Chapter 2. Atomic Structure

Transcription

1 Atomic Structure Chapter 2 What particles compose the substances around us? When you use the term particle to explain the different properties of a solid, liquid or gas, you are referring to the simplest, most basic unit of that substance. If you took a sample of that substance and started to divide it into smaller and smaller portions eventually you would reach this basic unit that still has the properties of you substance. The type of basic unit (particle) will depend on how the atoms are arranged in your substance. For example the simplest unit in a sheet of aluminium foil is the aluminium atom. The sheet of foil consists of many billions of atoms arranged in a network structure. Molecules Molecules are formed in many substances when small groups of atoms join together with a unique structural conformation. The atoms can be all the same or there can be several different types. A chemical formula gives information about the types and numbers of atoms present in each of these molecules. Examples: The simplest unit in the substance water is the water molecule, H 2 O which is a cluster of two hydrogen atoms and one oxygen atom. Water is therefore composed of 2 different elements, classifying it also as a compound. In oxygen gas, O 2 there are two oxygen atoms per molecule. 1

2 Molecules in simple 3-D Chemical molecules are discrete structures representing everyday substances in their most simple form (ie; atomic ratio). You will use a molecular model kit a bit later to represent a variety of different chemical structures. The lines or sticks between individual atoms represent Water, H 2 O Ammonia, NH 3 Carbon Tetrachloride, CCl 4 Other substances are made up of a large network of ions. Ions are atoms or groups of atoms which carry a positive or negative electric charge. Common table salt (which has the chemical name of sodium chloride) is made of a large network of sodium ions, Na+ and chloride ions, Cl - 2

3 Modelling Atomic Structure In order to be able to distinguish these different sorts of particles you must understand some fundamental ideas about the structure of an atom. Since atoms are too small to be seen even with the best microscopes, scientists have developed models of atoms to represent them. No model can be exactly like an atom in all respects and so models are being constantly revised as further research reveals more about atoms. In order to explain the properties you are observing, use the simplest possible model. Some models of the atom A Basic Timeline of historical development of models of the atom 3

4 A simple Bohr Model of the Atom This model of the atom will be of particular use to you in this module. Atoms are units composed of much smaller subatomic particles: protons, neutrons and electrons. Electrons: orbiting about the nucleus of the atom Nucleus: A dense mass of protons and neutrons at the centre of the atom KEY: + Proton (mass = 1 a.m.u.) Neutron (mass = 1 a.m.u.) Electron (mass = a.m.u) These subatomic particles are measured in arbitrary units called atomic mass units, shortened to a.m.u. The periodic table lists the atomic weights of all elements in these units, as individual atoms are far too small and light to weigh individually. 4

5 Inside the nucleus are found protons (positive charge) and neutrons (neutral charge). Almost all the mass of an atom is in the nucleus with an overall positive charge. Negatively charged electrons orbit the nucleus and define the volume of the atom. This would indicate that atoms are, by far, mostly empty space! A simple calculation based on relative atomic masses above will reveal the nucleus as containing around 99.9% by mass of an entire atom! Data Table of Subatomic Particles: Subatomic Particle Charge Relative mass (a.m.u.) Position Electron: e orbiting the nucleus Proton: p nucleus Neutron: n 0 1 nucleus The electrons form a negatively charged cloud around the nucleus. Atoms are neutral, so that the number of positively charged protons must equal the number to negatively charges electrons. N o of protons (p + ) = N o of electrons (e ) When electrons are lost an atom becomes a positively charged ion or cation. 5

