Chemistry Midterm Essay
Explain how a pure metal is held together. Include a definition of a metallic bond in your explanation.
- A piece of pure metal, such as copper or iron, consists not of metal atoms, but of closely packed cations. The cations are surrounded by mobile valence electrons that are free to drift from one part of the metal to another. Metallic bonds result from the attraction between the free-floating valence electrons and the positively charged metal ions.
Describe the trends in the atomic size of elements within groups and across periods in the periodic table. Provide examples
- Atomic size increases with increasing atomic number within a group. For example, sodium atoms are larger than lithium atoms, and potassium atoms are larger than sodium atoms. Atomic size decreases with increasing atomic number across a period. For example, lithium atoms are larger than beryllium atoms, and beryllium atoms are larger than boron atoms.
Describe the trends in electronegativity within groups and across periods in the periodic table. Provide examples
- Electronegativity values decrease from top to bottom within a group, and from right to left across a period. For example, rubidium is less electronegative than lithium. Lithium is less electronegative than fluorine.
Explain how ions form. Provide examples
- Ions form when electrons are transferred among atoms. For example, a group 1A element, such as potassium, tends to transfer one electron to other atoms, causing it to form a net positive charge, in this case K . Such ions with net positive charges are called cations. Ions with net negative charges are anions. Nonmetal atoms, such as chlorine, tend to accept electrons from other atoms. Chlorine tends to gain a single electron, forming the anion Cl .
Explain how the atoms of one element differ from those of another element
- Isotopes of the same element have different numbers of neutrons, and therefore, different mass numbers and different atomic masses. Isotopes of the same element have the same number of protons and electrons. The electrons, not the neutrons, are responsible for an element's chemical behavior.
What observations by Rutherford led to the hypothesis that atoms are mostly empty space, and that almost all of the mass of the atom is contained in an atomic nucleus?
- Rutherford's gold-foil experiment led to this hypothesis. Alpha particles were observed to mostly pass through a gold foil, which suggests that the volume of individual gold atoms consists mainly of empty space. The observation that some alpha particles were scattered at large angles led to the suggestion that the gold atom has a central core, or nucleus, composed of a concentrated mass capable of deflecting the alpha particles.
Positive ions are smaller than the atoms from which they are formed, but negative ions are larger than the atoms from which they are formed. Explain why this is so.
- When an electron is added to an atom, the attraction of the nucleus for any one electron decreases and the size of the ion's radius increases. When an electron is removed from an atom, there is an increase in the nuclear attraction experienced by the remaining electrons. Consequently, the remaining electrons are drawn closer to the nucleus.
In what way are two isotopes of the same element different? Explain why isotopes of the same element have the same chemical behavior.
Isotopes of the same element have different numbers of neutrons and therefore, different mass numbers and different atomic masses. Isotopes of the same element have the same number of portones and electrons. The electrons, not the neutrons are responsible for an element's chemical behavior.
Explain the octet rule and give an example of how it is used
The electron configurations of the noble gases are extremely stable. The octet rule states that, in chemical reactions, elements gain or lose electrons to achieve a noble gas configuration. This stable configuration is called an octet because it consists of 8 valence electrons (s p ), 2 from the outermost s orbital and 6 from the outermost p orbital. Oxygen has the electron configuration 1s 2s 2p . When oxygen reacts to form ionic compounds, it completes its octet by gaining two electrons from the element it reacts with. These two electrons add to the p orbital of oxygen, giving it the electron configuration (1s 2s 2p ) of neon.
Why is it possible to calculate the density of a gas at STP, knowing only its molar mass, but it is not possible to make the same calculation for a solid or a liquid?
The molar volume of any gas is 22.4L at STP. The density can be calculated in the following way: molar mass of ANY gas (g)/1 mol/22.4L=density of gas. The molar volumes of different solids and liquids are not uniformly the same at any prescribed conditions.
In what way are two isotopes of the same element different? Explain why isotopes of the same element have the same chemical behavior.
a pseudo-noble-gas configuation is a form s2 p6 d10. It has 18 electrons in the outer energy level and is a relatively stable configuration. Examples of ions with this configuration ar Ag+, Cu+, Cd2+ and Hg2+.
