It is important to recognize which numbering system is being used and to be able to find the number of valence electrons in the main block elements regardless of which numbering system is being used. You will come across periodic tables with both numbering systems. In this numbering system, group 1A is group 1 group 2A is group 2 the halogens (7A) are group 17 and the noble gases (8A) are group 18. The elements in this group are also gases at room temperature.Īn alternate numbering system numbers all of the \(s\), \(p\), and \(d\) block elements from 1-18. We will learn the reason for this later, when we discuss how compounds form. These elements also have similar properties to each other, the most significant property being that they are extremely unreactive, rarely forming compounds. This group contains very reactive nonmetal elements. Group 7A (or 17) elements are also called halogens. It is important to recognize a couple of other important groups on the periodic table by their group name. Remember, Mendeleev arranged the table so that elements with the most similar properties were in the same group on the periodic table. The same pattern is true of other groups on the periodic table. Once again, because of their similarities in electron configurations, these elements have similar properties to each other. Group 2A is also called the alkaline earth metals. As a result, the elements on the left side of the periodic table generally lose electrons when forming bonds. Although most metals tend to be very hard, these metals are actually soft and can be easily cut. Because elements on the left side of the periodic table have less than a half-full valence shell, the energy required to gain electrons is significantly higher compared with the energy required to lose electrons. Group 1A is also known as the alkali metals. Because of their similarities in their chemical properties, Mendeleev put these elements into the same group. The elements in 1A are all very reactive and form compounds in the same ratios with similar properties with other elements. This is what causes these elements to react in the same ways as the other members of the family. All of the 1A elements have one valence electron. A group is a vertical column of the periodic table. Remember that Mendeleev arranged the periodic table so that elements with the most similar properties were placed in the same group. This is the reason why H is always a terminal atom and never a central atom.\) Hydrogen only needs to form one bond to complete a duet of electrons. Atom (Group number)īecause hydrogen only needs two electrons to fill its valence shell, it follows the duet rule. Table showing 4 different atoms, each of their number of bonds, and each of their number of lone pairs. In each case, the sum of the number of bonds and the number of lone pairs is 4, which is equivalent to eight (octet) electrons. The number of electrons required to obtain an octet determines the number of covalent bonds an atom can form. Oxygen and other atoms in group 16 obtain an octet by forming two covalent bonds: To obtain an octet, these atoms form three covalent bonds, as in NH 3 (ammonia). Group 15 elements such as nitrogen have five valence electrons in the atomic Lewis symbol: one lone pair and three unpaired electrons. The transition elements and inner transition elements also do not follow the octet rule since they have d and f electrons involved in their valence shells. Because hydrogen only needs two electrons to fill its valence shell, it is an exception to the octet rule and only needs to form one bond. These four electrons can be gained by forming four covalent bonds, as illustrated here for carbon in CCl 4 (carbon tetrachloride) and silicon in SiH 4 (silane). For example, each atom of a group 14 element has four electrons in its outermost shell and therefore requires four more electrons to reach an octet. The number of bonds that an atom can form can often be predicted from the number of electrons needed to reach an octet (eight valence electrons) this is especially true of the nonmetals of the second period of the periodic table (C, N, O, and F). From left to right: water molecule, ammonia molecule, and methane molecule
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