DAT Booster Chemistry Chapter 2. Periodic Properties PDF

Chapter 2 1 Periodic Properties This chapter will cover the periodic properties that are relevant to the DAT. Although the elements in the periodic table are arranged in order o...

Chapter 2 1 Periodic Properties This chapter will cover the periodic properties that are relevant to the DAT. Although the elements in the periodic table are arranged in order of increasing atomic number, there are several important trends in atomic radius, ionization energy, effective nuclear charge, electron affinity, and electronegativity. 1 Introduction to the Periodic Table Groups 13-17 elements: includes metalloids, which have a combination of Two important terms to know when looking at a both metallic and non-metallic periodic table are groups and periods. A characteristics as highlighted in orange group consists of elements in the same column. on the periodic table These elements have the same number of Group 17 elements: halogens valence electrons, which are the electrons in Group 18 elements: noble gases the outermost subshell. A period consists of elements that occupy the same row and these The oxidation state of an element is related to elements have the same number of electron the number of electrons that an atom loses, shells. The atomic number of an element is gains, or appears to use when bonding with equal to the number of protons in the atom. another atom. Almost all transition metals have For example, nitrogen has an atomic number of multiple oxidation states. They have this 7 because it has 7 protons. property because they have several electrons with similar energies, meaning that one or all of them can be removed, depending on the circumstances. Manganese, for example, shows oxidation states from +2 to +7. Although most transition metals have color, row 4 transition metals are an exception as they are colorless. The two rows (which belong in periods 6 and 7) at the bottom of the periodic table are known as the inner transition metals. Period 6 inner transition metals are the lanthanides and The periodic table. Notice the highlighted groups. Period 7 inner transition metals are the actinides. Inner transition metals are far less On the DAT, it is important to know the names abundant on earth compared to transition of the different groups on the periodic table: metals and these elements are separated from Group 1 elements (excluding the transition metals as their properties differ. hydrogen): alkali metals Transition metal atoms have their valence Group 2 elements: alkaline earth metals electrons in the outermost d-orbital, whereas Groups 3-12 elements: transition inner transition metal atoms have their valence metals electrons in the f-orbital. DAT Booster | Booster Prep™ Chapter 2 2 The most prominent oxidation state for transition metals is +2, whereas the most prominent oxidation state for inner transition metals is +3. It is also good to know your diatomic atoms for the DAT. Diatomic atoms are atoms that are usually found paired due to their unstable nature. For example, carbon is a monatomic atom, which means its atoms are not usually paired. On the other hand, oxygen gas, when found in nature, is a molecule In nature, carbon is monatomic, which means it is found consisting of two oxygen atoms and therefore unpaired. Oxygen, on the other hand, is a diatomic is a diatomic atom. The diatomic atoms you atom that is paired. need to know are: Hydrogen, Nitrogen, Fluorine, Oxygen, Iodine, Chlorine, and It is also important to know that metallic Bromine. character on the periodic table increases going from right to left across a period and increases going down a group. DAT Pro-Tip: A mnemonic device to remember the diatomic atoms is: Have No Fear Of Ice Cold Beer Metals Non-Metals Malleable, lustrous Brittle, dull Good conductors of electricity/heat Poor conductors of electricity/heat Form basic oxides Form acidic oxides Lose electrons to form cations Gain electrons to form anions Usually solid at room temperature, with the Gas or solid at room temperature, with the exception of mercury (Hg), which is liquid exception of bromine (Br), which is liquid Generally, high melting and boiling points Generally, low melting and boiling points Important: Know the key differences in properties of metals and non-metals for the DAT listed in the table above! DAT Booster | Booster Prep™  Chapter 2 3 2 Periodic Trends Atomic Radius De nition: Half the distance between the nuclei of two identical atoms bonded together. Atomic radius is half the distance between the nuclei of two identical atoms bonded together. Trend: Atomic radius increases from right to left across a period and increases going down a group. Effective Nuclear Charge Increases right to left across a period: Moving De nition: The effective nuclear charge (Zeff) is from right to left across a period, the number the amount of positive charge experienced by of protons in an atom decreases. The an electron. The shielding effect of lower decreasing number of protons results in a orbital electrons prevents higher orbital weaker nuclear attraction between the protons electrons from experiencing a strong attraction and electrons, which results in electron shells to the nucleus. This effect explains why valence being further apart from the nucleus, therefore electrons are more easily removed. The increasing the radius. effective nuclear charge is calculated given