CLASS 9 SCIENCE · CHAPTER 9 · STUDY NOTES
Atomic Foundations of Matter
Understand atomic foundations of matter through short explanations, labelled models and original practice. Read one topic, explain it in your own words, then check.
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Count everything before and after
A gas that leaves the balance still has mass.Captured gas remains inside the total
- Law of conservation of mass
- In ordinary chemical reactions, the total mass of a closed material system is conserved to the accuracy of school measurements.
- Closed material system
- A chosen system across whose boundary matter does not enter or leave.
Dissolving salt redistributes particles; it does not remove their mass. In a reaction producing gas, an open container can appear lighter because some product escapes. Include the gas to compare the full system.
An original closed-system reaction starts with 15 g total reactants and leaves 4 g captured gas plus 11 g other products: 15=4+11 g. Tare consistently and account for material stuck to containers. Real measurements have uncertainty; not every instrument has exactly the same uncertainty.
Take a moment. What do you think?
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Question
An open reaction cup loses mass on the balance. Does that alone disprove conservation?
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A pure compound has a fixed composition
Do not confuse an atom-count ratio with a mass ratio.Same molecule; different ways to count
- Law of definite proportions
- A particular pure compound has its constituent elements in a fixed mass ratio in the introductory chemical model.
Water has H:O atom count 2:1, but rounded mass ratio 1:8 because each oxygen atom is about sixteen times as massive as hydrogen. An original 27 g pure-water sample contains 3 g hydrogen and 24 g oxygen in that rounded model.
A salt-and-water mixture can be concentrated or dilute. Its ratio is adjustable; the compound water itself is not. “Purified” matters: seawater contains other substances.
Nine mass parts in water
Pure water; rounded atomic masses H=1, O=16.
H mass=27×1/9=3 g; O mass=27×8/9=24 gThe parts must add to the 27 g sample.
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Question
Why is the mass ratio in water not 2:1?
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Chemical change rearranges atoms
Keep the useful chemical idea and recognise the old limits.Preserve each kind; change the grouping
- Dalton’s chemical picture
- Atoms of different elements combine in characteristic whole-number counts and are rearranged during ordinary reactions.
Dalton’s theory linked conservation and fixed composition: the atoms are regrouped, rather than created or destroyed, in an ordinary chemical reaction. The simple whole-number ratio concerns counts of atoms, not always their masses.
Subatomic particles showed that atoms are divisible. Isotopes showed that atoms of one element need not have identical mass. Nuclear changes go beyond ordinary chemical rearrangement.
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Question
Which Dalton claim needs revision because of isotopes?
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A formula tells which atoms are joined
An element’s molecule and a compound’s molecule are different.Same-element pair versus a compound
- Molecule
- An electrically neutral entity consisting of more than one atom.
- Chemical bond
- An interaction that holds atoms or ions together in a chemical structure.
O is an oxygen-atom symbol; O₂ represents a molecule with two oxygen atoms. H₂ and O₂ are molecules of elements; H₂O has different elements and is a compound. Helium normally exists as separate atoms.
A molecule can carry a substance’s chemical identity, but one molecule does not have a bulk melting point or state by itself. Bond formation can lower a system’s energy; breaking a bond needs energy. The overall reaction depends on all bonds broken and formed.
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Question
Is O₂ a compound because it has two atoms?
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Sharing can give each atom a fuller outer shell
Each single covalent bond represents one shared electron pair.Shared pairs and lone pairs in water
- Covalent bond
- A bond involving a shared electron pair between atoms.
- Single / double / triple bond
- One / two / three shared pairs in the simple bonding picture.
| Molecule | Bond picture | Shared pairs between the atoms |
|---|---|---|
| H₂ | H–H | 1 |
| Cl₂ | Cl–Cl | 1 |
| O₂ | O=O | 2 |
| N₂ | N≡N | 3 |
In H₂, the two atoms share one pair, giving each access to two outer electrons. Cl₂ also has one shared pair, with three lone pairs around each Cl. O₂ has two shared pairs; N₂ has three.
Hydrogen needs a duplet, not an octet. In HCl one shared pair joins unlike atoms. In H₂O oxygen shares a pair with each H and retains two lone pairs. Sharing does not mean electrons are fixed halfway between nuclei.
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Question
How many shared electrons are represented by a double bond?
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Electron transfer produces charged ions
The nucleus stays unchanged while electron count changes.One electron changes two charge counts
- Cation
- A positively charged ion, formed in a simple case by losing electrons.
- Anion
- A negatively charged ion, formed in a simple case by gaining electrons.
- Ionic bond
- Electrostatic attraction between oppositely charged ions.
Na has 11 protons and 11 electrons; losing one leaves Na⁺ with 10 electrons. Cl has 17 protons and 17 electrons; gaining one makes Cl⁻ with 18 electrons. Their opposite charges attract.
Ionic bonding is the attraction after ion formation, rather than merely the act of passing an electron. Electron gain does not increase the atom’s Z or change it into another element.
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Question
An oxygen atom gains two electrons. What charge does it have?
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