Therefore the mass number is the sum of the protons and neutrons in the atom. Also, recall that the mass of each proton as well as each neutron is 1 atomic mass unit. Since the mass of an electron is extremely small (negligible) it isn’t used in computation of the mass number. The mass number of an atom, as the name implies, tells the total mass of the atom. For example carbon would be expressed as 6C. In chemical notation the atomic number for an element is expressed as a subscript preceding the symbol for the element. In addition, since atoms have a neutral charge, the atomic number also tells us the number of electrons in the atom. Therefore, all hydrogen atoms have 1 proton and all carbon atoms have 6 protons. The significance of the atomic number is that it tells us the number of protons in the nucleus of each element. For carbon, the atomic number is 6 and, again, no other element has an atomic number of 6. No other element has an atomic number of 1. For example, the atomic number for hydrogen is 1. This number is the atomic number for the element and is unique for each different element. Take a look at the periodic table again and notice the number at the top of each box. Image created by BYU-I student Hannah Crowder Fall 2013 Atomic Number It has been suggested that if the nucleus were the size of a basketball the electrons would be about six kilometers or 3¾ miles away!) Electrons (blue) will be found somewhere within these orbitals. The shaded areas around the nucleus represent the electron orbitals (clouds). The nitrogen nucleus contains 7 protons (orange) and 7 neutrons (green). The image below represents our current model of a Nitrogen atom. For simplicity, however, we often think of these as satellite-like circular orbitals. This area is often referred to as the electron “cloud.” True, it is still a specific area, but it is a bit more amorphous than a spherical orbit. These “orbitals” are actually areas in space around the nucleus where the electrons will be located most of the time. Although we often describe the electrons as residing in orbits that circle the nucleus, like planets orbiting the sun, modern physics teaches us that this model is incorrect. Electrons move around the nucleus at tremendously high speeds, actually travelling at near the speed of light. They are so small that for practical purposes they do not contribute to the mass of the atom. Electrons have a negative charge but are extremely small and have a mass only 1/1850 that of a proton or neutron. The third type of stable particle is the electron. Neutrons and protons constitute almost all of an atom’s mass. Although the mass of the neutron is slightly greater than that of a proton we can assign both of them the relative mass of 1 (1 atomic mass unit or amu). We refer to these atoms as being radioactive. Too many or too few neutrons may result in an atomic nucleus that is unstable and may decay to form other elements. Neutrons bind with protons in a way that helps stabilize the nucleus. Neutrons have no charge, have mass, and are also located in the nucleus of the atom. Protons are positively charged particles, have mass, and are located in the center, or nucleus of the atom. These are the protons, neutrons, and electrons. Physicists have succeeded in blasting atoms apart into dozens of different sub-atomic particles, however, only 3 of them are stable. Therefore all of the atoms that make up the element carbon have the same chemical properties. Chemical properties include the physical state of the element (gas, liquid, or solid), the types of bonds the element can form, how it reacts with other elements, etc. We can define an atom as the simplest particle of an element that has the chemical properties of that element. Inorganic Chemistry (Atomic Structure) AtomsĪll elements are composed of extremely small particles of matter called atoms.
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