the following equation: Increases going down a group: Moving down a group, the number of electron shells Zeff = Z − S increases. Each additional electron level gets where Z = number of protons further and further away from the nucleus, S = number of shielding which causes the atomic radius to increase. (non-valence) electrons Trend: Effective nuclear charges increases left to right across a period and increases going up a group. Increases left to right across a period: Moving from left to right across a period, the numbers of protons increase with no increase in electron shells, and thus no increase in shielding effect. This results in electrons being pulled closer to the nucleus due to a stronger attraction. Increases going up a group: Moving up a Atomic radius increases from right to left group, the number of electron shells decreases across a period and going down a group. which brings outer shell electrons closer to the positively charged nucleus. This increases effective nuclear charge. DAT Booster | Booster Prep™ fi fi  Chapter 2 4 Anions have a larger radius: When a neutral atom gains electrons and becomes an anion, the increased electron number results in increased electron-electron repulsions. This expands the size of the electron cloud, resulting in a larger radius. Cations have a smaller radius: When a neutral In the shielding effect, because of other electrons between atom loses electrons and becomes a cation, the the electron of interest and the nucleus, the electron of decreased electron number results in interest experiences less attraction to the nucleus. decreased electron-electron repulsions. This reduces the size of the electron cloud, resulting in a smaller radius. Example 2.21: What is the effective nuclear charge for a valence Note: Metals typically form cations, resulting in electron of Gallium? ionic radii smaller than their atomic radii. Solution: The first step in determining effective Meanwhile, non-metals typically form anions, nuclear charge is to find the number of shielding resulting in ionic radii greater than their atomic electrons (non-valence electrons). Looking at the radii. periodic table, we can determine that Gallium (group 13) has 3 valence electrons. Ga has 31 protons so its non-valence electrons would be Example 2.22: 31 – 3 = 28. Use the equation: Which of the following ions has the smallest Zeff = Z – S radius? Zeff = 31 – 28 = +3 A) Na+ Note: Notice that the effective nuclear is equal to B) F- the number of valence electrons. This is often the C) O2- case, but not always. D) N3- E) Al3+ Isoelectronic Series Solution: In this case, all the elements provided are isoelectronic, which means that they all have De nition: These are atoms that have an the same number of electrons. We can eliminate identical number of electrons, but different the anions as a potential answer because anions numbers of protons. Anions are ions that have have a larger radius than cations. Since Al3+ has gained electrons and have more electrons than the most protons, the electrons will experience a protons, making them negatively charged. greater pull resulting in a smaller radius. Cations are ions that have lost electrons and Therefore, Al3+ has the smallest radius. have more protons than electrons, making them positively charged. DAT Booster | Booster Prep™ fi  Chapter 2 5 Ionization Energy De nition: The energy needed to remove an electron from an atom. Trend: Ionization energy increases going from left to right across a period and increases up a group. Increases left to right across a period: Across Ionization energy increases from left to right a period, the atomic number, or the number of across a period and going up a group. protons, increases. As the valence shell continues to ll, the electrons become harder Example 2.23: to remove (require more energy) due to an Which of the following elements has the highest increase in effective nuclear charge. Argon, for ionization energy? example, has a high ionization energy because it is a noble gas and has a completely lled A) Sodium valence shell. Due to this high stability, it would B) Calcium take a lot of energy to remove an electron. C) Chlorine D) Carbon Increases going up a group: Moving up a E) Argon group, there are fewer electron shells and subsequently less of a shielding effect from the Solution: Argon has the highest ionization energy because it is a noble gas. It is nearly impossible to inner electrons. This creates dif culty in extract an electron from argon’s valence shell removing the electrons from valence shells. because it is completely filled and therefore VERY Furthermore, the distance decreases between stable. Noble gases are chemically inert because the nucleus and the highest-energy electron, their high degree of stability makes it unlikely for strengthening the nuclear attraction to that them to lose or gain electrons. Therefore, the electron, and therefore requiring more energy. correct answer is Option E. Multiple ionization energies: The rst ionization energy is the energy required to remove the outermost electron. Following the removal of the rst electron, elements can have second, third, fourth, etc. ionization energies. The energy associated with removing each successive electron from an atom or ion increases. For example, the rst ionization energy of sodium is 496 kJ mol-1. However, following that, there is a huge jump to 4562 kJ mol-1 for the second ionization