficients) in front of formulae
We NEVER change the subscripts in formulae when balancing
Example: 2Mg + O₂ → 2MgO
State symbols may be included to show the physical state:
- (s) = solid
- (l) = liquid
- (g) = gas
- (aq) = aqueous solution (dissolved in water)
Example with state symbols: 2Mg(s) + O₂(g) → 2MgO(s)

Steps to Balance Chemical Equations

Write the word equation
Convert to symbol equation using correct formulae
Count atoms of each element on both sides
Add coefficients (numbers in front) to balance
Check that all elements are balanced
Ensure coefficients are in the simplest whole number ratio

Important Definitions

Atom: The smallest particle of an element that can exist and still retain the properties of that element.

Proton: A positively charged subatomic particle found in the nucleus of an atom, with a relative charge of +1 and relative mass of 1.

Neutron: A neutral subatomic particle (no charge) found in the nucleus of an atom, with a relative mass of 1.

Electron: A negatively charged subatomic particle that orbits the nucleus of an atom in electron shells, with a relative charge of -1 and negligible mass.

Nucleus: The dense central core of an atom containing protons and neutrons, where almost all the atom’s mass is concentrated.

Atomic number (Z): The number of protons in the nucleus of an atom, which defines the element.

Mass number (A): The total number of protons and neutrons in the nucleus of an atom.

Isotopes: Atoms of the same element that have the same number of protons (same atomic number) but different numbers of neutrons (different mass numbers).

Chemical formula: A representation using element symbols and subscript numbers to show which elements are present and how many atoms of each element are in a substance.

Reactants: The starting substances in a chemical reaction that react together (written on the left side of the equation).

Products: The substances formed in a chemical reaction (written on the right side of the equation).

Balanced equation: A symbol equation where the number of atoms of each element is the same on both sides of the equation.

Coefficient: A number placed in front of a chemical formula in an equation to balance it.

Diagrams and Structures

Diagram 1: Structure of an Atom

How to draw:

Draw a small circle in the center — label this “Nucleus”
Inside the nucleus, draw several circles marked with “+” (protons) and circles marked with “n” (neutrons)
Draw 2-3 concentric circles (circular shells) around the nucleus at different distances
On these shells, draw small circles marked with “-” (electrons) or “e⁻”
Label the shells as “Electron shell 1” (innermost), “Electron shell 2”, etc.
Add a label pointing to an electron: “Electron (negatively charged)”
Add a label pointing to a proton: “Proton (positively charged)”
Add a label pointing to a neutron: “Neutron (no charge)”
Add a note: “Not to scale — nucleus is much smaller in reality”

Diagram 2: Sodium Atom ($$\ce{^{23}_{11}Na}$$)

How to draw:

Draw a nucleus containing 11 protons (+) and 12 neutrons (n)
Draw first electron shell close to nucleus with 2 electrons
Draw second electron shell with 8 electrons
Draw third electron shell with 1 electron
Label: “Atomic number = 11 (11 protons, 11 electrons)”
Label: “Mass number = 23 (11 protons + 12 neutrons)”

Diagram 3: Isotopes of Carbon

How to draw two atoms side by side:

Carbon-12 ($$\ce{^{12}_{6}C}$$):

Nucleus with 6 protons and 6 neutrons
Two electron shells: 2 electrons in first shell, 4 in second shell
Label: “6 protons, 6 neutrons, 6 electrons”

Carbon-14 ($$\ce{^{14}_{6}C}$$):

Nucleus with 6 protons and 8 neutrons
Two electron shells: 2 electrons in first shell, 4 in second shell
Label: “6 protons, 8 neutrons, 6 electrons”
Note between them: “Same number of protons and electrons (same chemical properties), different number of neutrons”

Worked Examples

Example 1: Determining Subatomic Particles

Question: An atom of aluminium is represented as $$\ce{^{27}_{13}Al}$$. Determine the number of protons, neutrons, and electrons in this atom.

Solution:

Step 1: Identify the atomic number and mass number

Atomic number (bottom number) = 13
Mass number (top number) = 27

Step 2: Determine the number of protons

Number of protons = atomic number = 13 protons

Step 3: Determine the number of electrons

For a neutral atom, number of electrons = number of protons
Number of electrons = 13 electrons

Step 4: Determine the number of neutrons

Number of neutrons = mass number - atomic number
Number of neutrons = 27 - 13 = 14 neutrons

Answer: The aluminium atom has 13 protons, 14 neutrons, and 13 electrons.

