Ch.7 - Periodic Properties of the Elements

Problem 106

Chapter 7, Problem 106

A historian discovers a nineteenth-century notebook in which some observations, dated 1822, were recorded on a substance thought to be a new element. Here are some of the data recorded in the notebook: 'Ductile, silver-white, metallic looking. Softer than lead. Unaffected by water. Stable in air. Melting point: 153 °C. Density: 7.3 g>cm3. Electrical conductivity: 20% that of copper. Hardness: About 1% as hard as iron. When 4.20 g of the unknown is heated in an excess of oxygen, 5.08 g of a white solid is formed. The solid could be sublimed by heating to over 800 °C.' (a) Using information in the text and the CRC Handbook of Chemistry and Physics, and making allowances for possible variations in numbers from current values, identify the element reported.

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Identify the physical properties of the unknown element: ductile, silver-white, metallic, softer than lead, unaffected by water, stable in air, melting point of 153 °C, density of 7.3 g/cm³, electrical conductivity 20% of copper, hardness 1% of iron.

Compare these properties with known elements from the CRC Handbook of Chemistry and Physics, focusing on elements with similar melting points, densities, and conductivity.

Consider the chemical reaction: 4.20 g of the unknown element reacts with oxygen to form 5.08 g of a white solid. Calculate the mass of oxygen that reacted by subtracting the mass of the unknown from the mass of the product.

Determine the molar mass of the unknown element by using the mass of the unknown and the mass of oxygen to find the empirical formula of the oxide formed.

Cross-reference the calculated molar mass and empirical formula with known elements to identify the element that matches the properties and reaction data.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Physical Properties of Metals

The observations recorded in the notebook describe several physical properties of the unknown element, such as ductility, density, and melting point. Metals typically exhibit high electrical conductivity, malleability, and a shiny appearance. Understanding these properties helps in identifying the element, as they can be compared to known metals to narrow down possibilities.

Chemical Reactivity and Oxidation

The reaction of the unknown element with oxygen, resulting in a white solid, indicates its chemical reactivity. The formation of a solid product upon heating in excess oxygen suggests that the element may form an oxide. Analyzing the mass change during the reaction can provide insights into the element's identity and its oxidation state.

Density and Element Identification

Density is a critical physical property used to identify elements. The recorded density of 7.3 g/cm³ can be compared to known densities of metals to help pinpoint the unknown element. Since density is a characteristic property, it can significantly narrow down the list of potential candidates when combined with other observed properties.

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Related Practice

Open Question

Elemental barium reacts more violently with water than does elemental calcium. Which of the following best explains this difference in reactivity? (i) Calcium has greater metallic character than does barium. (ii) The electron affinity of calcium is smaller than that of barium. (iii) The first and second ionization energies of barium are less than those of calcium. (iv) The atomic radius of barium is smaller than that of calcium. (v) The ionic radius of the barium ion is larger than that of the calcium ion.

Open Question

(a) One of the alkali metals reacts with oxygen to form a solid white substance. When this substance is dissolved in water, the solution gives a positive test for hydrogen peroxide, H2O2. When the solution is tested in a burner flame, a lilac-purple flame is produced. What is the likely identity of the metal? (b) Write a balanced chemical equation for the reaction of the white substance with water.

Open Question

Zinc in its 2+ oxidation state is an essential metal ion for life. Zn2+ is found bound to many proteins that are involved in biological processes, but unfortunately, Zn2+ is hard to detect by common chemical methods. Therefore, scientists interested in studying Zn2+-containing proteins frequently substitute Cd2+ for Zn2+, since Cd2+ is easier to detect. On the basis of the properties of the elements and ions discussed in this chapter and their positions on the periodic table, describe the pros and cons of using Cd2+ as a Zn2+ substitute. Proteins that speed up (catalyze) chemical reactions are called enzymes. Many enzymes are required for proper metabolic reactions in the body. One problem with using Cd2+ to replace Zn2+ in enzymes is that Cd2+ substitution can decrease or even eliminate enzymatic activity. Can you suggest a different metal ion that might replace Zn2+ in enzymes instead of Cd2+? Justify your answer.

Open Question

In April 2010, a research team reported that it had made Element 117. This discovery was confirmed in 2012 by additional experiments. Write the ground-state electron configuration for Element 117 and estimate values for its first ionization energy, electron affinity, atomic size, and common oxidation state based on its position in the periodic table.

Textbook Question

We will see in Chapter 12 that semiconductors are materials that conduct electricity better than nonmetals but not as well as metals. The only two elements in the periodic table that are technologically useful semiconductors are silicon and germanium. Integrated circuits in computer chips today are based on silicon. Compound semiconductors are also used in the electronics industry. Examples are gallium arsenide, GaAs; gallium phosphide, GaP; cadmium sulfide, CdS; and cadmium selenide, CdSe. (a) What is the relationship between the compound semiconductors' compositions and the positions of their elements on the periodic table relative to Si and Ge?

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Textbook Question

We will see in Chapter 12 that semiconductors are materials that conduct electricity better than nonmetals but not as well as metals. The only two elements in the periodic table that are technologically useful semiconductors are silicon and germanium. Integrated circuits in computer chips today are based on silicon. Compound semiconductors are also used in the electronics industry. Examples are gallium arsenide, GaAs; gallium phosphide, GaP; cadmium sulfide, CdS; and cadmium selenide, CdSe. (b) Workers in the semiconductor industry refer to 'II–VI' and 'III–V' materials, using Roman numerals. Can you identify which compound semiconductors are II–VI and which are III–V?

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