Atoms, elements and compounds: read the chemical symbols
הוצג| English | משמעות |
|---|---|
| compound/ˈkɒmpaʊnd/ | תרכובת |
| element/ˈelɪmənt/ | יסוד |
What would explain this observation?
- O is the symbol for oxygen, while O₂ represents an oxygen molecule. Two atoms joined together do not necessarily make a compound 化合物: their element 元素 identities matter.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- An atom is the smallest part of an element that can exist. An element contains one type of atom, defined by proton number. A compound contains two or more elements chemically combined in fixed proportions. Water, H₂O, contains hydrogen and oxygen in a 2:1 atom ratio; oxygen, O₂, is still an element. A chemical reaction forms new substances and often produces a detectable energy change. Separating a compound into its elements requires chemical change.
- element: A substance containing only one type of atom defined by proton number; compound: A substance containing two or more elements chemically combined in fixed proportions.
Which substance is a compound?
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A compound contains two or more DIFFERENT elements chemically joined: water has hydrogen and oxygen. O2 and helium each contain one element, so they are not compounds, whatever their particle count.
Match each technical term to its precise meaning.
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Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Symbols use an uppercase first letter and, where present, a lowercase second letter: Co is cobalt, whereas CO is carbon monoxide. For the first twenty elements learn H hydrogen, He helium, Li lithium, Be beryllium, B boron, C carbon, N nitrogen, O oxygen, F fluorine, Ne neon, Na sodium, Mg magnesium, Al aluminium, Si silicon, P phosphorus, S sulfur, Cl chlorine, Ar argon, K potassium and Ca calcium. Group 1 also includes Rb rubidium, Cs caesium and Fr francium; Group 7 includes F fluorine, Cl chlorine, Br bromine, I iodine and At astatine. Use the supplied periodic table for other specified elements, including Fe iron, Cu copper and Zn zinc. Very radioactive francium and astatine are names to interpret from the table, not school reaction specimens.
- Read a formula by identifying each element symbol before applying its subscript. A subscript applies to the symbol immediately before it, or to an entire bracketed group. Name MgO magnesium oxide, NaCl sodium chloride and Ca(OH)₂ calcium hydroxide. Compare diagrams of separate atoms, molecules of one element and compound particles; keep particle identity distinct from the number of particles.
Which two habits make the investigation or model in this case more defensible?
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Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: one formula unit of Ca(OH)₂ contains one Ca, two O and two H, five atoms in all. Three formula units contain 3 calcium, 6 oxygen and 6 hydrogen atoms, fifteen total. The coefficient multiplies the entire formula; it does not change the fixed Ca:O:H ratio of 1:2:2.
How many oxygen atoms are represented by four formula units of Ca(OH)₂? Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
How many oxygen atoms are represented by four formula units of Ca(OH)₂?
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The result is 8 atoms. Known: each Ca(OH)2 unit contains 2 oxygen atoms (the subscript 2 multiplies both atoms inside the brackets). Four formula units contain 4 x 2 = 8 oxygen atoms. The coefficient multiplies the whole formula.
Check the conclusion and its limits
- A molecule can contain only one element, and some compounds form lattices rather than separate molecules. Heating a mixture to separate it is not automatically a chemical reaction. New substances, rather than an energy change alone, establish chemical change.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Every particle containing two atoms is a compound. This claim is false: O2 has two atoms but one element, so it is a molecule of an element, not a compound. A compound requires at least two different elements chemically combined.
Atoms, elements and compounds: read the chemical symbols: Symbols use an uppercase first letter and, where present, a lowercase second letter: Co is cobalt, whereas CO is carbon monoxide. For the first twenty elements learn H hydrogen, He helium, Li lithium, Be beryllium, B boron, C carbon, N nitrogen, O oxygen, F fluorine, Ne neon, Na sodium, Mg magnesium, Al aluminium, Si silicon, P phosphorus, S sulfur, Cl chlorine, Ar argon, K potassium and Ca calcium. Group 1 also includes Rb rubidium, Cs caesium and Fr francium; Group 7 includes F fluorine, Cl chlorine, Br bromine, I iodine and At astatine. Use the supplied periodic table for other specified elements, including Fe iron, Cu copper and Zn zinc. Very radioactive francium and astatine are names to interpret from the table, not school reaction specimens.
Every particle containing two atoms is a compound.
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O2 has two atoms but one element, so it is a molecule of an element, not a compound. A compound requires at least two different elements chemically combined.
A substance containing only one type of atom defined by proton number: write the technical term.
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element means A substance containing only one type of atom defined by proton number.