Atomic Foundations of Matter

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Pure water has the same chemical composition, H₂O, regardless of where it comes from. Natural water may contain different dissolved substances, so it should be purified before comparing it.

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O represents one oxygen atom. O₂ represents one oxygen molecule made of two oxygen atoms bonded together.

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Salt separates into charged ions in water. These moving ions carry electric current. Sugar dissolves as neutral molecules, so its solution has no mobile charged particles to carry current.

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The mass stays the same. The solution’s mass equals the mass of the water plus the salt, because dissolving changes how the salt is spread out, not the total amount of matter.

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They fizz and produce carbon dioxide gas. In the open set-up, some gas escapes, so the balance may show less mass even though mass is conserved when all products are counted.

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The balloon traps the carbon dioxide. The flask, balloon, and all reaction products stay on the balance, so the total mass before and after should be the same within measurement error.

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A white barium sulfate solid forms. If both flasks and all material remain on the balance, the total mass stays the same; the atoms have only rearranged into new substances.

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Put zinc and dilute hydrochloric acid in a sealed flask connected to a balloon or gas syringe. Weigh the complete set-up before mixing, let the reaction finish without losing gas, then weigh it again. The masses should be equal within measurement error. Use teacher supervision and safety goggles.

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Reactants: 4.0 + 2.92 = 6.92 g. Products: 1.76 + 0.72 + 4.44 = 6.92 g. The totals are equal, so the measurements support conservation of mass.

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The ratio is 44/12 g carbon dioxide per gram of carbon. So 2.4 × 44/12 = 8.8 g carbon dioxide.

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No. Ethanol reacts with oxygen and forms gases, mainly carbon dioxide and water vapour, which escape into the air. We must count the gases as products; in a closed system the total mass remains constant.

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By conservation of mass, water mass = 20 g + 160 g = 180 g.

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46 g sodium is twice 23 g, so the chlorine needed is twice 35.5 g = 71 g.

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Sulfur-to-oxygen mass ratio is 40:60 = 2:3. Therefore oxygen mass = 20 × 60/40 = 30 g.

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9 g carbon is three times 3 g, so oxygen mass = 3 × 4 = 12 g.

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A compound has a fixed chemical formula, so its elements always occur in a fixed mass ratio. A mixture can contain different proportions of its ingredients.

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Yes. The ratios are equivalent: 8:2 simplifies to 4:1. Both samples have the same copper-to-oxygen mass ratio.

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Option (ii): both statements are true, but the reason does not by itself explain the exact masses. Conservation of mass gives 2 + 16 = 18 g; the simple whole-number atom ratio explains the water formula H₂O.

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Each nitrogen atom has five valence electrons and needs three more for an octet. Two nitrogen atoms share three pairs of electrons, forming a triple covalent bond: N≡N (N₂).

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A fluorine atom has electron arrangement 2,7 and needs one electron to complete its outer shell. Two fluorine atoms share one pair, forming a single covalent bond: F—F (F₂).

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CO₂: carbon shares two pairs with each oxygen, giving O=C=O. H₂S: sulfur shares one pair with each of two hydrogen atoms, giving H—S—H. NH₃: nitrogen shares one pair with each of three hydrogen atoms; it also keeps one lone pair.

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Neon has electron arrangement 2,8. Its outer shell is already full, so it is stable and does not usually gain, lose, or share electrons.

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Examples are lithium (2,1), sodium (2,8,1), magnesium (2,8,2), and aluminium (2,8,3). They can lose their outer electrons to expose a full shell underneath.

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No. It then has 11 protons but 10 electrons, giving one more positive charge than negative charge. It becomes Na⁺, a cation.

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Oxygen has six valence electrons and gains two to complete its outer shell. It forms the oxide ion O²⁻.

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Chlorine gains one electron to become Cl⁻. One Mg²⁺ ion combines with two Cl⁻ ions; their charges balance to form MgCl₂.

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Potassium loses one electron to form K⁺; chlorine gains one to form Cl⁻. They combine 1:1 as KCl. Calcium loses two electrons to form Ca²⁺; two chlorine atoms each gain one to form two Cl⁻ ions. They combine as CaCl₂.

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Each sodium atom loses one electron to form Na⁺. Sulfur gains two electrons to form S²⁻. Two Na⁺ ions balance one S²⁻ ion, so the formula is Na₂S.

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Write the positive ion’s name first, then the negative ion’s name. A simple negative ion usually ends in “-ide”: for example, MgCl₂ is magnesium chloride. Keep the usual name for polyatomic ions, such as sulfate.

