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. However, this idea is complicated by the fact that not all atoms are normally bound together in the same way. Some are bound by covalent bonds in molecules, some are attracted to each other in ionic crystals, and others are held in metallic crystals. 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.
The covalent radii of these molecules are often referred to as atomic radii. This distance is measured in picometers. Atomic radius patterns are observed throughout the periodic table. Atomic size gradually decreases from left to right across a period of elements. This is because, within a period or family of elements, all electrons are added to the same shell. However, at the same time, protons are being added to the nucleus, making it more positively charged.
The effect of increasing proton number is greater than that of the increasing electron number; therefore, there is a greater nuclear attraction. This means that the nucleus attracts the electrons more strongly, pulling the atom's shell closer to the nucleus.
The valence electrons are held closer towards the nucleus of the atom. As a result, the atomic radius decreases. D own a group, atomic radius increases. The valence electrons occupy higher levels due to the increasing quantum number n.
Electron shielding prevents these outer electrons from being attracted to the nucleus; thus, they are loosely held, and the resulting atomic radius is large. The melting points is the amount of energy required to break a bond s to change the solid phase of a substance to a liquid. Generally, the stronger the bond between the atoms of an element, the more energy required to break that bond. Because temperature is directly proportional to energy, a high bond dissociation energy correlates to a high temperature.
Melting points are varied and do not generally form a distinguishable trend across the periodic table. The metallic character of an element can be defined as how readily an atom can lose an electron. From right to left across a period, metallic character increases because the attraction between valence electron and the nucleus is weaker, enabling an easier loss of electrons. Metallic character increases as you move down a group because the atomic size is increasing. When the atomic size increases, the outer shells are farther away.
The principal quantum number increases and average electron density moves farther from nucleus. 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.
In a pure covalent bond, the electrons are held on average exactly half way between the atoms. In a polar bond, the electrons have been dragged slightly towards one end. How far does this dragging have to go before the bond counts as ionic? There is no real answer to that. Sodium chloride is typically considered an ionic solid, but even here the sodium has not completely lost control of its electron. Because of the properties of sodium chloride, however, we tend to count it as if it were purely ionic.
Lithium iodide, on the other hand, would be described as being "ionic with some covalent character". In this case, the pair of electrons has not moved entirely over to the iodine end of the bond.
Lithium iodide, for example, dissolves in organic solvents like ethanol - not something which ionic substances normally do. In a simple diatomic molecule like HCl, if the bond is polar, then the whole molecule is polar. What about more complicated molecules?
Consider CCl 4 , left panel in figure above , which as a molecule is not polar - in the sense that it doesn't have an end or a side which is slightly negative and one which is slightly positive. The whole of the outside of the molecule is somewhat negative, but there is no overall separation of charge from top to bottom, or from left to right.
In contrast, CHCl 3 is a polar molecule right panel in figure above. The hydrogen at the top of the molecule is less electronegative than carbon and so is slightly positive. This means that the molecule now has a slightly positive "top" and a slightly negative "bottom", and so is overall a polar molecule. The distance of the electrons from the nucleus remains relatively constant in a periodic table row, but not in a periodic table column.
In this expression, Q represents a charge, k represents a constant and r is the distance between the charges.
It is readily seen from these numbers that, as the distance between the charges increases, the force decreases very rapidly. This is called a quadratic change. The result of this change is that electronegativity increases from bottom to top in a column in the periodic table even though there are more protons in the elements at the bottom of the column. Elements at the top of a column have greater electronegativities than elements at the bottom of a given column.
The overall trend for electronegativity in the periodic table is diagonal from the lower left corner to the upper right corner. Since the electronegativity of some of the important elements cannot be determined by these trends they lie in the wrong diagonal , we have to memorize the following order of electronegativity for some of these common elements. The most electronegative element is fluorine.
If you remember that fact, everything becomes easy, because electronegativity must always increase towards fluorine in the Periodic Table. Note: This simplification ignores the noble gases. Historically this is because they were believed not to form bonds - and if they do not form bonds, they cannot have an electronegativity value.
Even now that we know that some of them do form bonds, data sources still do not quote electronegativity values for them. The positively charged protons in the nucleus attract the negatively charged electrons. As the number of protons in the nucleus increases, the electronegativity or attraction will increase. Therefore electronegativity increases from left to right in a row in the periodic table.
This effect only holds true for a row in the periodic table because the attraction between charges falls off rapidly with distance. The chart shows electronegativities from sodium to chlorine ignoring argon since it does not does not form bonds.
As you go down a group, electronegativity decreases. If it increases up to fluorine, it must decrease as you go down. The chart shows the patterns of electronegativity in Groups 1 and 7. Consider sodium at the beginning of period 3 and chlorine at the end ignoring the noble gas, argon. As you can see, electronegativity increases as you go across a period, while electronegativity decreases as you go down a group.
However, if this is true which it is , we can also say many other factors occur, such as:. As you go across a period from left to right, electronegativity increases, ionization energy increases, and atomic radius decreases.
In order for energy to increase, radius must decrease. As you go up and down a period, electronegativity decreases, ionization energy decreases, and atomic radius increases.
In order for energy to decrease, radius must increase. Ionization Energy is the complete opposite. These strive to make cations while electronegative ions usually involve anions hence the phrase electronegativity.
May 18,
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