LET Elementary Chemistry — Matter, Atomic Structure and the Periodic TableRevision Notes

Exam Tips

• If a LET item asks whether something is a compound or a mixture, ask: Can it be separated by physical means? If YES → mixture. Does it have a fixed chemical formula? If YES → compound.
• Memorize the four categories in order from simplest to most complex: Element → Compound → Homogeneous Mixture → Heterogeneous Mixture.
• When the LET asks for intensive vs. extensive properties, remember: DENSITY is always intensive; MASS and VOLUME are always extensive.
• Connect content to Grade 5–6 Science learning competencies (e.g., DepEd's K-12 Science Curriculum Guide: 'describe the properties of matter'). This helps you link LET content to actual classroom teaching contexts.

Key Points

• Matter is anything that has MASS and occupies SPACE (volume). Air, water, and even your own body are matter.
• Physical properties can be observed WITHOUT changing the identity of the substance. Examples: color, density, melting point, boiling point, hardness, solubility.
• Chemical properties describe HOW a substance reacts to form new substances. Examples: flammability, ability to rust, reactivity with acid.
• Extensive properties DEPEND on the amount of matter present (mass, volume, length). Intensive properties are INDEPENDENT of amount (density, temperature, boiling point). This is a frequent LET test point.
• PURE SUBSTANCES have a fixed composition: Elements (one kind of atom) and Compounds (two or more elements chemically combined in a fixed ratio).
• MIXTURES have variable composition: Homogeneous mixtures (solutions – uniform throughout) and Heterogeneous mixtures (non-uniform, parts visibly distinct).
• A compound can only be separated by CHEMICAL means; a mixture can be separated by PHYSICAL means.
• Everyday Philippine examples: salt water (homogeneous mixture), pinakbet (heterogeneous mixture), table salt/NaCl (compound), gold (element).

Definitions

Section Title

1. Properties and Classification of Matter

Common Mistakes

• Confusing compound with mixture – remember a compound has a FIXED ratio and requires a CHEMICAL process to separate; a mixture has a variable ratio and is separated by PHYSICAL means.
• Saying that air is a compound – air is actually a HOMOGENEOUS MIXTURE (mainly nitrogen and oxygen that are NOT chemically bonded).
• Treating density as an extensive property – density is INTENSIVE because it is mass per unit volume and does not change with the amount of substance.
• Classifying salt water as a pure substance – dissolving salt in water creates a MIXTURE, not a compound, because the ratio of salt to water can vary.

Formulas

Exam Tips

• Memorize the density formula triangle: cover the quantity you want to find – D = m/V, m = D×V, V = m/D.
• For LET items with floating/sinking scenarios, quickly compare the object's density to 1.0 g/cm³ (the density of water at room temperature).
• Philippine context: bancas (outrigger boats) float because their overall density (including hollow parts and air) is less than water – a practical teaching analogy.

Key Points

• MASS is the total amount of matter in an object, measured in grams (g) or kilograms (kg). Mass does NOT change with location.
• WEIGHT is the force of gravity pulling on an object's mass. Weight DOES change with location – you would weigh less on the Moon but your mass stays the same.
• VOLUME is the amount of space matter occupies, measured in liters (L), milliliters (mL), or cubic centimeters (cm³).
• DENSITY is the ratio of mass to volume. It tells us how tightly packed matter is in a given space.
• Floating and sinking: an object FLOATS if its density is LESS than the liquid's density; it SINKS if its density is GREATER.
• Oil floats on water because oil (density ≈ 0.8 g/cm³) is less dense than water (density = 1.0 g/cm³) – a classic everyday Philippine demonstration.

Definitions

Section Title

2. Measuring Matter – Mass, Weight, Volume, and Density

Common Mistakes

• Saying mass changes when you go to a different planet – only WEIGHT changes, not mass.
• Using the density formula incorrectly: always divide mass by volume (m ÷ V), never volume by mass.
• Forgetting that floating is determined by DENSITY COMPARISON, not just the size or weight of an object.

