25. from left to right across a period in the periodic table, elements become less and more in their properties. 26. francium has 36 isotopes, but only francium-223 occurs in nature. francium-223 spontaneously emits particles and energy, so francium-223 is a(an) of francium. 27. at sea level, water at 100°c. 28. cooking requires continuous addition of energy to the chemical reactions that are taking place. the chemical reactions involved in cooking can be described as 29. the theory of matter states that all particles of matter are in constant motion. 30. in rutherford’s experiments, some of the aimed at gold atoms bounced back, suggesting that a solid mass was at the center of the atom. 31. you are given the melting points of three unknown substances and are asked to predict which one is an ionic compound. you would select the compound with the melting point. 32. a flight simulator that astronauts prepare for a shuttle launch is an example of a(an) 33. in the symbol ■([email  protected]) he, the superscript 4 is the for helium, and the subscript 2 is the for helium.

25. From left to right across a period in the periodic table, elements become less electropositive and more electronegative in their properties. 

26. Francium has 36 isotopes, but only francium-223 occurs in nature. Francium-223 spontaneously emits particles and energy, so francium-223 is a(an) radioisotope of francium. 

27. At sea level, water boils at 100°C. 

28. Cooking requires continuous addition of energy to the chemical reactions that are taking place. The chemical reactions involved in cooking can be described as endothermic. 

29. The Kinetic theory of matter states that all particles of matter are in constant motion. 

30. In Rutherford’s experiments, some of the alpha particles aimed at gold atoms bounced back, suggesting that a solid mass was at the center of the atom. 

31. You are given the melting points of three unknown substances and are asked to predict which one is an ionic compound. You would select the compound with the highest melting point. 

32. A flight simulator that helps astronauts prepare for a shuttle launch is an example of a(an) _scientific model. 

33. In the symbol 

explanation:

the largest database of organic compounds lists about 10 million substances, which include compounds originating from living organisms and those synthesized by chemists. the number of potential organic compounds has been estimated at 1060—an astronomically high number. the existence of so many organic molecules is a consequence of the ability of carbon atoms to form up to four strong bonds to other carbon atoms, resulting in chains and rings of many different sizes, shapes, and complexities. the simplest organic compounds contain only the elements carbon and hydrogen, and are called hydrocarbons. even though they are composed of only two types of atoms, there is a wide variety of hydrocarbons because they may consist of varying lengths of chains, branched chains, and rings of carbon atoms, or combinations of these structures. in addition, hydrocarbons may differ in the types of carbon-carbon bonds present in their molecules. many hydrocarbons are found in plants, animals, and their fossils; other hydrocarbons have been prepared in the laboratory. we use hydrocarbons every day, mainly as fuels, such as natural gas, acetylene, propane, butane, and the principal components of gasoline, diesel fuel, and heating oil. the familiar plastics polyethylene, polypropylene, and polystyrene are also hydrocarbons. we can distinguish several types of hydrocarbons by differences in the bonding between carbon atoms. this leads to differences in geometries and in the hybridization of the carbon orbitals.