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SCH 3U Unit 1: Matter, Chemical Trends, Chemical Bonds Notes on Trends in the Periodic Table (pg. 48 – 60) Factor Atomic radius Definition A measurement from the centre of the nucleus to the outermost electron (often in nm where 1 nm = 0.000 000 001 m). [This is a tough measurement to do. It is often estimated as ½ the distance between the nuclei of two atoms of the same element. See Figure 3 pg. 51.] Trend 1) Decreases Right Left across a period. 2) Increases Top Bottom in a group. Explanation 1) As one moves from left to right in a period, one valence electron is added to the same ring (or energy level or shell) with each subsequent atom. Because each electron is added to the same energy level, the amount of screening does not change. At the same time, one more proton is successively added to the nucleus. These changes result in an increased ENC and an increased force of attraction between the nucleus and the electrons. Consequently, the electrons are pulled in closer to the nucleus and the atomic radius decreases. 3Li 4Be 5B Decreasing radius as electrons are added into the same energy level while feeling more EFC from a nucleus with more and more positve protons. Screening stays constant as all valence electrons are screened by two inner electrons.... 2) As one moves from top to bottom in a group, each new element has another ring or energy level with electrons added to it. This means the valence electrons are further and further from the nucleus. The attractive force to the nucleus is screened even more each time another energy level is added. So, the further one goes down the group, the lower the ENC felt by the valence electrons. The electrons are held less and less tightly in the atom and the added energy levels increase its size. Group 1 3Li period 1 increasing atomic radius 11Na period 2 Factor Ionic radius ionization energy Definition A measurement from the centre of the nucleus to the outermost electron in the ion. - the amount of energy required to remove an electron from an atom or an ion in its gaseous state - 1st ionization energy is associated with the removal of the 1st valence electron, 2nd ionization energy with the removal of the second, etc. Generally speaking, ionization energies increase as more electrons are removed. Example: X(g) + 1st ionization energy X1+(g) + 1e1Where 1e1- = 1 free electron Trend 1) Positive ions tend to be smaller than the atoms they came from. As you move L—>R across a period, the more positive the ion becomes and the smaller it is. e.g. Na1+ < Mg2+ < Al3+ 2) Negative ions tend to be larger than the atoms they came from. As you move L R across a period, the more negative the ion becomes and the larger it is. e.g. N3- >O2- > F11) Increases R L across a period. [So, generally, ionization energies for metals are lower than for nonmetals.] 2) Decreases T B in a group Explanation 1) Removal of an electron from a neutral atom reduces the strength of the electron – electron repulsions. This causes the attractive force from the nucleus for the electrons to become strong enough to pull the electrons in more closely. 2) Adding an electron to a neutral atom increases the electron – electron repulsions while holding the nuclear charge the same. The electron – electron repulsions overcome the attractive force between electrons and protons enough to make the ionic radius larger than the radius of the atom it came from. 1) Across a period, nuclear charge increases by increments of one while each new valence electron is added. This means the shielding provided by nonvalence electrons remains the same, so ENC increases and the atomic radius decreases. Thus, the further to the right we go, the more strongly the valence electrons are attracted to the nucleus and the greater the ionization energy. 2) Going down a group, a new energy level is added with each subsequent atom, ensuring the valence electrons are moved further and further from the nucleus. This increases the shielding provided by non-valence electrons, decreases the ENC (even though the number of protons in the nucleus is increasing) and causes the atomic radius to increase. Thus, the further down the group we go, the less strongly the valence electrons are held and the lower the ionization energy. Factor Electronegativity - - Definition a derived number on a scale of 1 – 4 that describes the relative ability of an atom , when bonded, to attract electrons the higher the number, the greater the attraction of the atom for electrons in a bond; the electronegativity value for an atom can change depending on the atom it is going to bond with Trend 1) Increases R L across a period. 2) Decreases T B in a group Electron affinity - the energy involved (either required or released) when an electron is accepted by an atom in its gaseous state Example: X(g) + 1e1- X1+(g) + electron affinity energy Where 1e1- = 1 free electron 1) Increases R L across a period. 2) Decreases T B in a group Chemical reactivity When comparing elements in the same group, chemical reactivity refers to how quickly or violently one element reacts with a given other substance compared to another element. For example, fluorine has a rapid reaction with sodium to form a white solid. Iodine and sodium have to be heated to react. Fluorine and iodine are both group 17 elements. Fluorine is the more chemically reactive. 1) For metals, chemical reactivity tends to increase as one moves T B in a group, 2) For non-metals, chemical reactivity tends to increase as one moves B T in a group. Explanation 1) Across a period, nuclear charge increases by increments of one while each new valence electron is added. This means the shielding provided by nonvalence electrons remains the same, so ENC increases and the atomic radius decreases. Thus, the further to the right we go, the more strongly the valence electrons are attracted to the nucleus and the more likely the atom will pull electrons toward itself in a bond, so the greater the electronegativity value. 2) Going down a group, a new energy level is added with each subsequent atom, ensuring the valence electrons are moved further and further from the nucleus. This increases the shielding provided by non-valence electrons, decreases the ENC (even though the number of protons in the nucleus is increasing) and causes the atomic radius to increase. Thus, the further down the group we go, the less strongly the valence electrons are held and the less likely the atom will pull electrons toward itself in a bond, so the lower the electronegativity value. 1) Across a period, nuclear charge increases by increments of one while each new valence electron is added. This means the shielding provided by nonvalence electrons remains the same, so ENC increases and the atomic radius decreases. Thus, the further to the right we go, the more strongly the valence electrons are attracted to the nucleus so (generally) the more (electron affinity) energy is released. 2) Going down a group, a new energy level is added with each subsequent atom, ensuring the valence electrons are moved further and further from the nucleus. This increases the shielding provided by non-valence electrons, decreases the ENC (even though the number of protons in the nucleus is increasing) and causes the atomic radius to increase. The larger the atom, the smaller the attractive force felt between the nucleus and its valence electrons so adding in another valence electron releases less (electron affinity) energy. 1) When reacting chemically, metals tend to lose one or more valence electrons to form positive ions. Going down a group, a new energy level is added with each subsequent atom, ensuring the valence electrons are moved further and further from the nucleus. This increases the shielding provided by nonvalence electrons, decreases the ENC (even though the number of protons in the nucleus is increasing) and causes the atomic radius to increase. The larger the atom, the smaller the attractive force felt between the nucleus and its valence electrons so the easier it becomes to lose an electron – hence the increase in reactivity as metallic atoms get larger. 2) When reacting chemically, non-metals tend to gain one or more electrons to form negative ions. We know that the smaller the atom, the greater the attractive force is between the valence electrons and the nucleus, the lower the shielding, and the greater the ENC. So – the smaller the non-metallic atom, the more reactive it is.