Counting Atoms

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Coefficient Noun

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[koh-uh-fish-uhnt]

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Coefficient

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A number placed before a chemical formula that indicates the total number of molecules or formula units of that compound.

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Example: The large red number '2' in front of a molecule like H₂O is the coefficient, indicating there are two of that entire molecule.

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Subscript Noun

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[suhb-skript]

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Subscript

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A number written to the bottom right of a chemical symbol, indicating the number of atoms of that specific element.

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Example: The small number '2' in H₂O is a subscript, indicating there are two hydrogen atoms in one molecule of water.

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Chemical Symbol Noun

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[kem-i-kuhl sim-buhl]

Back

Chemical Symbol

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A one- or two-letter abbreviation used to represent a specific chemical element on the periodic table.

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Example: The image shows that 'H' is the chemical symbol for a Hydrogen atom and 'O' is the symbol for an Oxygen atom.

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Atom Noun

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[at-uhm]

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Atom

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The smallest fundamental unit of an element that maintains the chemical identity and properties of that element.

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Example: An atom consists of a central nucleus (with protons and neutrons) and electrons orbiting in shells. This oxygen atom has 8 protons, 8 neutrons, and 8 electrons.

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Element Noun

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[el-uh-muhnt]

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Element

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A pure substance consisting of only one type of atom, distinguished by its unique number of protons.

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Example: This diagram shows a single atom, the building block of an element. An element is defined by the number of protons (green) in its nucleus.

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Compound Noun

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[kom-pound]

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Compound

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A substance formed when two or more different chemical elements are chemically bonded together in a fixed ratio.

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Example: This image shows a model of carbon dioxide (CO₂), a compound made of one carbon and two oxygen atoms chemically bonded together.

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Chemical Formula Noun

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[kem-i-kuhl fawr-myuh-luh]

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Chemical Formula

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A representation showing the types and numbers of atoms in the smallest representative unit of a substance.

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Example: A chemical formula like C₆H₁₂O₆ uses element symbols (C, H, O) and subscript numbers to show the exact number of atoms of each element in one molecule.

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Parentheses Noun

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[puh-ren-thuh-seez]

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Parentheses

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Used in a chemical formula to enclose a polyatomic ion, indicating the enclosed group is treated as a single unit.

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Example: This table shows how to count atoms in chemical formulas. For formulas with parentheses, like Fe(NO₃)₃, the subscript outside multiplies every atom inside.

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Molecule Noun

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[mol-i-kyool]

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Molecule

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An electrically neutral group of two or more atoms held together by covalent chemical bonds, forming the smallest unit of a compound.

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Example: This diagram shows a single water molecule (H₂O), which is made of two hydrogen (H) atoms and one oxygen (O) atom bonded together.