Exam Tips

• Key test question: 'Is a new substance formed?' If YES → chemical change. If NO → physical change.
• Remember the common LET traps: dissolving = physical; burning = chemical; melting = physical; cooking = chemical.
• When teaching Grade 4–5 pupils, use the analogy of cooking rice (bigas → kanin) as an irreversible chemical change – the bigas cannot be recovered.

Key Points

• A PHYSICAL CHANGE alters the form, shape, or state of matter but does NOT produce a new substance. The same substance remains.
• A CHEMICAL CHANGE (chemical reaction) produces one or more NEW substances with new properties.
• Signs of a chemical change: color change, gas production (bubbles), formation of a precipitate (solid), release or absorption of heat or light, production of odor.
• Physical changes are usually REVERSIBLE (ice melts → water freezes back to ice). Chemical changes are usually IRREVERSIBLE (cooked rice cannot be uncooked).
• MELTING and BOILING are PHYSICAL changes – the substance just changes state but its chemical identity remains the same.
• BURNING (pagsusunog), RUSTING (kalawang), COOKING, and DIGESTION are CHEMICAL changes – new substances are produced.
• Philippine classroom examples: Burning bamboo → chemical change (ash formed, new substance); shaping clay → physical change (same clay, different shape).

Definitions

Section Title

3. Physical and Chemical Changes

Common Mistakes

• Classifying DISSOLVING sugar in water as a chemical change – it is PHYSICAL because the sugar can be recovered by evaporation and no new substance is formed.
• Classifying RUSTING as a physical change – rusting produces iron oxide (a new substance), making it a CHEMICAL change.
• Thinking ALL irreversible changes are chemical – tearing paper is irreversible but still a PHYSICAL change.

Exam Tips

• LET items often present a scenario and ask which separation technique is best. Match the technique to the DIFFERENCE between the components: size difference → sieving; magnetic vs. non-magnetic → magnetic separation; soluble solid in liquid → evaporation; different boiling points → distillation.
• Philippine context anchors: salt from seawater = evaporation; cleaning tubig (water purification) = filtration or distillation; separating rice from stones = winnowing/sieving.

Key Points

• Because mixture components keep their individual properties, they can be separated using PHYSICAL methods that exploit differences between the parts.
• FILTRATION: separates an insoluble solid from a liquid (e.g., removing sand from water using filter paper).
• EVAPORATION: removes liquid by heating; recovers dissolved solids (e.g., obtaining rock salt/asin from seawater – a common Philippine livelihood in Pangasinan and Ilocos).
• DISTILLATION: separates liquids with different boiling points; used to purify water (tubig puro).
• MAGNETIC SEPARATION: uses a magnet to pull out magnetic materials (e.g., separating iron filings from sand).
• DECANTATION: carefully pouring off a liquid from a settled solid (e.g., pouring off clear water from settled mud).
• SIEVING/WINNOWING (paghahawi): separates solids of different sizes (e.g., removing stones from bigas/rice before cooking).
• IMPORTANT: Compounds CANNOT be separated by physical methods. A chemical reaction is required to break compounds into their elements.

Definitions

Section Title

4. Separation of Mixtures

Common Mistakes

• Thinking that distillation and evaporation are the same – distillation COLLECTS the purified vapor by condensation; evaporation just removes the liquid and discards it.
• Trying to separate a COMPOUND using physical means – this is impossible; only mixtures can be separated physically.
• Confusing filtration with decantation – filtration uses a filter medium; decantation simply pours off the liquid.

Exam Tips

• Memorize the six phase changes in PAIRS: Melting↔Freezing; Vaporization↔Condensation; Sublimation↔Deposition.
• Memory trick: changes that produce MORE freedom of movement (solid→liquid→gas) ABSORB heat (endothermic); the reverse RELEASES heat (exothermic).
• LET items may describe a scenario (e.g., 'mothballs gradually disappear without melting') and ask which phase change is occurring. Answer: SUBLIMATION.

