Chapter 2 PDF

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Chapter 2...

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Chapter 2 1. Define atom, element, molecule, compound 2. Identify the four major elements of living things 3. Distinguish between the following pairs of terms: neutron and proton, atomic number and atomic weight (mass number). 4. Describe the structure of an atom. Know what orbitals and shells are, and understand the connection between electron configuration and reactivity. 5. Distinguish between and discuss the biological importance of the following: non-polar covalent bonds, polar covalent bonds, ionic bonds, hydrogen bonds, and van der Waals interactions Matter- anything that takes up space and has mass. Element- is a substance that cannot be broken down to other substances by chemical reactions. Compound- is a substance consisting of two or more different elements combined in a fixed ratio. Four elements that make up living matter are oxygen, carbon, hydrogen, and nitrogen. 96 PERCENT. Atom- is the smallest unit of matter that retains properties of an element. Protons and electrons are electrically charged. Each proton has one unit of positive charge, and each electron has one unit of negative charge. A neutron, as its name implies, is electrically neutral. Protons and neutrons are packed together tightly in a dense core, or atomic nucleus, at the center of an atom; protons give the nucleus a positive charge The neutron and proton are almost identical in mass, each about 1.7 * 10-24 gram (g). A dalton is same as an amu. Protons and neutrons have a mass close to 1 dalton. And the mass of an electron is 1/2000 of a proton. This number of protons, which is unique to that element, is called the atomic number and is written as a subscript to the left of the symbol for the element. The abbreviation 2He, for example, tells us that an atom of the element helium has 2 protons in its nucleus. The atomic number tells you the amount of protons and electrons. The mass number tells you the total amount of protons and neutrons in an element. It is written as a superscript. 4-4 He. All atoms of a given element have the same number of protons, but some atoms have more neutrons than other atoms of the same element and therefore have greater mass. These different atomic forms of the same element are called isotopes of the element. ] Although the isotopes of an element have slightly different masses, they behave identically in chemical reactions. A radioactive isotope is one in which the nucleus decays spontaneously, giving off particles and energy. When the radioactive decay leads to a change in the number of protons, it transforms

the atom to an atom of a different element. For example, when an atom of carbon-14 ( 14C) decays, it becomes an atom of nitrogen (14N). Radioactive isotopes have many useful applications in biology Radioactive isotopes are often used as diagnostic tools in medicine. For example, certain kidney disorders are diagnosed by injecting small doses of radioactively-labeled substances into the blood and then analyzing the tracer molecules excreted in the urine. Radioactive isotopes are very useful in biological research and medicine, radiation from decaying isotopes also poses a hazard to life by damaging cellular molecules. When two atoms approach each other during a chemical reaction, their nuclei do not come close enough to interact. Of the three subatomic particles we have discussed, only electrons are directly involved in chemical reactions. The electrons of an atom have potential energy due to their distance from the nucleus. charged electrons are attracted to the positively charged nucleus. It takes work to move a given electron farther away from the nucleus, so the more distant an electron is from the nucleus, the greater its potential energy. An electron’s energy level is correlated with its average distance from the nucleus. Electrons are found in different electron shells, each with a characteristic average distance and energy level. The first shell is closest to the nucleus, and electrons in this shell have the lowest potential energy. Electrons in the second shell have more energy, and electrons in the third shell even more energy. An electron can move from one shell to another, but only by absorbing or losing an amount of energy equal to the difference in potential energy between its position in the old shell and that in the new shell. The chemical behavior of an atom depends mostly on the number of electrons in its outermost shell. We call those outer electrons valence electrons and the outermost electron shell the valence shell. In the case of lithium, there is only 1 valence electron, and the second shell is the valence shell. Atoms with the same number of electrons in their valence shells exhibit similar chemical behavior. For example, fluorine (F) and chlorine (Cl) both have 7 valence electrons, and both form compounds when combined with the element sodium (Na): Sodium fluoride (NaF) is commonly added to toothpaste to prevent tooth decay,  

An atom with a completed valence shell, like neon, is nonreactive. All other atoms are chemically reactive because they have incomplete valence shells.

