Unit 2.4 Understanding the Elements Listed on the Periodic Table

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1 Unit 2.4 Understanding the Elements Listed on the Periodic Table How many gallons of gas can fill a car's tank? The current system of measurement in the Unites States is a hodge-podge of different units, many of which are hard to interconvert, unlike the metric system. We at least have standardized units these days, unlike centuries past. At one time, measurements of length often were defined as the distance from the end of one appendage to another. For example, the yard would be defined as the distance from the king s nose to the tip of his thumb when his arm was stretched out. Standardized measurements make it possible for people everywhere to get the same amount of something. Note the red and white labels on the gas pump above. These labels certify that the gas pump has been checked and is pumping an accurate gallon of gas. Standard measurements in science are very important so that we can compare experimental data from one lab to another and make sure we all are talking about the same thing. Atomic Mass Units Masses of individual atoms are very, very small. Using a modern device called a mass spectrometer, it is possible to measure such minuscule masses. An atom of oxygen-16, for example, has a mass of g. While comparisons of masses measured in grams would have some usefulness, it is far more practical to have a system that will allow us to more easily compare relative atomic masses. Scientists decided on using the carbon-12 nuclide as the reference standard by which all other masses would be compared. By definition, one atom of carbon-12 is assigned a mass of 12 atomic mass units (amu). An atomic mass unit is defined as a mass equal to one twelfth the mass of an atom of carbon-12. The mass of any isotope of any element is expressed in relation to the carbon-12 standard. For example, one atom of helium-4 has a mass of amu. An atom of sulfur-32 has a mass of amu. What Is a Proton? A proton is one of three main particles that make up the atom. The other two particles are the neutron and electron. Protons are found in the nucleus of the atom. This is a tiny, dense region at the center of the atom. Protons have a positive electrical charge of one (+1) and a mass of 1 atomic mass unit (amu), which is about kilograms. Watch out! Lightning is extremely dangerous. A single bolt of lightning can carry a billion volts of electricity. That s enough energy to light a 100-watt light bulb for three months! As impressive as it is, lightning is nothing more than a sudden flow of extremely tiny particles. What are the particles that flow in a lightning bolt? The answer is electrons.

2 What Is an Electron? Electrons are extremely small. The mass of an electron is only about 1/1837 the mass of a proton or neutron, so electrons contribute virtually nothing to the total mass of an atom. Electrons have a negative electric charge of -1, which is equal but opposite to the charge of proton, which is +1. All atoms have the same number of electrons as protons, so the positive and negative charges cancel out, making atoms electrically neutral. Electrons are found outside of the nucleus in the electron cloud or energy levels. What Is a Neutron? Atoms of all elements except for most atoms of hydrogen have neutrons in their nucleus. Atoms generally have about the same number of neutrons as protons, or slightly more. Neutron have no electrical charge, or zero charge, however, they have a mass of 1 amu or about 1.67 x kilograms. Together with protons, they make up virtually all of the mass of an atom. Atomic Number The atomic number of an atom is the number of protons found in the nucleus. Atoms of the same element have the same number of protons, and atoms of different elements have a different number of protons. The modern periodic table uses atomic number to arrange the atoms. Each successive atom has one more proton, thus the atomic number increases by one with each element. History of Atomic Mass Determinations As a part of his research on atoms, John Dalton determined a number of atomic weights of elements in the early 1800s. Atomic masses were the basis for the periodic table that Mendeleev developed. Originally all atomic masses were based on a comparison to hydrogen, which has an atomic mass of one. After the discovery of the proton, scientists assumed that the weight of an atom was essentially that of the protons electrons were known to contribute almost nothing to the atomic weight of the element. This approach worked until we learned how to determine the number of protons in an element. We then saw that the atomic mass for an element was often twice the number of protons (or more). The discovery of the neutron provided the missing part of the picture. The atomic mass is now known to be the sum of the protons and neutrons in the nucleus.. Nuclear Notation Shows the information required to determine the element, as well as the number of protons, electrons, and neutrons. For example, all helium atoms have two protons, and no other elements have atoms with two protons. In the case of helium, the atomic number is 2. The atomic number of an element is usually written in front of and slightly below the element s symbol. The atomic mass, the number of protons plus neutrons is written above the atomic number, before the element symbol.

