Earth Lab: Atomic Structure By Trista L. Pollard When scientists design models of atoms, they usually show a simplified version of the atom's nucleus and its subatomic particles. The nucleus is made up of protons and neutrons (picture red and blue gumballs stuck together) with electrons moving at high speeds around the outside of the nucleus (imagine gumballs on a circular wire). Over the years, scientists have found out that the highly fast-moving electrons form a cloud around the nucleus. In fact, within this electron cloud, the electrons are spaced at different distances from the nucleus. These areas of electrons are called energy levels or shells. Each shell can only have a certain number of electrons. Since electrons are negatively charged and opposite charges attract, the electrons are attracted to the nucleus, which has a positive charge. It is this attraction that keeps the electrons inside the atom. Speaking of the nucleus, you should know that most of the atom's mass is the nucleus. However, the nucleus's size is extremely small compared to the size of the atom. Most of the atom's volume is empty space, due to the extremely small size of the electrons, protons, and neutrons. 1 Scientists use information in the atom to describe elements. The nucleus of an atom has a specific number of protons. This number of protons determines the atom's atomic number and the name of the element. An element's atomic number distinguishes it from other elements. Atoms that are "free" or uncharged have the same number of electrons as protons. Therefore an atom's atomic number is also equal to the number of electrons within that atom. If you think about it, an element is a huge collection of atoms (with neutral charges) that all have the same atomic number. Remember the name of the element? Well, all atoms with six protons are classified as carbon atoms, whereas all oxygen atoms have eight protons. All elements are organized on a periodic table of elements. Elements with similar arrangements of electrons in their atoms are put together in the same column of the periodic table or groups. All of the elements that are part of the group have similar chemical properties. Below is a diagram explaining how elements appear on the periodic table. 2 3 An atom's atomic mass refers to the sum of its protons and neutrons. Subatomic particles are too small to define the mass in units of grams. Therefore, atomic mass unit (amu) is used to describe the mass of protons, neutrons, and electrons. The mass of a proton and a neutron is about 1 amu each. Electrons, which are much smaller than protons and neutrons, are not included in the calculated atomic mass of an atom. In fact, if you were to compare the mass of electrons with the mass of protons, you would need about 1,840 electrons together to equal the mass of one proton. 4 Even though all atoms of an element have the same number of protons, they may have different amounts of neutrons. These atoms are called isotopes. Isotopes will have the same atomic number as other atoms of the same element. However, they will have a different atomic mass due to the different number of neutrons. Isotopes of a specific element will have different properties. To account for the different masses of an element's isotopes, the periodic table uses average atomic mass. The average atomic mass of each element is the weighted average of the three naturally occurring isotopes of that element. For example, there are two isotopes for carbon: carbon-12 and carbon-14. Since carbon-12 is more common, the average atomic mass on the periodic table is about 12 amu. Remember, all carbon atoms have six protons. This means that carbon-12 has six neutrons and carbon-14 has eight neutrons. The diagram below shows one of the naturally occurring isotopes for Hydrogen. 5 6 Although isotopes of the same element have different amounts of neutrons, they are chemically almost identical. Isotopes of an element react the same during chemical reactions. In fact, different isotopes from the same element may end up in the same mineral. (We'll talk about minerals later.) Keep in mind that isotopes may be similar, but they also have properties of their own. 7 1. The areas of electrons in electron clouds are called ______. Stages or views Shells or energy levels Power levels or valences All of the above 3. If the element Helium (He) has two protons and two neutrons, what are the atomic number and the atomic mass? 2. Scientists sometimes compare the orbiting of electrons around the nucleus of an atom to the planets orbiting around the sun. Explain the comparison. 4. Isotopes of elements have the same number of protons but a different number of ______. Electrons Amu's Charges Neutrons 7. Based on the diagram, how many 8. What is the difference between protons, neutrons, and electrons does this element's atom contain? the atoms of these two isotopes for sodium?