Key Points

• The three common STATES (phases) of matter are SOLID, LIQUID, and GAS. PLASMA is sometimes listed as the fourth state.
• SOLID: fixed shape, fixed volume; particles are closely packed and only vibrate in place.
• LIQUID: takes the shape of its container but has fixed volume; particles are close but can slide past each other.
• GAS: takes the shape and volume of its container; particles are far apart and move freely and rapidly.
• Phase changes occur when HEAT is added (endothermic – absorbs heat) or removed (exothermic – releases heat).
• MELTING (solid → liquid): heat absorbed; FREEZING (liquid → solid): heat released.
• VAPORIZATION/EVAPORATION or BOILING (liquid → gas): heat absorbed; CONDENSATION (gas → liquid): heat released.
• SUBLIMATION (solid → gas directly): heat absorbed. Example: dry ice (frozen CO₂), mothballs (naphthalene/parang-parang), solid air fresheners.
• DEPOSITION (gas → solid directly): heat released. Example: frost forming on grass or on the outside of a cold bottle.
• CRITICAL TEST POINT: During a phase change, TEMPERATURE REMAINS CONSTANT even as heat is added or removed because the energy is used to break or form intermolecular forces, not to raise temperature.

Definitions

Section Title

5. Phases of Matter and Phase Changes

Common Mistakes

• Thinking temperature rises continuously during boiling – temperature STOPS rising at the boiling point until all the liquid has vaporized.
• Confusing evaporation and boiling – evaporation occurs at the surface at any temperature; boiling occurs throughout the liquid at the boiling point.
• Saying sublimation goes from solid to GAS passing through liquid – it goes DIRECTLY from solid to gas, skipping the liquid phase.
• Mixing up endothermic and exothermic phase changes – phase changes that SPREAD particles apart (melt, vaporize, sublime) ABSORB heat; those that BRING particles together (freeze, condense, deposit) RELEASE heat.

Formulas

Exam Tips

• Quick calculation sequence for any LET atom problem: (1) Identify Z from the periodic table. (2) Find A from the isotope name (e.g., carbon-14 → A=14). (3) Neutrons = A – Z. (4) In a neutral atom, electrons = Z.
• Memorize: Protons and Neutrons are in the NUCLEUS (heavy, compact center). Electrons are OUTSIDE (in shells/cloud).
• The order of atomic model development is a frequent LET historical question: Dalton → Thomson → Rutherford → Bohr → Quantum/Modern Model.

Key Points

• An ATOM is the smallest unit of an element that retains the chemical properties of that element.
• Three SUBATOMIC PARTICLES: Proton (positive charge, in nucleus), Neutron (no charge, in nucleus), Electron (negative charge, in electron cloud around nucleus).
• The NUCLEUS (protons + neutrons) contains almost all the atom's mass and carries an overall positive charge.
• In a NEUTRAL atom, the number of PROTONS equals the number of ELECTRONS.
• ATOMIC NUMBER (Z) = number of PROTONS. This identifies the element. Every atom of carbon has exactly 6 protons (Z=6), no matter what.
• MASS NUMBER (A) = number of PROTONS + number of NEUTRONS.
• Number of NEUTRONS = Mass Number (A) – Atomic Number (Z). For example, Carbon-12: Z=6, A=12, Neutrons = 12–6 = 6.
• ISOTOPES are atoms of the SAME element (same number of protons, same atomic number) with DIFFERENT numbers of neutrons (different mass numbers). Carbon-12 and Carbon-14 are isotopes: both have 6 protons, but 6 and 8 neutrons respectively.
• Isotopes have the SAME chemical behavior because they have the same electron arrangement (and the same number of valence electrons).
• VALENCE ELECTRONS are the electrons in the OUTERMOST shell; they determine how atoms bond and react.
• When an atom LOSES electrons → becomes a positive ION (CATION). When it GAINS electrons → becomes a negative ION (ANION).