6 When electrons are gained by an atom it becomes a negatively charged ion or anion. 6

7 Isotopes Any element is identified by the number of protons in its nucleus, defined as its Atomic Number. The number of neutrons within an individual element can vary without changing its identity, hence the concept of isotopes. Different isotopes of a given element have the same number of protons, but different numbers of neutrons in the nucleus. Case Study 1: Carbon (Atomic Number 6) All atoms of Carbon will be identified by Atomic Number 6, having this number of protons within its nucleus. The nuclei of most carbon atoms contain also 6 neutrons giving a combines mass number of 12. But statistically, a very small proportion of carbon atoms in nature have 8 neutrons, thus giving a combined mass number of 14. These are two different isotopes of carbon which can be distinguished by their different masses; C-12 and C-14. C-14 is unstable and radioactive. Radioactivity is a feature of many different elemental isotopes. The residual intensity of C-14 radioactivity is used for carbon dating of ancient fossils. So why do many elements appearing on the Periodic Table have mass number values listed with decimal places? Fractions of 1 a.m.u. do not occur in nature! Case Study 2: Chlorine (Atomic Number 17) Chlorine is listed with an atomic mass of All Chlorine atoms contain 17 protons in their nucleus. From the atomic mass above it could be concluded that a chlorine nucleus contains neutrons; but there is no such thing as 0.45 of a neutron. An atomic mass, as appears on the periodic table is an average mass of all isotopes occurring in nature. This may have decimal places A mass number is a round figure, summing the protons + neutrons of a single atom. 7

8 Statistically in nature, chlorine atoms are composed of 2 isotopes: Chlorine-35 (having 18 neutrons) accounts for around 76% of chlorine atoms in nature. Chlorine-37 (having 20 neutrons) accounts for around 24% of chlorine atoms in nature. Now, given these statistics of two chlorine isotopes, calculate its atomic mass Quantifying Subatomic Particles in Atoms The Periodic Table of the Elements is a complete database of known atomic structures. Atoms are listed in order of atomic number, being the number of protons contained within the nucleus of the atom. An Element is a substance composed of a single specific atom. The Periodic Table lists over 100 different elements providing information on their composition of protons, neutrons and electrons. FEATURES OF ELEMENTS IN THE PERIODIC TABLE: 1. Chemical Symbol: Shortened form of name. 2. Atomic Number: Number of Protons in the Nucleus. 3. Mass Number: Combined mass of Protons and Neutrons. Atomic Number Atomic Mass 20 Ca Chemical Symbol Chemical Name of Element DETERMINING ATOMIC STRUCTURE Number of Protons = Atomic Number Number of Electrons = Number of Protons, ie Atomic Number Number of Neutrons = Atomic Mass (rounded) Number of Protons 8

9 2.5 How are Electrons organised around the Nucleus? Protons and neutrons make up the almost the entire mass of each atom and are located in the nucleus. In our model of the atom, the electrons are arranged in orbits or shells around the nucleus and equal the number of protons in a neutral atom. However, because they may be a long way from the nucleus, outer electrons may be readily removed or added and then an ion is formed. Electrons which are in the same shell are about the same distance from the nucleus and they have about the same energy. Shells are sometimes referred to as energy levels. More complicated models of the atoms refer to subshells, suborbitals, standing waves and so on. Shells are numbered from the nucleus. Electron shells (or orbits) around the nucleus The first four shells contain 2 (first shell), 8, 8 and 18 electrons as a maximum. The next two shells, contain 18 and 32 electrons. The rule for assigning electrons to the shells is that the first shell is filled before an electron is added to the next one.the electron configuration for an element (or ion) is simply the number of electrons in each shell, separated by a full stop and reading from the first shell outwards. Element Atomic number Electron configuration Hydrogen 1 1 Helium 2 2 Lithium Carbon Sodium Silicon Chlorine Calcium When drawing the electrons of a particular atom, the shells are depicted as concentric circles around the nucleus, and the electrons as dashes, dots or e symbols. For example, a hydrogen atom with 1 electron 9

10 Written Exercise: Draw structural models of the following elements: Aluminium Beryllium Calcium Nitrogen 10

11 THE FIRST 20 ELEMENTS OF THE PERIODIC TABLE Use a periodic table and your knowledge of electron shell structures to complete the following table. Some information has been completed for you. Element Symbol Atomic Number Mass Number Neutron Number Electron Number Electron Configuration Charge of Ion valency Hydrogen H Helium 2 Lithium 7 Beryllium 5 Boron 5 Carbon C 2 : 4 Nitrogen Oxygen 16 Fluorine 10 Neon 10 Sodium Na 2 : 8 : 1 Magnesium Aluminium 27 Silicon 14 Phosphorous 15 Sulfur 2 : 8 : 6 Chlorine 1 - Argon Ar Potassium 19 Calcium 40 11