Determining the atomic radii is rather difficult because there is an uncertainty in the position of the outermost electron — we do not know exactly where the electron is. This phenomenon can be explained by the Heisenberg Uncertainty Principle.
To get a precise measurement of the radius, but still not an entirely correct measurement, we determine the radius based on the distance between the nuclei of two bonded atoms.
The radii of atoms are therefore determined by the bonds they form. An atom will have different radii depending on the bond it forms; so there is no fixed radius of an atom. When a covalent bond is present between two atoms, the covalent radius can be determined. When two atoms of the same element are covalently bonded, the radius of each atom will be half the distance between the two nuclei because they equally attract the electrons.
The distance between two nuclei will give the diameter of an atom, but you want the radius which is half the diameter. Covalent radii will increase in the same pattern as atomic radii. The reason for this trend is that the bigger the radii, the further the distance between the two nuclei. The covalent radius depicted below in Figure 1 will be the same for both atoms because they are of the same element as shown by X.
The ionic radius is the radius of an atom forming ionic bond or an ion. The radius of each atom in an ionic bond will be different than that in a covalent bond. This is an important concept. The reason for the variability in radius is due to the fact that the atoms in an ionic bond are of greatly different size. One of the atoms is a cation, which is smaller in size, and the other atom is an anion which is a lot larger in size. So in order to account for this difference, one most get the total distance between the two nuclei and divide the distance according to atomic size.
The bigger the atomic size, the larger radius it will have. If we were able to determine the atomic radius of an atom from experimentation, say Se, which had an atomic radius of pm, then we could determine the atomic radius of any other atom bonded to Se by subtracting the size of the atomic radius of Se from the total distance between the two nuclei.
So, if we had the compound CaSe, which had a total distance of pm between the nucleus of the Ca atom and Se atom, then the atomic radius of the Ca atom will be pm total distance - pm distance of Se , or pm.
The electrons of the valence shell have less attraction to the nucleus and, as a result, can lose electrons more readily. This causes an increase in metallic character. Another easier way to remember the trend of metallic character is that moving left and down toward the bottom-left corner of the periodic table, metallic character increases toward Groups 1 and 2, or the alkali and alkaline earth metal groups.
Likewise, moving up and to the right to the upper-right corner of the periodic table, metallic character decreases because you are passing by to the right side of the staircase, which indicate the nonmetals. These include the Group 8, the noble gases , and other common gases such as oxygen and nitrogen.
Based on the periodic trends for ionization energy, which element has the highest ionization energy? Answer: C. Helium He Explanation: Helium He has the highest ionization energy because, like other noble gases, helium's valence shell is full. Therefore, helium is stable and does not readily lose or gain electrons. Answer: A. True Explanation: Atomic radius increases from right to left on the periodic table.
Therefore, nitrogen is larger than oxygen. Answer: Lead Pb Explanation: Lead and tin share the same column. Metallic character increases down a column. Lead is under tin, so lead has more metallic character. Answer: Bromine Br Explanation: In non-metals, melting point increases down a column.
Because chlorine and bromine share the same column, bromine possesses the higher melting point. Answer: Sulfur S Explanation: Note that sulfur and selenium share the same column. Electronegativity increases up a column. This indicates that sulfur is more electronegative than selenium. Answer: Most noble gases have full valence shells.
Explanation: Because of their full valence electron shell, the noble gases are extremely stable and do not readily lose or gain electrons.
Explanation: The electrons above a closed shell are shielded by the closed shell. S has 6 electrons above a closed shell, so each one feels the pull of 6 protons in the nucleus. Oxygen O Explanation: Periodic trends indicate that atomic radius increases up a group and from left to right across a period.
Therefore, oxygen has a smaller atomic radius sulfur. Answer: B. False Explanation: The reasoning behind this lies in the fact that a metal usually loses an electron in becoming an ion while a non-metal gains an electron.
This results in a smaller ionic radius for the metal ion and a larger ionic radius for the non-metal ion. Electronegativity Trends Electronegativity can be understood as a chemical property describing an atom's ability to attract and bind with electrons. From left to right across a period of elements, electronegativity increases.
If the valence shell of an atom is less than half full, it requires less energy to lose an electron than to gain one. Conversely, if the valence shell is more than half full, it is easier to pull an electron into the valence shell than to donate one. From top to bottom down a group, electronegativity decreases.
This is because atomic number increases down a group, and thus there is an increased distance between the valence electrons and nucleus, or a greater atomic radius. Important exceptions of the above rules include the noble gases, lanthanides , and actinides.
The noble gases possess a complete valence shell and do not usually attract electrons. The lanthanides and actinides possess more complicated chemistry that does not generally follow any trends. Therefore, noble gases, lanthanides, and actinides do not have electronegativity values.
As for the transition metals, although they have electronegativity values, there is little variance among them across the period and up and down a group. This is because their metallic properties affect their ability to attract electrons as easily as the other elements.
Ionization Energy Trends Ionization energy is the energy required to remove an electron from a neutral atom in its gaseous phase. Trends The ionization energy of the elements within a period generally increases from left to right. This is due to valence shell stability. The ionization energy of the elements within a group generally decreases from top to bottom. This is due to electron shielding. The noble gases possess very high ionization energies because of their full valence shells as indicated in the graph.
Note that helium has the highest ionization energy of all the elements. Electron Affinity Trends As the name suggests, electron affinity is the ability of an atom to accept an electron.
Electron affinity increases from left to right within a period. This is caused by the decrease in atomic radius. Electron affinity decreases from top to bottom within a group. This is caused by the increase in atomic radius. Atomic Radius Trends 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.
Atomic radius decreases from left to right within a period. Why does atomic radius decrease as you go across a period? Oct 20, Explanation: This is a very important periodic phenomenon: the contraction of atomic radii across the period.
What happens to atomic radii down a Group? Explanation: The following diagram shows the periodic trends of atomic radius of the representative elements main group elements for the first six periods. Related questions Why do periodic trends exist for electronegativity? Why does atomic size increase down a group? What do periodic trends of reactivity occur with the halogens?
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