![]() ![]() The valence electrons are held closer towards the nucleus of the atom. This means that the nucleus attracts the electrons more strongly, pulling the atom's shell closer to the nucleus. The effect of increasing proton number is greater than that of the increasing electron number therefore, there is a greater nuclear attraction. However, at the same time, protons are being added to the nucleus, making it more positively charged. This is because, within a period or family of elements, all electrons are added to the same shell. Atomic radius patterns are observed throughout the periodic table.Ītomic size gradually decreases from left to right across a period of elements. The covalent radii of these molecules are often referred to as atomic radii. Nevertheless, it is possible for a vast majority of elements to form covalent molecules in which two like atoms are held together by a single covalent bond. Some are bound by covalent bonds in molecules, some are attracted to each other in ionic crystals, and others are held in metallic crystals. However, this idea is complicated by the fact that not all atoms are normally bound together in the same way. The atomic radius is one-half the distance between the nuclei of two atoms (just like a radius is half the diameter of a circle). This is caused by the increase in atomic radius. Electron affinity decreases from top to bottom within a group. ![]() This is caused by the decrease in atomic radius. Electron affinity increases from left to right within a period.This causes the electron to move closer to the nucleus, thus increasing the electron affinity from left to right across a period. Moving from left to right across a period, atoms become smaller as the forces of attraction become stronger. With a larger distance between the negatively-charged electron and the positively-charged nucleus, the force of attraction is relatively weaker. This means that an added electron is further away from the atom's nucleus compared with its position in the smaller atom. The "7" is added four times since there are four fluorine atoms with 7 valence electrons each.\( \newcommand\): Periodic Table showing Electron Affinity TrendĮlectron affinity generally decreases down a group of elements because each atom is larger than the atom above it (this is the atomic radius trend, discussed below). For main group elements, the number of valence electrons usually ranges between 1 and 8 because eight electrons forms a complete octet. Then add 4 + 7 + 7 + 7 + 7 to get a total of 32 valence electrons. For instance, if you are trying to find the total number for valence electrons in CF 4, then you will need to first find out that carbon (C) has four valence electrons and fluorine (F) has seven valence electrons. To do so, add up all the valence electrons for each atom in the molecule of that compound. Once you have determined the number of valence electrons for the atoms of each element in the compound, you are ready to determine the total number of valence electrons in the molecule. Similar logic can be applied to all other groups to obtain the results shown in the graphic below. Group 13 elements have an s 2p 1 electron configuration and three valenece electrons. All Group 2 elements have an s 2 electron configuration and two valenece electrons. As shown in the graphic below, all Group 1 elements have an s 1 electron configuration and thus have one valence electron. Since the number of valence electrons is a periodic property, the value can be easily determined by locating the element in the periodic table. How does one Determine the # of Valence Electrons?įirst you need to be able to determine the number of valence electrons present in each element of the compound.
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