3 Atoms of the First Six Elements Name Symbol Atomic Number Protons Neutrons Electrons Atomic Mass Hydrogen H Helium He Lithium Li Beryllium Be Boron B Carbon C Consider the element helium. Its atomic number is 2, so it has two protons in its nucleus. Its nucleus also contains two neutrons. Since = 4, we know that the mass number of the helium atom is 4. Finally, the helium atom also contains two electrons since the number of electrons must equal the number of protons. This example may lead you to believe that atoms have the same number of protons and neutrons, but further examination of the table above will show that this is not the case. Lithium, for example has three protons and four neutrons, leaving it with a mass number of 7. Knowing the mass number and the atomic number of an atom allows you to determine the number of neutrons present in that atom by subtraction. Number of neutrons = atomic mass - atomic number Atoms of the element chromium (Cr) have an atomic number of 24 and an atomic mass of 52. How many neutrons are in the nucleus of a chromium atom? = 28 neutrons in a chromium atom The number of electrons would be equal to the number of protons in neutral atoms. How many electrons would be in the chromium atom? Chromium has 24 protons and 24 electrons. The composition of any atom can be illustrated with a shorthand notation using the atomic number and the mass number. Both are written before the chemical symbol, with the mass number written as a

4 superscript and the atomic number written as a subscript. The chromium atom discussed above would be written as: Another way to refer to a specific atom is to write the mass number of the atom after the name, separated by a hyphen. The above atom would be written as chromium-52. What Are Isotopes? All atoms of the same element have the same number of protons, but some may have different numbers of neutrons. For example, all carbon atoms have six protons, and most have six neutrons as well. But some carbon atoms have seven or eight neutrons instead of the usual six. Atoms of the same element that differ in their numbers of neutrons are called isotopes. All elements have isotopes. Many isotopes occur naturally. Usually one or two isotopes of an element are the most stable and common. Different isotopes of an element generally have the same physical and chemical properties. That s because they have the same numbers of protons and electrons. An Example: Hydrogen Isotopes Hydrogen is an example of an element that has isotopes. Three isotopes of hydrogen are modeled in the figure below. Most hydrogen atoms have just one proton and one electron and lack a neutron. These atoms are just called hydrogen. Some hydrogen atoms have one neutron as well. These atoms are the isotope named deuterium. Other hydrogen atoms have two neutrons. These atoms are the isotope named tritium e - Q: The mass number of an atom is the sum of its protons and neutrons. What is the mass number of each isotope of hydrogen shown in the Figure above? A: The mass numbers are: hydrogen = 1, deuterium = 2, and tritium = 3. (See the top number of the nuclear notation.) For most elements other than hydrogen, isotopes are named for their mass number. For example, carbon atoms with the usual 6 neutrons have a mass number of 12 (6 protons + 6 neutrons = 12), so they are called carbon-12. Carbon atoms with 7 neutrons have an atomic mass of 13 (6 protons + 7 neutrons = 13). These atoms are the isotope called carbon-13.

5 Q: Some carbon atoms have 8 neutrons. What is the name of this isotope of carbon? A: Carbon atoms with 8 neutrons have an atomic mass of 14 (6 protons + 8 neutrons = 14), so this isotope of carbon is named carbon-14. Q: The element oxygen has 8 protons. How many protons and neutrons are there in oxygen-17? A: Oxygen-17 like all atoms of oxygen has 8 protons. Its name provides the clue that it has a total of 17 protons and neutrons. Therefore, it must have 9 neutrons (8 + 9 = 17). Elements Listed on the Periodic Table Each square representing an element on the periodic table shows information about the number of particles found in that element. Atomic mass is the average number of protons plus neutrons (p + + n) Atomic number is the number of protons (p + ) found in the nucleus 6 Carbon C Chemical Symbol: 1 or 2 letters used to represent the element. The 1 st letter is always capital. If there is a 2 nd letter it is lower case. When ( ) is used around the atomic mass, it means that very little of that elements exists at any given time. Scientists have not been able to attain a good average atomic mass. Remember that all elements have isotopes. The atomic mass listed is the average mass.