Definitions

Section Title

6. Atomic Structure

Common Mistakes

• Confusing atomic number with mass number – atomic number = PROTONS only; mass number = PROTONS + NEUTRONS.
• Thinking isotopes have different chemical properties – WRONG. Isotopes of the same element have IDENTICAL chemical properties because they have the same number of electrons.
• Forgetting that in a neutral atom, protons = electrons. Only IONS have unequal protons and electrons.
• Saying an atom that gains electrons becomes a cation – gaining electrons makes a NEGATIVE ion (ANION); losing electrons makes a positive ion (CATION).

Exam Tips

• Use the mnemonic 'Dalton The Rascal Builds Quantum Machines' → Dalton, Thomson, Rutherford, Bohr, Quantum/Modern – to remember the order of atomic models.
• LET historical science questions often ask which experiment disproved which model. Rutherford's gold foil experiment disproved Thomson's plum pudding model.
• Connecting to classroom teaching: When teaching Grade 6 Science (The Structure of Matter), the Bohr model (shells and energy levels) is the most commonly used model in elementary textbooks because it is visual and easy to understand.

Key Points

• JOHN DALTON (early 1800s): Atoms are tiny, indivisible, solid particles – like billiard balls. All atoms of the same element are identical.
• J.J. THOMSON (1897): Discovered the ELECTRON through cathode ray tube experiments. Proposed the 'plum pudding' model – electrons embedded in a positively charged sphere (like raisins in bread/tinapay).
• ERNEST RUTHERFORD (1911): Gold foil experiment showed that most of an atom is EMPTY SPACE and that there is a small, dense, positively charged NUCLEUS at the center. Proposed the nuclear model.
• NIELS BOHR (1913): Electrons travel in fixed, circular ORBITS or ENERGY LEVELS around the nucleus (like planets around the sun). Electrons in specific orbits have specific energies.
• MODERN QUANTUM MODEL (post-1920s): Electrons do not travel in fixed orbits but occupy REGIONS OF PROBABILITY called ORBITALS (electron cloud). This is the currently accepted model.
• Each model improved upon the previous based on new experimental evidence – a good example of the scientific method in action.

Definitions

Section Title

7. Historical Models of the Atom

Common Mistakes

• Mixing up Thomson and Rutherford – Thomson discovered the electron and proposed plum pudding; Rutherford discovered the nucleus.
• Saying Bohr's model is the currently accepted model – the QUANTUM/MODERN model (electron cloud) is the current accepted model.
• Confusing Dalton's model (indivisible atom) with later models – Dalton did NOT know about subatomic particles.

Exam Tips

• Memory trick for Group names: 'All Alkali Metals Are Happy Noble' → Group 1 (Alkali), 2 (Alkaline Earth), 17 (Halogens), 18 (Noble Gases).
• LET may ask: Which group contains the most reactive metals? → Group 1 (Alkali Metals). Which group contains the most reactive nonmetals? → Group 17 (Halogens).
• Practice reading periodic table notation: ²³Na (sodium) means atomic number = 11 (from the table), mass number = 23, neutrons = 23–11 = 12.
• Connect to Grade 6 Science DepEd curriculum: 'Describe the arrangement of elements in the periodic table' is a K-12 learning competency.

Key Points

• DMITRI MENDELEEV (1869) created the first widely recognized periodic table, organized by increasing ATOMIC MASS and by repeating (periodic) properties. He even predicted the existence and properties of undiscovered elements.
• The MODERN periodic table is arranged by increasing ATOMIC NUMBER (not atomic mass).
• PERIODS are the HORIZONTAL ROWS. There are 7 periods. Across a period, atomic number increases by one for each element.
• GROUPS (also called FAMILIES) are the VERTICAL COLUMNS. There are 18 groups. Elements in the same group have SIMILAR CHEMICAL PROPERTIES because they have the same number of VALENCE ELECTRONS.
• KEY GROUPS to memorize for the LET: Group 1 = Alkali Metals (very reactive metals); Group 2 = Alkaline Earth Metals; Group 17 = Halogens (very reactive nonmetals); Group 18 = Noble Gases (stable, unreactive, full outer shell).
• METALS are found on the LEFT and CENTER of the periodic table. They are shiny, malleable, ductile, and good conductors of heat and electricity.
• NONMETALS are found in the UPPER RIGHT of the periodic table. They are dull, brittle (in solid form), and poor conductors.
• METALLOIDS (also called semi-metals) are found along the STAIRCASE LINE between metals and nonmetals. They have properties of BOTH metals and nonmetals. Example: Silicon (Si), Germanium (Ge).
• NOBLE GASES (Group 18) are the LEAST REACTIVE elements because they have a FULL outermost electron shell (stable electron configuration).
• HALOGENS (Group 17) are the MOST REACTIVE nonmetals because they have 7 valence electrons and need only one more to complete their outer shell.