energy because we would be removing an electron Notice the exceptions in the ionization energy from a stable con guration (a full outer shell). trends across different elements. DAT Booster | Booster Prep™ fi fi fi fi fi fi fi fi  Chapter 2 6 There are two notable exceptions to the Electron Af nity above rules for ionization energy: De nition: The amount of energy released 1. The rst is that alkaline earth metals when an electron is added to an atom. have lled orbitals, which gives them greater stability, leading to their higher Trend: Electron af nity increases going from ionization energy compared to Group left to right across a period and increases going 13 elements in the same period. This is up a group. why Be has higher ionization energy compared to B. Increases left to right across a period: Across a period, as the atom’s valence shell gets lled, 2. The second exception is that group 15 there is increased attraction between the elements have half- lled orbitals, which nucleus and the electrons of the atom. This gives it greater stability, leading to its creates a stronger af nity for electrons. higher ionization energy compared to Group 16 elements in the same period. Increases going up a group: Moving up a This is why N has a higher ionization group, there are fewer electron shells, leading energy compared to O. to decreased electron shielding and an increased proximity between the nucleus and valence electrons, increasing the nuclear Example 2.24: attraction and thereby increasing electron Which of the following elements has the highest af nity. second ionization energy? A) Beryllium There are three notable exceptions to the B) Calcium rules for electron af nity: C) Strontium D) Lithium 1. The rst is that group 2 elements have E) Potassium lled s-orbitals, so their electron af nities are very low Solution: The second ionization energy is the energy required to remove a second electron 2. The second exception is that group 15 from the valence shell. Beryllium, calcium, and elements have half- lled orbitals p- strontium are all group 2, so removing their orbitals, so their electron af nities are second electron would result in a full electron lower than group 14 elements of the shell and would not require as much energy. same period Lithium and potassium are in group 1, so removing their second electron would disturb the full electron shell and result in a higher second 3. The third exception is that noble gases ionization energy. Recalling the fact that ionization have lled electron shells, so their energy increases moving up a group, lithium has electron af nities are negligible the highest second ionization energy. Therefore, the correct answer is Option D. DAT Booster | Booster Prep™ fi fi fi fi fi fi fi fi fi fi fi fi fi fi fi fi fi  Chapter 2 7 Electronegativity De nition: The measurement of an atom’s ability to attract electrons in a bond. The higher the electronegativity of an atom, the greater ability to attract an electron pair. Trend: Electronegativity increases going from left to right across a period and increases up a Electron affinity increases from left to right group. The most electronegative element is across a period and going up a group. uorine, and if you remember this, the trend becomes very easy to remember. Example 2.25: Increases left to right across a period: With Which of the following elements has the greatest increasing protons as you go from left to right electron affinity (EA)? across a period, the ability of an atom to attract A) Beryllium an electron pair is increased. This is similar to B) Lithium the electron af nity trend; however, this trend is C) Nitrogen speci c to electron pairs in a bond, not the D) Carbon addition of a single electron. E) Neon Increases going up a group: Moving up a Solution: Following the trend of EA, lithium and group, as the atomic radius decreases, and the beryllium (far left of the periodic table) will have valence electrons experience less shielding, the the lowest EA. Neon, a noble gas, has a ability of an atom to attract an electron pair is negligible EA due to its complete valence shell. It increased. may appear that nitrogen will have the largest EA, however carbon has an electron configuration 1 Note: The noble gases are an exception to this electron away from a half-filled subshell therefore trend as they have full valence shells and carbon has the highest electron affinity. Therefore, therefore no electronegativity value. the correct answer is Option D. Electronegativity increases from left to right across a period and going up a group. Electron affinities in kJ/mol for certain elements. DAT Booster | Booster Prep™ fl fi fi fi Chapter 2 8 Example 2.26: Rank the following elements in order of increasing electronegativity: Cl, P, Sr, Al, F Solution: Looking at the periodic table, Sr would have the least amount of electronegativity as it is in group 2. Since Al, P, and Cl are all in period 3, and electronegativity increases from left to right across a period, Al, P, and Cl, respectively, will have progressively greater electronegativity levels. Right away, we also know that fluorine is the most electronegative atom therefore the answer is Sr < Al < P < Cl < F. In a water molecule, oxygen is more electronegative, thus electron density is more concentrated towards oxygen. 3 Summary of Periodic Trends A summary of important periodic trends to know are shown. DAT Booster | Booster Prep™