6 Summary An atom s mass number is its mass in atomic mass units (amu), which is about equal to the total number of protons and neutrons in the atom. A proton one of three main particles that make up the atom. It is found in the nucleus. It has an electrical charge of one +1 and a mass of 1 atomic mass unit (amu). Atoms of any given element have a unique number of protons that is different from the numbers of protons of all other elements. Electrons are one of three main types of particles that make up the atom. They are extremely small and have an electric charge of -1, and virtually no mass. All neutral atoms have the same number of electrons as protons. Electrons are found outside of the nucleus in electron clouds or energy levels. A neutron is one of three main particles that make up the atom. It is found in the nucleus and is neutral in electric charge. It has about the same mass as a proton. Neutrons are found in all atoms except for most atoms of hydrogen. The nucleus is a small, dense region at the center of the atom. It consists of positive protons and neutral neutrons, so it has an overall positive charge. The nucleus is just a tiny part of the atom, but it contains virtually all of the atom s mass. The number of protons in an atom is called its atomic number. This is also unique for each element. The atomic mass is defined as the total number of protons and neutrons in an atom. The number of neutrons = mass number - atomic number. All the atoms of a given element have the same number of protons and electrons, but they may vary in their numbers of neutrons. Atoms of the same element that differ in their numbers of neutrons are called isotopes. Different isotopes of an element generally have the same physical and chemical properties because they have the same numbers of protons and electrons. Carbon-12 is the reference for all atomic mass calculations. Review 1. What is an atomic mass unit? What is it equal to? 2. Why is the nucleus positive in charge? 3. Explain why the nucleus is very dense. 4. Describe protons. 5. What is the relationship between protons and elements? 6. What are electrons? 7. What is the relationship between the number of electrons and protons in a neutral atom? 8. What is a neutron? 9. What is the atomic number of an atom? 10. What is the atomic mass of an atom? 11. A tin atom has an atomic number of 50 and an atomic mass of 118, how many protons, electrons and neutrons would be found in the tin atom?

7 12. What is the mass of a cobalt atom that has 27 protons and 30 neutrons? 13. The symbol below represents an isotope of helium. How many protons and neutrons does it have? 14. What are isotopes? 15. All carbon atoms have six protons. Most also have six neutrons, but some have seven or eight neutrons. What is the mass number of a carbon isotope that has seven neutrons? 16. Why do isotopes of an element generally have the same physical and chemical properties? Answers 1. An atomic mass unit is a unit of measure for subatomic particles. It is equal to 1.7 x kilograms. 2. The nucleus is positive because it is made of protons which carry a +1 charge, and neutrons which are neutral. 3. The nucleus is a tightly packed area in the center of the atoms containing both protons and neutrons, which constitute most of the mass of an atom. 4. Protons are a particle found in the nucleus of an atom, with a +1 charge and a mass of 1 amu. 5. The number of protons is unique to each element. 6. Electrons are a particle found around the nucleus of an atom, with a -1 charge and virtually no mass. 7. In a neutral atom, the number of electrons is equal to the number of protons. 8. Neutrons are particles found in the nucleus of an atom, with no charge and a mass of 1 amu. 9. Atomic number is the number of protons found in the atom. 10. Atomic mass is the number of protons plus the number of neutrons found in the atom protons, 50 electrons (number of protons is equal to the number of electrons), 68 neutrons (atomic mass atomic number, ) 12. Atomic mass is 57 (protons plus neutrons, ) protons, 3 neutrons (and 2 electrons) 14. Isotopes are atoms of the same element with a different atomic mass because they have a different number of neutrons. 15. With 7 neutrons it is carbon Isotopes have the same properties because they have the same number of protons and electrons.