Example 2: Understanding Isotopes

Question: Chlorine has two common isotopes: chlorine-35 and chlorine-37. Complete the table below:

Isotope	Symbol	Protons	Neutrons	Electrons	Mass Number
Chlorine-35	$$\ce{^{35}_{17}Cl}$$	?	?	?	?
Chlorine-37	$$\ce{^{37}_{17}Cl}$$	?	?	?	?

Solution:

For Chlorine-35:

Atomic number of chlorine = 17 (this is constant for all chlorine atoms)
Number of protons = 17
Mass number = 35 (given in the name)
Number of neutrons = 35 - 17 = 18
Number of electrons = 17 (neutral atom)

For Chlorine-37:

Atomic number of chlorine = 17
Number of protons = 17
Mass number = 37
Number of neutrons = 37 - 17 = 20
Number of electrons = 17

Completed table:

Isotope	Symbol	Protons	Neutrons	Electrons	Mass Number
Chlorine-35	$$\ce{^{35}_{17}Cl}$$	17	18	17	35
Chlorine-37	$$\ce{^{37}_{17}Cl}$$	17	20	17	37

Key point: Both isotopes have the same number of protons and electrons (same chemical properties) but different numbers of neutrons (different masses).

Example 3: Balancing Chemical Equations

Question: Balance the following equation for the reaction between hydrogen and oxygen to form water: H₂ + O₂ → H₂O

Solution:

Step 1: Count atoms on each side

Left side (reactants):

H: 2 atoms
O: 2 atoms

Right side (products):

H: 2 atoms
O: 1 atom

Step 2: Identify which elements are unbalanced

Hydrogen is balanced (2 = 2)
Oxygen is NOT balanced (2 ≠ 1)

Step 3: Balance oxygen first

Put a coefficient of 2 in front of H₂O: H₂ + O₂ → 2H₂O

Step 4: Recount all atoms

Left side:

H: 2 atoms
O: 2 atoms

Right side:

H: 2 × 2 = 4 atoms
O: 2 × 1 = 2 atoms

Step 5: Balance hydrogen

Hydrogen is now unbalanced (2 ≠ 4)
Put a coefficient of 2 in front of H₂: 2H₂ + O₂ → 2H₂O

Step 6: Final check

Left side:

H: 2 × 2 = 4 atoms
O: 2 atoms

Right side:

H: 2 × 2 = 4 atoms
O: 2 × 1 = 2 atoms

Answer: The balanced equation is 2H₂ + O₂ → 2H₂O

Example 4: Writing and Balancing Equations from Word Equations

Question: Write a balanced symbol equation for the following reaction: magnesium + hydrochloric acid → magnesium chloride + hydrogen

Solution:

Step 1: Write the symbol equation with correct formulae

Magnesium = Mg
Hydrochloric acid = HCl
Magnesium chloride = MgCl₂
Hydrogen = H₂

Unbalanced equation: Mg + HCl → MgCl₂ + H₂

Step 2: Count atoms on each side

Left side:

Mg: 1 atom
H: 1 atom
Cl: 1 atom

Right side:

Mg: 1 atom
Cl: 2 atoms
H: 2 atoms

Step 3: Balance chlorine and hydrogen

Put a coefficient of 2 in front of HCl: Mg + 2HCl → MgCl₂ + H₂

Step 4: Final check

Left side:

Mg: 1 atom
H: 2 × 1 = 2 atoms
Cl: 2 × 1 = 2 atoms

Right side:

Mg: 1 atom
Cl: 2 atoms
H: 2 atoms

Answer: The balanced equation is Mg + 2HCl → MgCl₂ + H₂

Common Mistakes to Avoid

Atomic Structure Mistakes

Confusing mass number with atomic number: Remember, atomic number is the number of protons (bottom number), mass number is protons + neutrons (top number)
Forgetting that neutral atoms have equal protons and electrons: The number of electrons equals the number of protons in a neutral atom
Thinking electrons have significant mass: Electrons have negligible mass (1/1840 of a proton)
Placing neutrons outside the nucleus: Both protons AND neutrons are in the nucleus; only electrons orbit outside

Isotope Mistakes

Thinking isotopes have different numbers of protons: Isotopes ALWAYS have the same number of protons (same element) but different numbers of neutrons
Saying isotopes have different chemical properties: Isotopes have the SAME chemical properties because they have the same number of electrons
Confusing isotope notation: In $$\ce{^{A}_{Z}X}$$, A (top) is mass number, Z (bottom) is atomic number

Chemical Formula Mistakes

Miscounting atoms in brackets: In Ca(OH)₂, remember to multiply BOTH O and H by 2 to get 2 O atoms and 2 H atoms
Forgetting to multiply: In Mg(NO₃)₂, there are 2 N atoms and 6 O atoms (not 2 N and 3 O)
Writing capital letters incorrectly: “Co” is cobalt, “CO” is carbon monoxide — case matters!