A covalent bond is the sharing of a pair of valence electrons by two atoms.\ When the bonds form, they give the atom a full complement of electrons in the valence shell. The bonding capacity of oxygen, for example, is 2. This bonding capacity is called the atom’s valence and usually equals the number of unpaired electrons required to complete the atom’s outermost (valence) shell The molecules H2 and O2 are pure elements rather than compounds because a compound is a combination of two or more different elements. Water, with the molecular formula H2O, is a compound.

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Electron configuration influences the chemical behavior of an atom. Simplified models of the atom greatly distort the atom’s relative dimensions. To gain an accurate perspective of the relative proportions of an atom, if the nucleus was the size of a golf ball, the electrons would be moving about 1 kilometer from the nucleus. o Atoms are mostly empty space. When two elements interact during a chemical reaction, it is actually their electrons that are involved. The nuclei do not come close enough to interact. The electrons of an atom vary in the amount of energy they possess. Energy is the ability to do work. Potential energy is the energy that matter stores because of its position or location. o Water stored behind a dam has potential energy that can be used to do work turning electric generators. o Because potential energy has been expended, the water stores less energy at the bottom of the dam than it did in the reservoir. Electrons have potential energy because of their position relative to the nucleus. o The negatively charged electrons are attracted to the positively charged nucleus. o The farther electrons are from the nucleus, the more potential energy they have. Changes in an electron’s potential energy can only occur in steps of a fixed amount, moving the electron to a fixed location relative to the nucleus. o An electron cannot exist between these fixed locations. The different states of potential energy that the electrons of an atom can have are called energy levels or electron shells. o The first shell, closest to the nucleus, has the lowest potential energy. o Electrons in outer shells have more potential energy. o Electrons can change their position only if they absorb or release a quantity of energy that matches the difference in potential energy between the two levels. The chemical behavior of an atom is determined by its electron configuration—the distribution of electrons in its electron shells. o The first 18 elements, including those most important in biological processes, can be arranged in 8 columns and 3 rows.  Elements in the same row fill the same shells with electrons.  Moving from left to right, each element adds one electron (and proton) from the element before. The first electron shell can hold only 2 electrons. o The two electrons of helium fill the first shell. Atoms with more than two electrons must place the extra electrons in higher shells. o For example, lithium, with three electrons, has two in the first shell and one in the second shell. The second shell can hold up to 8 electrons. o Neon, with 10 total electrons, has two in the first shell and eight in the second, filling both shells. The chemical behavior of an atom depends mostly on the number of electrons in its outermost shell, the valence shell. o Electrons in the valence shell are known as valence electrons. o Lithium has one valence electron; neon has eight. Atoms with the same number of valence electrons have similar chemical behaviors. An atom with a completed valence shell, like neon, is nonreactive.

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All other atoms are chemically reactive because they have incomplete valence shells. The paths of electrons are often portrayed as concentric paths, like planets orbiting the sun. In reality, an electron occupies a more complex three-dimensional space, an orbital. The orbital represents the space in which the electron is found 90% of the time. o Each orbital can hold a maximum of two electrons. o The first shell has room for a single spherical 1s orbital for its pair of electrons. o The second shell can pack pairs of electrons into a spherical 2s orbital and three dumbbell-shaped 2p orbitals. The reactivity of atoms arises from the presence of unpaired electrons in one or more orbitals of their valence shells. o Electrons occupy separate orbitals within the valence shell until forced to share orbitals.  The four valence electrons of carbon each occupy separate orbitals, but the five valence electrons of nitrogen are distributed into three unshared orbitals and one shared orbital.

When atoms interact to complete their valence shells, it is the unpaired electrons that are involved....