If two or more elements or chemical compounds come into contact with one another and there is enough __________ present, a chemical change may take place.
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When two distinct elements are chemically combined—i.e., chemical bonds form between their atoms—the result is called a chemical compound. ... Most elements on Earth bond with other elements to form chemical compounds, such as sodium (Na) and Chloride (Cl), which combine to form table salt (NaCl).
The atoms of the various chemical elements can be likened to the letters of the alphabet: just as the letters of the alphabet are combined to form thousands of words, the atoms of the elements can combine in various ways to form a myriad of compounds. In fact, there are millions of chemical compounds known, and many more millions are possible but have not yet been discovered or synthesized. Most substances found in nature—such as wood, soil, and rocks—are mixtures of chemical compounds. These substances can be separated into their constituent compounds by physical methods, which are methods that do not change the way in which atoms are aggregated within the compounds. Compounds can be broken down into their constituent elements by chemical changes. A chemical change (that is, a chemical reaction) is one in which the organization of the atoms is altered. An example of a chemical reaction is the burning of methane in the presence of molecular oxygen (O2) to form carbon dioxide (CO2) and water.
CH4 + 2O2 → CO2 + 2H2O
In this reaction, which is an example of a combustion reaction, changes occur in the way that the carbon, hydrogen, and oxygen atoms are bound together in the compounds.
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Chemical
For example, the atoms of the elements in Group 1 of the periodic table all have one valence electron, the atoms of the elements in Group 2 have two valence electrons, and so on, until Group 18, whose elements contain eight valence electrons, is reached. The simplest and most important rule for predicting how atoms form compounds is that atoms tend to combine in ways that allow them either to empty their valence shell or to complete it (i.e., fill it up), in most cases with a total of eight electrons. Elements on the left side of the periodic table tend to lose their valence electrons in chemical reactions. Sodium (in Group 1), for example, tends to lose its lone valence electron to form an ion with a charge of +1. Each sodium atom has 11 electrons (e−), each with a charge of −1, to just balance the +11 charge on its nucleus. Losing one electron leaves it with 10 negative charges and 11 positive charges to give a net +1 charge: Na → Na+ + e−. Potassium, located directly beneath sodium in Group 1, also forms +1 ions (K+) in its reactions, as do the remaining members of Group 1: rubidium (Rb), cesium (Cs), and francium (Fr). The atoms of the elements toward the right end of the periodic table tend to undergo reactions such that they gain (or share) enough electrons to complete their valence shell. For example, oxygen in Group 16 has six valence electrons and thus needs two more electrons to complete its outermost shell. Oxygen achieves this arrangement by reacting with elements that can lose or share electrons. An oxygen atom, for instance, can react with a magnesium (Mg) atom (in Group 2) by taking magnesium’s two valence electrons, producing Mg2+ and O2− ions. (When a neutral magnesium atom loses two electrons, it forms the Mg2+ ion, and, when a neutral oxygen atom gains two electron
metallic elements in the periodic table
metallic elements in the periodic table
Metals, nonmetals, and metalloids are represented in different regions of the periodic table.
Encyclopædia Britannica, Inc.
The nonmetals, which are relatively few in number, are found in the upper right-hand corner of the periodic table—except for hydrogen, the only nonmetallic member of Group 1. The physical properties characteristic of metals are absent in nonmetals. In chemical reactions with metals, nonmetals gain electrons to form negative ions. Nonmetallic elements also react with other nonmetals, in this case forming molecular compounds. Chlorine is a typical nonmetal. At ordinary temperatures, elemental chlorine contains Cl2 molecules and reacts with other nonmetals to form molecules such as HCl, CCl4, and PCl3. Chlorine reacts with metals to form ionic compounds containing Cl− ions.
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