Balancing Equations Mistakes

Changing subscripts in formulae when balancing: NEVER change H₂O to H₂O₂ — only add coefficients in front
Not checking all elements at the end: Always recount atoms of EVERY element after balancing
Having coefficients that aren’t in simplest form: 4H₂ + 2O₂ → 4H₂O should be simplified to 2H₂ + O₂ → 2H₂O
Forgetting to write “1” as nothing: Don’t write “1Mg”, just write “Mg”
Balancing one element but unbalancing another: Always recount everything after each change

General Mistakes

Missing units or labels: Always include proper notation like $$\ce{^{23}_{11}Na}$$ with both numbers
Incorrect use of state symbols: Remember: (s) solid, (l) liquid, (g) gas, (aq) aqueous — use brackets, not parentheses for atoms
Confusing the direction of the arrow: Reactants → Products (arrow shows direction of reaction)

Exam Tips

For Atomic Structure Questions

Always show your working when calculating neutrons: write “neutrons = mass number - atomic number”
Use the phrase “neutral atom” when explaining why electrons equal protons
Key phrases to earn marks:
- “The nucleus contains protons and neutrons”
- “Electrons orbit the nucleus in shells”
- “Protons are positively charged, neutrons have no charge, electrons are negatively charged”
- “The atom is electrically neutral because the number of positive charges equals the number of negative charges”

For Isotope Questions

Always mention both similarities and differences:
- Similarities: same number of protons, same number of electrons, same atomic number, same chemical properties
- Differences: different number of neutrons, different mass number, slightly different physical properties
Key phrase: “Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons”
When comparing isotopes, present your answer in a clear table or list

For Chemical Formula Questions

Count carefully when brackets are involved
Show your working: For Ca(OH)₂, write “Ca = 1, O = 2 × 1 = 2, H = 2 × 1 = 2”
Double-check your atom count before finalizing

For Equation Questions

Word equations: Always use full names, not symbols; use “+” between multiple substances; use “→” not “=”
Symbol equations:
- Write correct formulae first (don’t invent formulae!)
- Balance by adding coefficients ONLY
- Check your answer by counting atoms on both sides
- Show your final balanced equation clearly
State symbols: If asked for them, include them in brackets immediately after each formula
Common equations to memorize:
- Combustion of magnesium: 2Mg + O₂ → 2MgO
- Formation of water: 2H₂ + O₂ → 2H₂O
- Reaction of magnesium with acid: Mg + 2HCl → MgCl₂ + H₂

Mark-Earning Keywords

For structure: “nucleus”, “electron shells”, “protons and neutrons in nucleus”, “electrons orbit”
For charge: “electrically neutral”, “equal number of positive and negative charges”
For isotopes: “same element”, “same protons”, “different neutrons”, “same chemical properties”
For equations: “balanced”, “same number of atoms on both sides”, “reactants”, “products”

Presentation Tips

Draw clear diagrams with labels pointing to specific parts
Use proper notation for isotopes: $$\ce{^{A}_{Z}X}$$ format
Write chemical equations on one line with proper spacing: 2Mg + O₂ → 2MgO
Underline or box your final answers to make them clear to the examiner
Use a pencil for diagrams so you can correct mistakes
Number your working (Step 1, Step 2) to show clear logical progression

Quick Summary

Essential points to remember for revision:

✓ Atomic structure: Atoms consist of a nucleus (containing protons and neutrons) with electrons orbiting in shells
✓ Charges and masses: Protons (+1, mass 1), neutrons (0, mass 1), electrons (-1, negligible mass)
✓ Neutral atoms: Number of protons = number of electrons, giving zero overall charge
✓ Atomic number (Z): Number of protons in an atom; defines the element
✓ Mass number (A): Total number of protons and neutrons; neutrons = A - Z
✓ Isotopes: Same element (same protons, same atomic number) but different neutrons (different mass numbers); same chemical properties
✓ Chemical formulae: Subscripts show number of atoms; brackets multiply everything inside
✓ Counting atoms in formulae: Ca(OH)₂ = 1 Ca, 2 O, 2 H; Mg(NO₃)₂ = 1 Mg, 2 N, 6 O
✓ Word equations: Show names of reactants and products: reactants → products
✓ Symbol equations: Use chemical formulae and must be balanced
✓ Balancing equations: Add coefficients (numbers in front) to make atoms equal on both sides; NEVER change subscripts
✓ State symbols: (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous solution