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TOPIC 2 ELEMENTAL & ENVIRONMENTAL CHEMISTRY

TOPIC 2 : ELEMENTAL AND ENVIRONMENTAL CHEMISTRYThis topic deals with some underlying principles of chemistry and then considers the chemistry of the environment. The elemental chemistry focuses on the periodic table and the concept of electronegativity, which underlie most of the other topics in this curriculum statement. The environmental chemistry focuses on a small number of inorganic molecular substances and their impact on the environment. When the chemical elements are arranged in a periodic table, similarities and trends in properties become apparent. This topic examines the properties of compounds and elements. These properties can be explained in terms of the electronegativities of the elements and their positions in the periodic table. The effects of human beings on the environment have not always been for the better. In the last hundred years concern about these effects has extended from local to global matters. Students are often exposed to environmental concerns about life on Earth, sometimes presented in emotive language. In this topic students are exposed to factual information, and consider causes and solutions of environmental problems.

Subtopic 2.1: The Periodic TableKey IdeasThe arrangement of electrons in any atom can be described in terms of shells and subshells.

Intended Student LearningWrite, using subshell notation, the electron configuration for an atom or monatomic ion of any of the first thirty-eight elements in the periodic table.

The position of an element in the periodic table reflects Identify the s, p, d, and f block elements in the its electron configuration. periodic table. The periodic table is the unifying framework for the study of the chemical elements and their compounds. Elements within each group of the periodic table have similar chemical properties that can be explained in terms of their similar outer-shell electron configurations. Predict the following properties of the s and p block elements of any of the first thirty-eight elements in the periodic table: Metal, metalloid, or non-metal nature of the element. Charge of the monatomic ions. Likely oxidation state(s) of the element in its compounds (including octet expansion for phosphorus, sulfur, and chlorine). Find regions with elements of high, intermediate, and low electronegativity in the periodic table. Predict the acidicbasic character of the oxides of an element from the position of the element in the periodic table. Write equations for the reactions of oxides of non-metals such as SiO2, CO2, SO2, SO3, and P4O10 with hydroxide ions and with water, where a reaction occurs. Write equations for the reactions of oxides of metals such as MgO, Na2O, CuO, and Fe2O3 with acids and with water, where a reaction occurs.

The electronegativities of non-metallic atoms are higher than those of metals; non-metallic atoms tend to gain electrons in chemical reactions. The trend from metallic to non-metallic behaviour across a period is related to the increase in electronegativity. These trends are reflected in changes in the acidicbasic character of the oxides. The oxides of non-metals are acidic. Their acidic character can be displayed by reaction with hydroxide ions to produce an oxyanion and, in most cases, by reaction with water to produce an oxyacid. The oxides of metals are basic. Their basic character can be displayed by reaction with an acid to produce a cation and, in some cases, by reaction with water to produce OH in solution.

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TOPIC 2 ELEMENTAL & ENVIRONMENTAL CHEMISTRY

Key Ideas

Intended Student Learning

Metalloids form amphoteric oxides. Amphoteric oxides Write equations for the reaction of an amphoteric can display basic character by reaction with hydrogen oxide such as Al2O3, ZnO with hydrogen ions or ions and acidic character by reaction with hydroxide hydroxide ions. ions. Small molecules are formed from elements in a small Predict whether or not a compound or element is section of the periodic table. Small molecules are those likely to be molecular, given its properties, name, either of non-metallic elements or of compounds of elemental composition, or formula. non-metallic elements. Atoms in a molecule are bound strongly to each other by covalent bonds. Molecules interact weakly with each other. The strengths of secondary interactions between non-polar molecules depend on their molar mass. Compare the strengths of covalent bonds with the strengths of secondary interactions. Explain the higher melting-points and boiling-points of substances of large molar mass.

The shape of molecules can be explained and predicted Draw diagrams showing covalent bonds, by repulsion between pairs of bonding and non-bonding pairs, and shapes for two-element non-bonding electrons. molecules and ions containing no more than five atoms. Examples that involve valence shell octet tetrahedra, SO2 expansion are limited to and SO3. The polarity of a molecule results from the polar character of the bonds and their spatial arrangement. Predict whether or not a molecule is polar, given its spatial arrangement.

