Atoms, periodic patterns, bonding and states
Read a formula and particle model, use a supplied periodic pattern, and connect states of matter with particle motion.
What you will tackle
- Use atomic particles and atomic number
- Interpret groups and periods
- Distinguish ionic and covalent models
- Read names, formula ratios and balanced equations
- Explain state changes with particle models
Atomic structure and the periodic table
Protons have positive charge, electrons negative charge and neutrons no net charge. Atomic number is proton count; a neutral atom has equal proton and electron counts. Isotopes share proton count but differ in neutron count. The periodic table orders elements by atomic number; a group is a column and a period a row. Similar outer-electron patterns can help explain related chemistry within a group, using the pattern supplied in a passage.
Bonding and chemical language
In a simple ionic model, electron transfer produces oppositely charged ions attracted to one another; a covalent bond involves shared electron pairs. Formula subscripts give atom ratios, while a coefficient multiplies the whole formula. Sodium chloride, NaCl, has a 1:1 ion ratio; carbon dioxide, CO₂, has one carbon per two oxygens. Balanced equations conserve each type of atom, not the number of molecules.
States and changes
Particles in solids remain near fixed positions while vibrating; liquids have close particles able to rearrange; gases have widely separated particles moving freely. Melting and boiling change physical state without necessarily changing chemical identity. During a phase change of a pure substance at fixed pressure, energy transfer can occur while temperature remains constant. Chemical changes rearrange bonding; physical mixtures and solutions can vary in composition.
Build your notes, then practise
Work on paper before opening an explanation. Guided tasks provide a starting point; independent tasks remove the scaffolding; application tasks ask you to justify a method or interpretation in context.
Guided practice
Guided 1
An atom has 11 protons, 12 neutrons and 11 electrons. Find atomic number and net charge; what changes when it loses one electron? Read 2CO₂ as atom counts and balance H₂ + O₂ → H₂O.
Show a starting hint
Do not change formula subscripts to balance an equation.
Compare with the explained answer
- Atomic number is 11 and the atom is neutral; losing one electron gives net charge +1 without changing atomic number.
- 2CO₂ contains two carbon atoms and four oxygen atoms.
- 2H₂ + O₂ → 2H₂O conserves four hydrogens and two oxygens.
Answer / response criteria: 11, neutral, then +1; C=2 and O=4; balanced equation as stated.
Independent practice
Independent 1
A supplied table lists elements X and Y in the same group, and Z in the next group. Which pair most directly invites a similar-outer-pattern hypothesis? Explain ionic versus covalent bonding and why boiling pure water does not by itself create a new compound.
Compare with the explained answer
- X and Y share the supplied group position; this supports a testable related-pattern hypothesis, not identity of every property.
- The simple ionic account involves charged ions and attraction; covalent bonding involves sharing electron pairs.
- Boiling separates water molecules into the gas phase while retaining H₂O identity under this physical-change model.
Answer / response criteria: X and Y; bonding distinction as stated; state change preserves chemical identity.
Application practice
Application 1
An unknown material melts at a reproducible temperature and conducts electricity when molten but poorly as a solid. Is this consistent with an ionic model, and does it uniquely prove that model? A sample has the same proton count but one extra neutron: is it a new element? Explain a temperature plateau while heating.
Compare with the explained answer
- Mobile ions in the molten state can account for conductivity, while fixed ions in a solid cannot carry charge freely; this is supporting evidence, not a uniquely complete identification.
- An extra neutron produces another isotope of the same element, not a new atomic number.
- During melting or boiling, energy can change particle arrangement/separation rather than increase average kinetic energy, yielding a temperature plateau under the stated conditions.
Answer / response criteria: Consistent but not unique evidence; an isotope; phase-change energy can produce a plateau.
What can you explain now?
An unknown material melts at a reproducible temperature and conducts electricity when molten but poorly as a solid. Is this consistent with an ionic model, and does it uniquely prove that model? A sample has the same proton count but one extra neutron: is it a new element? Explain a temperature plateau while heating.
Compare with the explanation
Consistent but not unique evidence; an isotope; phase-change energy can produce a plateau.
Explain the reasoning before comparing. This review records your self-assessment rather than an official score or mastery decision.
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Source and review status
Coverage follows the named requirements and examples in the 2026–2027 ACT test guide and subject descriptions. That guide explicitly does not list every possible topic. The broader coded College and Career Readiness Standards are also mapped to conceptual teaching and practice; this does not calibrate a learner to a score band. The separate scientific-background context list and reading-complexity rubric have their own stated limits.
Original teaching and explained self-check practice are available. Specialist review and difficulty calibration remain pending. Essays require self-check or teacher review and are not automatically marked or awarded official scores.
Official source specification / guide