1. Structure and Properties of the Atom

Transcription

1 SACE Stage 1 Chemistry - The Essentials 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:, 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. The nucleus of the atom is very small compared with the size of the atom. The atom is mainly empty space. Energy levels ( 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 SACE Stage 1 Chemistry - The Essentials 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 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, 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 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 Protons Electrons No. of Neutrons Na 19 F Magnesium H Calcium Pt Au Nitrogen State the number of, protons and neutrons in an atom of each of the following elements Cd Po Cr 9. Elements are sometimes referred to as Uranium or Uranium With the aid of a Periodic Table determine the number of, protons, and neutrons in these isotopes of uranium. Uranium- 238 Uranium

5 SACE Stage 1 Chemistry - The Essentials 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 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 neutrons Since each isotope has the same number of (8) and since the 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 Protons 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 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) (d) neon-22. an oxygen atom with equal numbers of protons and neutrons. platinum-195. a calcium atom with 24 neutrons. 6

7 SACE Stage 1 Chemistry - The Essentials 1.5 Electronic Configuration: Because the chemical properties of an element are determined by the number of in its atom, it is important to understand the arrangement of the - 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 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 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 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 in this level are called valence. The valence are the that are primarily involved in chemical reactions, and similarities among the configurations of the valence (the valence shell configurations) account for similarities of the chemical properties among groups of elements. 1 K 2 L Energy level 3 M other energy levels nucleus maximum of 2 maximum of 8 stable at 8 maximum of 18 Details Of The First Three Electron Energy Levels When a shell is filled a new electron shell is started for the remaining. 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. For example, as the third (M) shell has a capacity of 18 you would assume that potassium, containing 19, should have an electronic configuration of This, however would mean that the valence shell contains more than eight. Therefore once the M shell contains the stable number of eight, 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 is eight - an octet of. This is the basis of the octet rule, which states that any element is most stable when it has a set of eight valence. 7

8 Chapter 1: Structure & Properties of the Atom Energy level 1 2 K L Energy level K L M 10P 10N Neon atom 2e - 8e - 18P 22N 2e - 8e - 8e - Argon atom Energy level K L M Energy level 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 which each of the first four main energy shells of an atom may contain. 17. Write the electronic configuration of: (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 SACE Stage 1 Chemistry - The Essentials 19. Which of the following pairs are isotopes? Explain your answers. 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

10 Chapter 1: Structure & Properties of the Atom Summary Test 1: Structure & Properties of the Atom 1. Name the three sub-atomic particles (dealt with in Chemistry), state their charge and positions in the atom. (3 marks) 2. Consider the neutral atoms listed: O, 11 Na, 17 Cl. For each atom state: atomic number mass number number of protons, electron and neutrons (d) electronic configuration Present your answer in table form. (9 marks) 3. Consider K. Hence complete the spaces. (d) (e) (f) (g) (h) name of this element. atomic number mass number number of neutrons electronic configuration mass number of another isotope containing one more neutron atomic number of another isotope of potassium name another element with similar properties to K (8 marks) 10

11 4. Explain carefully in your own words all the information conveyed by the symbol U. SACE Stage 1 Chemistry - The Essentials 5. Complete the following table: (3 marks) Symbol At. No. Mass No. No. of No. of neutrons Electronic configuration 12 C 6 2, S O2-2, 8 Na F- 9 (6 marks) 6. What is an isotope? State the information needed to determine whether atoms are isotopes of a particular element. (4 marks) 7. Write down the electronic configuration of the following elements. (Use a copy of the Periodic Table to look up the atomic numbers of the elements). (d) (e) (f) (g) (h) beryllium oxygen calcium magnesium fluorine sodium nitrogen chlorine (8 marks) 11

12 Chapter 1: Structure & Properties of the Atom 8. Complete the following table by classifying each of the elements as either metal/non-metal and state how many these elements lose/gain to have a full outer shell. Element Metal/Nonmetal Loses/Gains No. of transferred Formula of ion formed sodium magnesium oxygen bromine nitrogen barium aluminium sulfur (8 marks) 9. Examine the following list of atomic species A, B, C, E, G, From this list identify the species which: J, 13 K, 21 M represents an atom of the element with atomic number 15. (d) (e) (f) (g) represents a sodium atom. has more protons than neutrons. represents an atom of the most common element in the Earth s atmosphere. is an isotope of the metal magnesium. represents an atom of an element in Group VIII (the Inert gases). represents an atom of a metallic element used in saucepans and kitchen foil. (7 marks) 10. Complete the following table: Atomic Name of Isotope Chemical symbol protons neutrons Mass 4 Beryllium-9 Iodine Cr (7 marks) [Total 66 marks] 12