The strengths of secondary interactions between Explain the higher melting-points and molecules of similar molar mass depend on the polarity boiling-points of polar substances when of the molecules. compared with those of non-polar substances of similar molar mass. Molecules containing NH or OH groups can form hydrogen bonds to N or O atoms in other molecules. 1. Electron Configurations (up to atomic number 38) (a) The arrangement of electrons in any atom can be described in terms of shells and subshells. Each electron in an atom, or monotomic ion, has a certain amount of potential energy arising from the attraction between its negative charge and the Main Shell Subshells positive charge of the nucleus 1 1s Electrons in the atoms or monatomic ions of a particular element have energy values that are unique to that 2 2s element, and these electrons are said to exist at certain 2p energy levels. 3 3s Each allowed energy level is represented by a main shell 3p number (1,2,3 etc) and a subshell represented by the 3d letters s, p, d, f or g. For each main shell (n), there are n 4 4s subshells. The subshells for the first four shells are 4p shown in the table. 4d 4f Describe, with the aid of diagrams, hydrogen bonding between molecules.

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TOPIC 2 ELEMENTAL & ENVIRONMENTAL CHEMISTRY

(b)

When writing electron configurations, the following principles must be observed:y

in the most stable state of any atom (or ion), the electrons occupy the lowest available energy level. They are allocated to subshells in order of increasing energy as showing in the following energy sequence 1s 2s 2p 3s 3p 4s 3d 4p 5s lower energy p higher energy for each subshell, there is a maximum number of electrons which can occupy that subshell as shown below Subshell Maximum number of electrons s p d f 2 6 10 14

y

A diagram to illustrate the sequence is shown below.

(c)

Using subshell notation The electron configuration is written in energy sequence order the main shell number is written, followed by the letter for the subshell and then the number of electrons (as a superscript). This is illustrated in the following examples:y y y

sodium (11 electrons) iron (26 electrons) strontium (38 electrons) chromium (24 electrons) copper (29 electrons)

1s2 2s2 2p6 3s1 1s2 2s2 2p6 3s2 3p6 4s2 3p6 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 1s2 2s2 2p6 3s2 3p6 4s1 3d5 1s2 2s2 2p6 3s2 3p6 4s1 3d10

Note that chromium and copper do not conform to the principles exactlyy y

(d)

Using subshell notation to write electron configurations for monatomic ions (i) (ii) For positive ions of the main group elements determine the number of electrons left. eg Ca2+ has lost 2 electrons and has 18 left and so is 1s2 2s2 2p6 3s2 3p6 For negative ions of the main group elements, determine the total number of electrons. eg the S2- has gained 2 electrons and so has 18 electrons so its configuration is 1s2 2s2 2p6 3s2 3p6. 3

TOPIC 2 ELEMENTAL & ENVIRONMENTAL CHEMISTRY

(iii)

For positive ions of the transition metals (eg Fe2+ and Fe3+) write the electron configuration of the atom eg Fe (26 electrons) - 1s2 2s2 2p6 3s2 3p6 4s2 3d6 Decrease the number electrons equal to the number of positive charges on the ion (ie 2 for Fe2+ and 3 for Fe3+), by removing 4s electrons first, with any further deletions being made from the 3d subshell. Thus Fe2+ (24 electrons) becomes 1s2 2s2 2p6 3s2 3p6 3d6 and Fe3+ (23 electrons) becomes 1s2 2s2 2p6 3s2 3p6 3d5

EXERCISE 11. Write the electron configuration of the following elements (a) (b) (c) (d) 2. (a) (b) (c) (d) 2. Lithium Nitrogen Magnesium Sulphur Beryllium Oxygen Sodium Chlorine (e) (f) (g) (h) (e) (f) (g) (h) Argon Manganese Arsenic Rubidium Potassium Manganese II Copper I Bromine

Write the electron configuration of the following ions

Electron configuration and the Periodic table (a) An elements position in the Periodic Table is determined by its electron configuration. (i) (ii) Horizontal rows are called Periods and the number of the period in which an element is placed is equal to the highest numbered main shell that is occupied by electrons. Vertical columns are called Groups (the main groups are normally numbered with Roman Numerals) and the number of electrons occupying its highest numbered main (or outer) shell determines the group number (for main group elements). Those with incomplete 3d subshells go into a group called the transition elements.

ExamplesPhosphorus (15 electrons) - 1s2 2s2 2p6 3s2 3p3 has electrons in the 3rd shell and is in Period 3 and has 5 electrons in that main shell so is in Group V. Bromine (35 electrons) - 1s2 2s2 2p6 3s2 3p