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It could help scientists inspect atomic arrangements and materials more directly. Atoms are extremely small and constantly interact, so making clear, accurate images and interpreting them would still be challenging.

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CO₂: carbon dioxide. NO₂: nitrogen dioxide. SF₆: sulfur hexafluoride. PCl₃: phosphorus trichloride.

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Sodium hydrogencarbonate: NaHCO₃. Sulfur dioxide: SO₂. Ferric chloride (iron(III) chloride): FeCl₃. Cuprous oxide (copper(I) oxide): Cu₂O.

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Fe³⁺ needs three OH⁻ ions for a neutral compound: Fe(OH)₃. Two K⁺ ions balance one CO₃²⁻ ion: K₂CO₃.

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Typical results: sodium chloride and copper sulfate dissolve in water, not kerosene/petrol; they do not conduct as solids but their water solutions conduct. Sugar dissolves in water but not kerosene/petrol and does not conduct as a solid or solution. Camphor and naphthalene are generally insoluble in water, dissolve in non-polar solvents, and do not conduct. Actual observations depend on sample and conditions.

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An ionic bond is likely. In the solid, ions are locked in place; in water, the ions can move and carry electric current.

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M tends to form M²⁺; oxygen forms O²⁻, so the simplest formula is MO. The bond is ionic. If the compound dissolves, its mobile ions allow the solution to conduct electricity.

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H₂O has two H atoms and one O atom: (2 × 1) + 16 = 18 u.

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CO₂ has one C and two O atoms: 12 + (2 × 16) = 44 u.

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HNO₃ mass = 1 + 14 + (3 × 16) = 63 u.

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CH₄ mass = 12 + (4 × 1) = 16 u.

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Na₂O mass = (2 × 23) + 16 = 62 u.

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One nitrate group has mass 14 + (3 × 16) = 62 u. Two groups give 124 u; add Ca: 124 + 40 = 164 u.

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KCl mass = 39 + 35.5 = 74.5 u.

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Each OH group has mass 16 + 1 = 17 u. Mg(OH)₂ mass = 24 + (2 × 17) = 58 u.

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Sodium loses 1 electron; oxygen gains 2. The compound is Na₂O.

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Oxygen forms O₂ with a double bond (O=O); beryllium and oxygen form ionic BeO.

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Option (iii): 2 Fe³⁺ = +6 and 3 O²⁻ = −6.

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(iv) is correct; (i), (ii), and (iii) are incorrect.

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Aluminium nitrate: Al(NO₃)₃; calcium oxide: CaO; ferric oxide: Fe₂O₃.

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• (i) CaBr₂;
• (ii) Al₂(CO₃)₃;
• (iii) K₂SO₄;
• (iv) NH₄Cl.

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Option (iii): Cl⁻ has 17 protons and 18 electrons arranged 2, 8, 8.

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NH₄NO₃ = 80 u; H₃PO₄ = 98 u; NaHCO₃ = 84 u.

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• (i) Mg₃N₂;
• (ii) Li₃N;
• (iii) Na₂S;
• (iv) Al₂O₃.

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Li⁺: LiNO₃, Li₂SO₄, Li₃PO₄; Al³⁺: Al(NO₃)₃, Al₂(SO₄)₃, AlPO₄; Cu²⁺: Cu(NO₃)₂, CuSO₄, Cu₃(PO₄)₂; NH₄⁺: NH₄NO₃, (NH₄)₂SO₄, (NH₄)₃PO₄.

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Reactants = 11.3 g; products = 11.3 g. Mass is conserved.

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Atomic number 11; mass number 23; Na⁺ with electron arrangement 2, 8.

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B is more reactive; A and B form a covalent compound AB₃ (PCl₃ for P and Cl).

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Option (iii): the assertion is true, but the reason is false.

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²⁷Al: 13 electrons, 14 neutrons; ⁸⁰Br⁻: 36 electrons, 45 neutrons; ²⁰¹Hg²⁺: 78 electrons, 121 neutrons.

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Use equal measured amounts of purified water from different sources and a teacher-approved method to separate and measure the gases. Compare the hydrogen and oxygen masses; the mass ratio should be about 1:8. Record method, measurements, and experimental error.

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Choose three elements. For each, draw one bar for the neutral atom’s electron count and another for a common ion’s count. Label the charge and show whether electrons were lost or gained. Example: Na 11→10, Mg 12→10, Cl 17→18.

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Make separate cation and anion cards with charges. Players pair cards so total charge is zero, then write the simplest formula and name. Give points only when both the charge balance and formula are correct.