Chemical Elements and Atomic Structure

The simplest forms of matter that have unique chemical properties are called [1], 92 of which occur naturally. Each has a unique atomic number, which states how many [2] there are in an atom. These, together with uncharged [3], constitute the nucleus. Their combined mass is called the [4] of the element. Electrons orbit around this nucleus in layers called [5]. Not all atoms of an element are identical; [6] differ from each other in number of neutrons, but are still the same element and behave the same chemically. The [7] of an element takes into account the fact that a sample of it contains various isotopes with different atomic masses.

When an atom gains or loses electrons, it becomes a/an [8]. If it is negatively charged it is called a/an [9], and if positively charged, a/an [10]. The charge on one of these particles, such as 2+, is called its [11]. Some of these particles, especially those that are small and inorganic, are called [12] because they enable the body fluids to conduct electricity. Uncharged particles with an unpaired electron are highly reactive and destructive particles called [13]. Some elements and compounds such as selenium and vitamin E protect the body from oxidation by these particles. These protective chemicals are therefore called [14].

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A [15] is formed when two or more atoms are joined together by a chemical bond. If it consists of two or more different elements, it is called a/an [16]. Molecules are represented by [17] that show how many atoms of each kind are present. Some compounds, called [18] of each other, have the same number and kind of atoms, but with different arrangements.  Except for hydrogen, the biologically important elements form stable bonds when they have [19] electrons in the valence shell. If one atom fills this shell by completely removing electrons from another and the particles cling to each other because of their opposite charges, a/an [20] bond is formed between them. If a pair of electrons orbit around both nuclei, a type of bond called a/an [21] bond is formed. This bond is [22] if the electrons spend about equal amounts of time around both nuclei, and [23] if they spend significantly more time in the proximity of one nucleus than of the other. When two molecules have bonds of the latter type, their polar regions may be weakly attracted to each other and form [24] bonds.

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A mixture is a substance composed of two or more substances that are physically but not [25] combined with each other. When the particles of a mixture are so small they will pass through semipermeable membranes and will not settle out of the mixture, the mixture is called a [26]. The most abundant component of such a mixture (usually water) is called the [27]. Slightly larger particles, such as proteins in water, form mixtures called [28], where the particles usually scatter light and make the mixture cloudy, but do not spontaneously settle out. Mixtures whose particles will settle out if the mixture stands long enough are called [29].

Solutions of equal percentage composition may not have the same number of particles per unit volume. When it is important that they do so, a measure of concentration called [30] may be more appropriate. A 1 M solution consists of a quantity of solute, one [31], in one liter of solution. Electrolyte solutions are more often measured in [32] per liter, because not only the number of particles but the number of unit charges is important to their physiological effect.

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A molecule that accepts protons is called a/an [33] and one that releases them is a/an [34]. pH is a measure of the negative [35] of hydrogen ion molarity. [36] are mixtures that resist changes in pH by neutralizing excess H⁺ or OH^-.

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A chemical reaction is the formation or breaking of a chemical [37]. The law of [38] says that all atoms in the reactants must be accounted for in the [39] of the reaction. A reaction in which a molecule is broken down to two or more simpler molecules is called a/an [40] reaction, and its opposite is a/an [41] reaction. When a group of atoms is merely transferred from one molecule to another, or molecules "swap" atoms, we call it a/an [42] reaction. Many reactions can go both ways depending on the relative amounts of substances present on either side of the equation. These are called [43] reactions and the direction in which they proceed is predicted from the law of [44]. When there is a stable ratio of reactants and products in such a reaction, we say the reaction is in a state of [45]. Among the factors that affect the rate of a chemical reaction, [46] are substances which temporarily bind reactants and allow them to interact, but which themselves are not permanently changed by the reaction.

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Energy is the capacity to [47], and exists in two basic forms: energy of motion, called [48], and stored or [49] which can be released by the interaction of two objects. [50] radiation is produced by the decay of radioisotopes, and is named for its ability to eject electrons from molecules. The form with greatest penetrating power is [51] rays, while [52] particles are the least penetrating but still very dangerous when emitted by radionuclides inside the body. We are continually exposed to about 295 mrem/year of [53] radiation from natural sources. The amount of time required for the body to rid itself of one-half of any internal radioisotope is called the isotope's [54]. It is determined by a combination of its own decay rate and the rate at which the body excretes it.

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The energy transfers in normal chemical reactions are described by the laws of [55]. The first of these laws states that under ordinary conditions, energy can be converted from one form to another, but cannot be [56]. The second states that some energy is lost as [57] in every energy transfer, and therefore no process can have 100% [58]. All chemical reactions in the body are collectively called its [59]. Any energy-releasing reaction is said to be [60], and all the physiological reactions of this type form a branch of 59 called [61]. Any reaction that requires an energy input is said to be [62], and reactions of this type in physiology are collectively called [63]. One common type of energy-releasing reaction is [64], which means the removal of electrons from a substance. This is done by electron acceptors called [65] agents. For this reaction to happen, however, the electron acceptor must be [66]-which means that an electron is added to it.

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