A neutron is a slightly heavier particle with a mass 1.0087 amu and a charge of zero as its name suggests, it is neutral.
(The Dalton (Da) and the unified atomic mass unit (u) are alternative units that are equivalent to the amu.) The fundamental unit of charge (also called the elementary charge) equals the magnitude of the charge of an electron (e) with e = 1.602 × 10 −19 C.Ī proton has a mass of 1.0073 amu and a charge of 1+. (This isotope is known as “carbon-12” as will be discussed later in a discussion of isotopes.) Thus, one amu is exactly \frac 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). Helium atom is comprised of two protons and two neutrons located in the nucleus and two electrons distributed over most the atom’s volume.Ītoms-and the protons, neutrons, and electrons that compose them-are extremely small. Graphical depiction of the electron cloud for the helium (He) atom. More details about the energies and distributions of electrons in the atom will be discussed in a later module on light, matter, and atomic structure. We use the term “electron cloud” because the exact positions of the electrons cannot be known, we simply know that they are distributed about the volume of the atom ( Figure 2). Most of the volume of an atom is occupied by the “cloud” of electrons. (credit middle: modification of work by “babyknight”/Wikimedia Commons credit right: modification of work by Paxson Woelber) If an atom could be expanded to the size of a football stadium, the nucleus would be the size of a single blueberry. A third sub-atomic component that bears no electrical charge-the neutrons (n 0)-have masses similar to protons and co-locate with protons in the nucleus.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 1). The surprising results of Rutherford, which we will explore in more detail in the next section, indicated that the positively charged subatomic particles-the protons (p +)-are much more massive than electrons (by a factor of about 2,000) and are highly concentrated in the atom’s nucl e us, which occupies just a tiny portion of the atom’s entire volume.
We designate electrons with the symbol e –, the superscript emphasizes their negative charge. Scientists established that the atom was not indivisible and due to the work of Thomson, Millikan, and others, the charge and mass of the negative, subatomic particles-the electrons-were known. | Key Concepts and Summary | Glossary | End of Section Exercises | The Structure of the Atom In the early part of the 20th century much of our current understanding of atoms and atomic structure had been established. Know the names and symbols of certain elements.Recognize the atom is comprised of protons, neutrons, and electrons distinguish among these building blocks.
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