1. Structure and Properties of the Atom

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1 1. Structure and Properties of the Atom 1.1 Atoms: A simple definition of the atom is that it is the smallest particle that contains the properties of that element. The idea of atoms was first suggested by Democritus, an ancient Greek who lived in the fourth century BC. John Dalton ( ) was an Englishman who improved on the atomic theory in order to explain chemical behaviour. This theory included the following ideas: 1. All elements are composed of tiny, indivisible particles called atoms. 2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. 3. Atoms of different elements can combine with one another in simple (whole number) ratios to form compounds. 4. Chemical reactions occur when atoms are separated, joined or rearranged. However, atoms of one element are not changed into atoms of another by a chemical reaction. Some of Dalton's atomic theory is still accepted today. One revision concerns his idea that atoms are indivisible. Using increasingly powerful atom smashers, physicists have found dozens of subatomic particles. In fact, so many particles have been found that no single theory of atomic 'structure' can account for all of them. In chemistry, we chemists concern ourselves with only three of these particles: electrons, protons and neutrons. 1.2 Fundamental Atomic Particles: We have seen that atoms are composed of even smaller particles. The differences between atoms (and therefore elements) are caused mainly by the fact that they contain different numbers of these fundamental atomic particles. The simplest model of the atom describes it as a very small positively charged nucleus (containing most of the mass of the atom ie. protons and neutrons) surrounded by moving electrons. The nucleus of the atom is very small compared with the size of the atom. The atom is mainly empty space. s (electrons orbit nucleus) Representation of an Atom. Nucleus contains protons & neutrons (nearly all of mass of atom) an electron is a particle which carries a single negative charge. It has virtually no mass compared with the neutron and proton. the proton is a particle which carries a single positive charge. It has a much larger mass than an electron. This mass can be defined as one atomic mass unit (1 a.m.u.). the neutron carries no charge (ie. it is electrically neutral). It has a mass approximately equal to that of the proton. Characteristics of the Fundamental Atomic Particles charge mass relative to an electron position electron -1 1 orbits the nucleus proton within the nucleus neutron within the nucleus 1

2 Chapter 1: Structure & Properties of the Atom QUESTIONS: 1. Describe the composition of the nucleus of the atom. 2. Compare the relative size and relative density of a nucleus to its atom. 3. Complete the following table: name of particle where found in atom relative mass charge proton +1 neutron in nucleus electron 4. Explain how atoms which have electrically charged particles still have no overall electrical charge. 5. Explain what is meant by the term subatomic particles Give two examples of a sub-atomic particle (1) (2) 2

3 1.3 Atomic and Mass Each type of atom contains a different number of protons in its nucleus. This means that a different number of protons in a nucleus gives rise to a different type of atom eg. hydrogen (atomic number 1) contains one proton in its nucleus. Fluorine (atomic number nine) contains nine protons in its nucleus. Therefore each of the elements in the periodic table has a different atomic number (usually represented with a Z). All the atoms of the different elements are electrically neutral ie. have no overall electric charge. Thus the number of protons in the nucleus of an atom must equal the number of electrons around its nucleus. Most of the mass of an atom is concentrated in its nucleus. Thus the total number of protons and neutrons in the nucleus is the mass number of an element (usually written as A). The full symbol for an atom X is written: A ZX The symbol identifies the element, while the atomic number and mass number allow determination of the number of protons, electrons and neutrons in the atom. Example: A sodium atom (with atomic number of 11 and a mass number of 23) number of protons + number of neutrons mass number 23 Na 11 symbol of element atomic number number of protons Table: Details Of The First Ten Elements Of The Periodic Table. Name Symbol Atomic Composition of Nucleus Mass of Electrons Protons Neutrons Hydrogen H Helium He Lithium Li Beryllium Be Boron B Carbon C Nitrogen N Oxygen O Fluorine F Neon Ne

4 Chapter 1: Structure & Properties of the Atom QUESTIONS: 6. Explain why the mass of an electron is ignored when the mass of an element (in atomic mass units) is being calculated. 7. Complete the following table. Name of Element Symbol Atomic Mass of Protons of Electrons No. of Neutrons Na 19 F Magnesium H Calcium Pt Au Nitrogen State the number of electrons, protons and neutrons in an atom of each of the following elements Cd Po (c) Cr 9. Elements are sometimes referred to as Uranium or Uranium With the aid of a Periodic Table determine the number of electrons, protons, and neutrons in these isotopes of uranium. Uranium- 238 Uranium

