(This isotope is known as “carbon-12” as will be discussed later in this module.) Thus, one amu is exactly 1 12 1 12 of the mass of one carbon-12 atom: 1 amu = 1.6605 × × 10 −24 g. Since 1961, it has been defined with regard to the most abundant isotope of carbon, atoms of which are assigned masses of exactly 12 amu.
The amu was originally defined based on hydrogen, the lightest element, then later in terms of oxygen. When describing the properties of tiny objects such as atoms, we use appropriately small units of measure, such as the atomic mass unit (amu) and the fundamental unit of charge (e). For example, a carbon atom weighs less than 2 × × 10 −23 g, and an electron has a charge of less than 2 × × 10 −19 C (coulomb). (credit middle: modification of work by “babyknight”/Wikimedia Commons credit right: modification of work by Paxson Woelber)Ītoms-and the protons, neutrons, and electrons that compose them-are extremely small. For a perspective about their relative sizes, consider this: If the nucleus were the size of a blueberry, the atom would be about the size of a football stadium ( Figure 2.11).įigure 2.11 If an atom could be expanded to the size of a football stadium, the nucleus would be the size of a single blueberry. The diameter of an atom is on the order of 10 −10 m, whereas the diameter of the nucleus is roughly 10 −15 m-about 100,000 times smaller. The nucleus contains the majority of an atom’s mass because protons and neutrons are much heavier than electrons, whereas electrons occupy almost all of an atom’s volume. It was learned that an atom contains a very small nucleus composed of positively charged protons and uncharged neutrons, surrounded by a much larger volume of space containing negatively charged electrons. The development of modern atomic theory revealed much about the inner structure of atoms. Calculate average atomic mass and isotopic abundance.Define the atomic mass unit and average atomic mass.Write and interpret symbols that depict the atomic number, mass number, and charge of an atom or ion.
This means that the most electronegative atom is Fluorine and the least electronegative is Francium.By the end of this section, you will be able to: The trend for electronegativity is to increase as you move from left to right and bottom to top across the periodic table. The main properties of an atom dictate it's electronegativity are it's atomic number as well as its atomic radius. The higher the electronegative of an element, the more that atom will attempt to pull electrons towards itself and away from any atom it bonds to. Linus Pauling described electronegativity as “the power of an atom in a molecule to attract electrons to itself.” Basically, the electronegativity of an atom is a relative value of that atom's ability to attract election density toward itself when it bonds to another atom. In 1936, Linus Pauling came up a method for estimating atomic electronegativities forms the basis of our understanding of electronegativity today. To quantify how much ionic character (and how much covalent character) a bond possesses, electronegativity differences between the atoms in the bond can be used. Generally, however, bonds are partially covalent and partially ionic, meaning that there is partial transfer of electrons between atoms and partial sharing of electrons. Only homonuclear bonds are truly covalent, and nearly perfect ionic bonds can form between group I and group VII elements, for example, KF. Most real chemical bonds in nature are neither truly covalent nor truly ionic. Recall that a lowercase Greek delta (\(δ\)) is used to indicate that a bonded atom possesses a partial positive charge, indicated by \(δ^+\), or a partial negative charge, indicated by \(δ^−\), and a bond between two atoms that possess partial charges is a polar bond. \) compares the electron distribution in a polar covalent bond with those in an ideally covalent and an ideally ionic bond.
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