CHEM1011 Lecture 1 6 th March 2018 States of matter Different states of a substance are different physical ways of packing its component particles: solid (closely packed together and organized), liquid (still closely packed together but disorganized), gas (far apart and disorganized) CHEM1011 Lecture 2 8 th March 2018 Physical and chemical changes Physical change is associated with a change of state. Physical properties are properties that a substance shows by itself, without changing or interacting with another substance. e.g. colour, melting point, conductivity, density, surface tension Chemical change involves changing one substance into another. Chemical properties are the properties of a substance that result in the formation of a new substance e.g. flammability, corrosiveness, reactivity with acid. An intensive property is independent of the amount of substance present e.g. ph An extensive property depends on how much of a substance you have got e.g. volume.
Allotropes elements with different bonding arrangements of atoms e.g. carbon CHEM1011 Lecture 3 9 th March 2018 Atomic structure Atomic theory John Dalton 1808 All matter consists of atoms are tiny indivisible particles of an element that cannot be created or destroyed. Atoms of one element cannot be converted into atoms of another element. Atoms of an element are identical and are different from atoms of any other element. Cathode ray JJ Thomson 1897 Found that all metals produced the same negatively charged particle which was 1000 lighter than a hydrogen atom Atoms are divisible, as such cathode rays renamed electrons. Plum pudding model of atom Thomson Suggested that atom was one positive particle covered in smaller, negative particles. Rutherford 1909 Atoms are mostly empty space occupied by electrons. All the positive charge and essentially all the mass lies in a tiny region in the centre the nucleus. The nucleus is made of positively charged particles, protons, and uncharged neutrons.
Elements Atomic number = number of protons Mass number = protons + neutron. Forces in the atom Nucleons are held together by the strong nuclear force only effective over very short distances These forces are strong enough to overcome the repulsion between the protons. CHEM1011 Lecture 4 13 th March 2018 Electron Structure Electromagnetic radiation
Electromagnetic radiation Although all radiation has the same speed, the energy can vary. The higher the frequency, the more rapidly the wave is oscillating and thus the higher the energy. Atomic spectra When an atom is supplied with energy (e.g. heat, electric current) it emits electromagnetic radiation of a relatively few, fixed frequencies that are characteristic of that element this is the atomic emission spectrum
e.g. Light emitted from a hydrogen lamp has only a few lines Light of certain energies are emitted, and the pattern is unique to hydrogen suggesting the process of emitting light from the atom is quantized i.e. comes in discrete amounts. Naturally, our eyes will see the combination of wavelengths. But when we pass the light through a prism, we see the lines.
Bohr Model Electrons in atoms can only occupy certain energy levels (orbits). When an electron moves from one energy level to another, energy is absorbed or emitted. This energy corresponds to light of a specific energy/frequency. The number of electrons that can pack into an orbit depends on the size of the orbit. Ground state electron structures - Ground state is the lowest energy state of an atom or particle. (2,8,18,32) - When electrons return to ground state, they release energy and thus light as seen above.
CHEM1011 Lecture 5 15 th March 2018 Problems with Bohr Model The problem with the Bohr Model is that when AES is performed on other elements that aren t hydrogen, some more complicated color combinations can occur. This suggests that within each shell, there are subshells, where electrons can move up into with sufficient energy, and then release a specific wavelength of light when returning to ground state. Subshells These subshells are called s,p, d, f. There is one extra subshell for each new shell
Filling the subshells Example Excited configurations
Note: Elements in the same group have the same valence electron configurations and, as a result, very similar chemical properties. CHEM1011 Lecture 6 16 th March 2018 Types of bonding Ionic bonding Neutral atoms always have the same number of electrons as the number of protons in the nucleus. Many atoms may gain or lose electrons to form ions, which have a charge. Ions form because elements want to achieve stable noble gas electron configuration can be achieved by gaining or loosing electrons. Noble gases example
Predicting the formation of Ions Atoms gain or lose electrons to become isoelectronic as the nearest noble gas i.e. have the same electron configuration. The electrostatic attraction of cations and anions result in the formation of ionic bonds. Ionic bonding must be between a metal (loose electron) and non-metal (gain electron) as such the production of ions through the transfer of electrons must occur. Ionic bonds produce a solid crystal lattice structure. The cations and anions are packed to maximize attraction and minimize repulsion forces. The ratio of the cations to anions ensures the overall charge of the compound is zero. The cation-anion attract is a non-directional bond i.e. the charge is uniform in all directions. Metallic bonding Outer electrons of a metal atom are not fixed to that atom but spread around surrounding atoms or delocalized. Malleable and ductile atoms able to slide past each other in a sea of electrons. Mobile electrons good electrical and thermal conductivity in solid and molten state.
CHEM1011 Lecture 7 20 th March 2018 Ionic bonding - Caused by electrostatic attraction between cations and anions. It will take energy to remove an electron from an atom this energy forms from the electrostatic attraction between positive and negative ions. Note: you rarely get get highly charged ions because the more electrons lost, the more energy is required. Covalent bonding In covalent compounds a Noble gas electron configuration is obtained by sharing of electrons. In ionic compounds a Noble gas electron configuration is obtained by transfer of electrons. Example H 2 gas When the nuclei are really far apart (as in 1) there is no energy. As you move them closer together (2) i.e. making a bond, they start to release energy until you reach the minimum i.e. bond is formed and at the most stable state (3). As you push them closer together they begin to repel and thus generating an exponential amount of energy (4).
Reasons for covalent bonding Reach noble gas configuration because it is most stable o Only valence electrons can be involved o Electrons tend to form pairs Electron density between the atoms are directional i.e. atoms will prefer specific orientations i.e. covalent molecules have definite shape. Valence Valence electrons are the outer most electrons. They are used in bonding. The valence of an atom is the size of the charge if it forms and ion and the number of bonds it forms with a neutral molecule. Note: positive numbers with no charge. Covalent bonding: C and H