5 1.4 Isotopes: The nuclei of the atoms of a given element must all contain the same number of protons, but the number of neutrons in such atoms may vary. Atoms that have the same number of protons but different numbers of neutrons are called isotopes ie. they have the same atomic number but a different mass number. Despite these differences isotopes are chemically alike. This is because the chemical properties of an element are largely due to the number of electrons in the outer shell. As an example let us consider the oxygen atom. Subatomic Particles Present In The Three Isotopes Of Oxygen O 8 O 8 O protons electrons neutrons Since each isotope has the same number of electrons (8) and since the electrons around the nucleus determine the chemical properties it can be seen that each isotope of oxygen has the same chemical properties. Since each isotope has a different number of neutrons they have slightly differing masses and hence their physical properties differ slightly (for example density, m. pt and b. pt). Almost all of the elements have isotopes but the relative abundance of each varies. For example the element chlorine has two isotopes: Cl at 75.77% natural abundance, and 17 Cl 24.23% natural abundance. While tin has 10 naturally occurring isotopes, more than any other element and none of which are radioactive: Sn (32.5%); Sn (24%); Sn (14.5%); Sn (8.5%); 50 Sn (7.5%); Sn (5%); Sn (4.5%); Sn (1%); Sn (0.5%); Sn (0.5%). Uranium consists mainly of three radioactive isotopes: U (99.3%); U (0.7%); U (0.005%) Uranium-235 and Uranium-238 both undergo radioactive decay via a sequence of other elements (all radioactive), until they arrive at lead 206 and lead 207 respectively, which are stable atoms. Uranium-234 is produced as part of the decay series of uranium-235. Uranium -238 has a half-life of 4.5 billion years; uranium -235 has a half-life of 700 million years; uranium -234 has a half-life of years. Half-life is defined as the time taken for the concentration of a substance to fall to half its initial value. QUESTIONS: 10. Complete the following table: Atom Name of Element Atomic Mass of Protons of Neutrons No. of Electrons A B C D E

6 Chapter 1: Structure & Properties of the Atom 11. Explain why isotopes of the same element have the same atomic number, but differing mass number. Write the symbols representing the isotopes of carbon having mass numbers 12, 13, and 14 respectively. 12. Using the periodic table, determine the number of neutrons in these atoms. 32 S 80 Br (c) 108 Ag (d) 207 Pb 13. If X is a general symbol representing various elements, state, with reasons, which ones of the following are isotopes of the same element X, 16 8 X, X, X, 9 X, 15 X. 14. State how isotopes of the same element are: identical to and different from one another 15. Write Z A X symbols for the following isotopes (hint; you may need to refer to a periodic table) (c) (d) neon-22. an oxygen atom with equal numbers of protons and neutrons. platinum-195. a calcium atom with 24 neutrons. 6

7 1.5 Electronic Configuration: Because the chemical properties of an element are determined by the number of electrons in its atom, it is important to understand the arrangement of the electrons - ie. electronic configuration. One common picture of an atom is to represent it as a miniature solar system with the nucleus at the centre and the electrons moving like planets in orbits around it. We replace the orbits with shells which are numbered n = 1, 2, 3,... 7 or given labels K, L, M,...Q. The number of electrons that can fit in these shells is governed by the rule 2n 2, where n = the number of the shell. Shell Designation Electron capacity 1 st 2 nd 3 rd 4 th K-shell L -shell M-shell N-shell The shells are said to represent different energy levels. This is the resultant energy due to the attraction and repulsion forces of all charged particles. If we use as a guiding principle the fact that electrons always occupy shells of lowest energy we can build up the electronic configuration of the various atoms. Thus the first (K) shell is filled first, then the second (L) shell and so on. This is shown in the table on the following page. The outermost energy level is called the valence level, and the electrons in this level are called valence electrons. The valence electrons are the electrons that are primarily involved in chemical reactions, and similarities among the configurations of the valence electrons (the valence shell configurations) account for similarities of the chemical properties among groups of elements. 1 K 2 L 3 M other energy levels nucleus maximum of 2 electrons maximum of 8 electrons stable at 8 electrons maximum of 18 electrons Details Of The First Three Electron Energy Levels When a shell is filled a new electron shell is started for the remaining electrons. There is a rule, however, which does not allow the outermost shell (valence shell) of an atom of an element to hold more than 8 electrons. For example, as the third (M) shell has a capacity of 18 electrons you would assume that potassium, containing 19 electrons, should have an electronic configuration of This, however would mean that the valence shell contains more than eight electrons. Therefore once the M shell contains the stable number of eight electrons, the next electron enters the N shell resulting in the configuration for potassium. In view of the inert (very stable) nature of the Group VIII (noble gases), it is clear that the optimum number of electrons is eight - an octet of electrons. This is the basis of the octet rule, which states that any element is most stable when it has a set of eight valence electrons. 7

8 Chapter 1: Structure & Properties of the Atom 1 2 K L K L M 10P 10N Neon atom 2e - 8e - 18P 22N 2e - 8e - 8e - Argon atom K L M K L M 4 N 11P 12N 2e - 8e - 1e - 19P 22N 2e - 8e - 8e - 1e - Sodium atom Potassium atom Electronic Structure of Four Elements: Neon, Sodium, Argon & Potassium QUESTIONS: 16. State the maximum number of electrons which each of the first four main energy shells of an atom may contain. 17. Write the electronic configuration of: (c) (d) (e) oxygen: calcium: beryllium: fluorine: magnesium: 18. Deduce the electronic configuration of the element whose atomic number is 18. Give the name of this element. If the predominant isotope of this element contains 22 neutrons in the nucleus of an atom, state its mass number. Electronic Configuration: Name: Mass No: 8

9 19. Which of the following pairs are isotopes? Explain your answers. (c) 50 Ti and 50 V 12 C and 13 C 40 Ar and 40 K 20. Explain why the atomic number of an element is more important than the mass number in identifying an element. Electronic Configuration Of The First 20 Elements Electronic Configuration Element Symbol At. No. 1 (K) 2 (L) 3 (M) 4 (N) Hydrogen H 1 1 Helium He 2 2 Lithium Li Beryllium Be Boron B Carbon C Nitrogen N Oxygen O Fluorine F Neon Ne Sodium Na Magnesium Mg Aluminium Al Silicon Si Phosphorus P Sulfur S Chlorine Cl Argon Ar Potassium K Calcium Ca

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