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| 2649243244 | matter | Anything that takes up space and has mass. | 0 | |
| 2649302014 | element | Any substance that cannot be broken down to any other substance by chemical reactions. | 1 | |
| 2649303727 | compound | A substance consisting of two or more different elements combined in a fixed ratio. | 2 | |
| 2649305563 | emergent properties | Properties not possessed by the constituents. | 3 | |
| 2649307669 | essential element | A chemical element required for an organism to survive, grow, and reproduce. | 4 | |
| 2649310195 | trace element | An element indispensable for life but required in extremely minute amounts. | 5 | |
| 2649326799 | atom | The smallest unit of matter that retains the properties of an element. | 6 | |
| 2649328760 | subatomic particles | The parts that make up an atom. | 7 | |
| 2649329466 | neutron | A subatomic particle having no electrical charge (electrically neutral), with a mass of 1 dalton, found in the nucleus of an atom. | 8 | |
| 2649329467 | proton | A subatomic particle with a single positive electrical charge, with a mass of 1 dalton, found in the nucleus of an atom. | 9 | |
| 2649330235 | electron | A subatomic particle with a single negative charge and a minute mass, and move around the nucleus of an atom. | 10 | |
| 2649332259 | atomic nucleus | An atom's dense central core, containing protons and neutrons. | 11 | |
| 2649338423 | dalton | A measure of mass for atoms and subatomic particles; the same as the atomic mass unit, or amu. | 12 | |
| 2649400845 | atomic number | The number of protons in the nucleus of an atom, unique for each element and designated by a subscript. | 13 | |
| 2649408375 | mass number | The sum of the number of protons and neutrons in an atom's nucleus. | 14 | |
| 2649409050 | atomic mass | The total mass of an atom, which is the mass in grams of 1 mole of the atom. | 15 | |
| 2649411786 | isotope | One of several atomic forms of an element, each with the same number of protons but a different number of neutrons, thus differing in atomic mass. | 16 | |
| 2649419504 | radioactive isotope | An isotope that is unstable; the nucleus decays spontaneously, giving off detectable particles of energy. | 17 | |
| 2649452257 | energy | The capacity to cause change, especially to do work (to move matter against an opposing force). | 18 | |
| 2649455024 | potential energy | The energy that matter possesses as a result of its location or spatial arrangement (structure). | 19 | |
| 2649455025 | electron shell | An energy level of electrons at a characteristic average distance from the nucleus of an atom. | 20 | |
| 2650692649 | periodic table of the elements | A tabular arrangement of the chemical elements, organized on the basis of their atomic number (number of protons in the nucleus), electron configurations, and recurring chemical properties. | 21 | |
| 2650694444 | period | A row of the periodic table, corresponding to all of the atoms with that number of electron shells. | 22 | |
| 2650698269 | group | A column of the periodic table, each having the sequential addition of an electron and a proton moving from left-to-right. | 23 | |
| 2650723710 | valence electron | An electron in the outermost electron shell. | 24 | |
| 2650723711 | valence shell | The outermost energy shell of an atom, containing the valence electrons involved in the chemical reactions of that atom. | 25 | |
| 2650725757 | inert | Chemically unreactive because the valence shell is full. | 26 | |
| 2650746991 | chemical bond | An attraction between two atoms, resulting from a sharing of outer shell electrons or the presence of opposite charges on the atoms. The bonded atoms gain complete outer electron shells. | 27 | |
| 2650748197 | covalent bond | A type of strong chemical bond in which two atoms share one or more pairs of valence electrons. | 28 | |
| 2650749737 | molecule | Two or more atoms held together by covalent bonds. | 29 | |
| 2650756920 | molecular formula | A way to represent just the atoms of a molecule. | 30 | |
| 2650756921 | structural formula | A way to represent the atoms of a molecule, and the bonds with lines. | 31 | |
| 2650757515 | single bond | The sharing of a pair of valence electrons by two atoms. | 32 | |
| 2650759896 | double bond | The sharing of two pairs of valence electrons by two atoms. | 33 | |
| 2650770001 | valence | The bonding capacity of a given atom; the number of covalent bonds an atom can form usually equals the number of unpaired electrons in its outermost shell. | 34 | |
| 2650825041 | electronegativity | The attraction of a given atom for the electrons of a covalent bond. | 35 | |
| 2650826348 | nonpolar covalent bond | A type of bond in which electrons are shared equally between two atoms of similar electronegativity. | 36 | |
| 2650827156 | polar covalent bond | A bond between atoms that differ in electronegativity. The shared electrons are pulled closer to the more electronegative atom, making it slightly negative and the other atom slightly positive. | 37 | |
| 2650836583 | ion | An atom or group of atoms that has gained or lost one or more electrons, thus acquiring a charge. | 38 | |
| 2650836584 | cation | A positively charged ion. | 39 | |