Definitions

Section Title

8. The Periodic Table – Structure and Organization

Common Mistakes

• Saying Mendeleev arranged the table by atomic NUMBER – he arranged it by atomic MASS; the modern table uses atomic NUMBER.
• Confusing periods (horizontal) with groups (vertical) – remember: GROUP sounds like a vertical club/family; PERIOD is horizontal like a row of seats.
• Saying hydrogen (H) is an alkali metal because it is in Group 1 – hydrogen is a NONMETAL despite being placed in Group 1.
• Forgetting that noble gases are in Group 18 and are stable – they are NOT in Group 8 (which is in the transition metals area).

Exam Tips

• Use a 2×2 table in your mind: trend across period vs. down group; then plug in the four trends (size, ionization energy, electronegativity, metallic character).
• The LET loves questions like: 'Which element has a larger atomic radius, Na or Cl?' Na is to the LEFT of Cl in Period 3 → Na has LARGER atomic radius.
• Another common item: 'Which element is most electronegative?' Answer: FLUORINE (F) – it is in the upper right corner of the periodic table.
• Remember: Francium (lower left) = largest atomic radius, lowest ionization energy, lowest electronegativity, most metallic. Fluorine (upper right) = smallest atomic radius, highest electronegativity, least metallic.

Key Points

• PERIODIC TRENDS are predictable patterns in element properties that occur as you move across a period or down a group. These are a MAJOR LET topic.
• ATOMIC SIZE (ATOMIC RADIUS): DECREASES across a period (left→right) because more protons pull electrons in more tightly. INCREASES down a group (top→bottom) because more electron shells are added.
• IONIZATION ENERGY (energy needed to remove an electron): INCREASES across a period (harder to remove electrons as nucleus pulls more strongly). DECREASES down a group (easier to remove electrons farther from nucleus).
• ELECTRONEGATIVITY (ability of an atom to attract electrons in a bond): INCREASES across a period (left→right). DECREASES down a group (top→bottom). FLUORINE (F) is the MOST ELECTRONEGATIVE element.
• METALLIC CHARACTER (tendency to lose electrons and behave as a metal): DECREASES across a period (left→right). INCREASES down a group (top→bottom). MOST metallic element: Francium (Fr), lower left. LEAST metallic (most nonmetallic): Fluorine (F), upper right.
• SUMMARY RULE: Properties that make an atom HOLD ONTO electrons (ionization energy, electronegativity) INCREASE toward the upper right (nonmetal corner). Properties that make an atom RELEASE electrons (metallic character, atomic size) INCREASE toward the lower left.
• Mnemonic for across a period (left→right): Atomic size DECREASES, Ionization energy INCREASES, Electronegativity INCREASES, Metallic character DECREASES – DIII-M (DOWN–INCREASE–INCREASE–INCREASE then Metallic Down).

Definitions

Section Title

9. Periodic Trends

Common Mistakes

• Saying atomic size increases across a period – it DECREASES because more protons are pulling the electrons in more tightly.
• Confusing ionization energy and electronegativity direction – both INCREASE left to right across a period and DECREASE top to bottom in a group.
• Saying cesium or francium is the most electronegative – FLUORINE (F) is the most electronegative; cesium and francium have the LOWEST electronegativity.
• Forgetting the reasoning behind the trends – always explain WHY: more protons = stronger pull (across period); more shells = electrons farther away (down group).