| 2650837378 | anion | A negatively charged ion. | 40 | |
| 2650838085 | ionic bond | A chemical bond resulting from the attraction between oppositely charged ions. | 41 | |
| 2650839432 | ionic compound | A compound resulting from the formation of an ionic bond; also called a salt. | 42 | |
| 2650844714 | salt | A compound resulting from the formation of an ionic bond; also called an ionic compound. | 43 | |
| 2650986156 | hydrogen bond | A type of weak chemical bond that is formed when the sightly positive hydrogen atom of a polar covalent bond in one molecule is attracted to the slightly negative atom of a polar covalent bond in another molecule or in another region of the same molecule. | 44 | |
| 2650987605 | van der Waals interactions | Weak attractions between molecules or parts of molecules that results from transient local partial charges. | 45 | |
| 2651034975 | chemical reaction | The making and breaking of chemical bonds, leading to changes in the composition of matter. | 46 | |
| 2651034976 | reactant | A starting material in a chemical reaction. | 47 | |
| 2651034977 | product | A material resulting from a chemical reaction. | 48 | |
| 2651043608 | chemical equilibrium | In a chemical reaction, the state in which the rate of the forward reaction equals the rate of the reverse reaction, so that the relative concentrations of the reactants and products do not change with time. | 49 | |
| 2651056292 | polar molecule | A molecule (such as water) with an uneven distribution of charges in different regions of the molecule. | 50 | |
| 2651062546 | cohesion | The linking together of like molecules, often by hydrogen bonds. | 51 | |
| 2651081340 | surface tension | A measure of how difficult it is to stretch or break the surface of a liquid. | 52 | |
| 2651783687 | kinetic energy | The energy associated with the relative motion of objects. Moving matter can perform work by imparting motion to other matter. | 53 | |
| 2651790924 | thermal energy | Kinetic energy due to the random motion of atoms and molecules; energy in its most random form. | 54 | |
| 2651799048 | temperature | A measure in degrees of the average kinetic energy (thermal energy) of the atoms and molecules in a body of matter. | 55 | |
| 2651799049 | heat | Thermal energy in transfer from one body of matter to another. | 56 | |
| 2651800217 | calorie | The amount of heat energy required to raise the temperature of 1g of water by 1C; also the amount of heat energy that 1 g of water releases when it cools by 1C. | 57 | |
| 2651801665 | kilocalorie | A thousand calories; the amount of heat energy required to raise the temperature of 1 kg of water by 1C. | 58 | |
| 2651801666 | joule | A unit of energy equal to 0.239 calories. | 59 | |
| 2651816650 | specific heat | The amount of heat that must be absorbed or lost for 1 g of a substance to change its temperature by 1C. | 60 | |
| 2651912108 | evaporation | Transformation from a liquid to a gas. | 61 | |
| 2651925355 | heat of vaporization | The quantity of heat a liquid must absorb for 1g of it to be converted from the liquid to the gaseous state. | 62 | |
| 2651935094 | evaporative cooling | The process in which the surface of an object becomes cooler during evaporation, a result of the molecules with the greatest kinetic energy changing from the liquid to the gaseous state. | 63 | |
| 2651982111 | solution | A liquid that is a homogenous mixture of two or more substances. | 64 | |
| 2651982112 | solvent | The dissolving agent of a solution. Water is the most versatile one. | 65 | |
| 2651982702 | solute | A substance that is dissolved in a solution. | 66 | |
| 2651986717 | aqueous solution | A solution in which water is the solvent. | 67 | |
| 2651986718 | hydration shell | The sphere of water molecules around a dissolved ion. | 68 | |
| 2651999933 | hydrophilic | Having an affinity for water. | 69 | |
| 2652003721 | hydrophobic | Having no affinity for water; tending to coalesce and form droplets in water. | 70 | |
| 2652030990 | molecular mass | The sum of the masses of all the atoms in a molecule; sometimes called molecular weight. | 71 | |
| 2652035626 | mole | The number of grams of a substance that equals its molecular weight in daltons and contains Avogadro's number of molecules. | 72 | |
| 2652036839 | molarity | A common measure of solute concentration, referring to the number of moles of solute per liter of solution. | 73 | |
| 2652775945 | hydrogen ion | A single proton with a charge of 1+. The dissociation of water leads to its generation, along with a hydroxide ion (OH-). | 74 | |
| 2652776708 | hydroxide ion | A water molecule that has lost a proton OH- | 75 | |
| 2652778116 | hydronium ion | A water molecule that has an extra proton bound to it; H3O+, commonly represented as H+. | 76 | |
| 2652793366 | acid | A substance that increases the hydrogen ion concentration of a solution. | 77 | |
| 2652812963 | base | A substance that reduces the hydrogen ion concentration of a solution. | 78 | |
| 2652855010 | pH | A measure of hydrogen ion concentration equal to -log [H+] and ranging in value from 0 to 14. | 79 | |
| 2652890776 | buffer | A solution that contains a weak acid and its corresponding base. Minimizes changes in pH when acids or bases are added to the solution. | 80 |
