All matter is composed of extremely small particles called atoms Atoms of a given element are identical in size, mass, and other properties, while atoms of different elements differ in size, mass, and other properties Atoms cannot be subdivided, created or destroyed. Atoms of different elements can combine in simple, whole number ratios to form chemical compounds In chemical reactions, atoms are combined, separated, or rearranged Parts of Dalton’s atomic theory are now altered to keep up with our current technology Atoms are now capable of being subdivided Not all atoms of the same element have the same masses (isotopes) Law of Conservation of Mass and Law of Multiple Proportions
If atoms are not separated in chemical reactions and atoms of different elements can combine in chemical reactions, then mass is conserved in chemical reactions (Law of Conservation of Mass) Law of Multiple Proportions If two or more different compounds are composed of the same two elements, the masses of the second element combined with a certain mass of the first element can be expressed as ratios of small whole numbers Only whole atoms combine in chemical compounds between the same two elements. Therefore, different compounds formed by the same two elements must result from combinations of different relative numbers of atoms Example: CO 12 g of C to 16 g of O CO2 12 g of C to 32 g of O
Structure of the Atom
The nucleus of the atom is made up of two primary particles, neutrons and protons Rutherford’s Experiment A beam of positively charge particles was aimed at a thin sheet of gold foil about 10,000 atoms thick Most of the particles passed straight through the foil or were only slightly deflected. About 1 in every 8000 particles was greatly deflected Conclusions: Positively charged particles coming very close to the nucleus were deflected strongly due to like charges Particles slightly deflected passed close to the nucleus causing like charges to be slightly deflected Most particles passed straight through suggesting that the majority of the atom is space Since a vast majority of the particles went undeflected, the nucleus must occupy a very small portion of the volume of an atom There is a small very dense positively charged area in an atom Protons are subatomic particles that have a positive charge and are located in the nucleus. Protons have a mass of 1.673 X 10-24 g Their mass is 1836 times greater than the mass of an electron Neutrons are subatomic particles that are electrically neutral and are found in the nucleus The mass of a neutron is 1.675 X 10-24 g, which is nearly the same mass as a proton The number of neutrons in an atom can be calculated by subtracting the number of protons in an atoms from the total mass Protons + Neutrons = Mass Isotopes are atoms of the same element with different masses caused by differing numbers of neutrons Nuclear forces hold the particles in the nucleus together If several positively charged protons are packed closely together in the nucleus, one would expect the protons to repel one another very strongly It appears that the short range proton-proton, proton-neutron, and neutron-neutron interactions hold the nucleus together The fact that there are also other subatomic particles may also help hold the nucleus together Electrons were discovered from experiments using cathode-ray tubes An electrical current passes through the tube from the cathode (negative terminal) to the anode (positive terminal) Several different experiments have been performed with cathode-ray tubes with several observable characteristics Different gases glow with different colors as color passes through the tube The part of the glass tube opposite from the cathode glows An object placed between the cathode and the opposite end of the tube casts a shadow on the glass Cathode rays are deflected by a magnetic fiels The rays are deflected away from a negative electrode The cathode ray is composed of particles that pass from the cathode to the anode Cathode-ray tube experiments suggest that all particles emitted are composed of identical negatively charged particles Since cathode rays always have the same properties, regardless of the element used to produce them, it was concluded that electrons are present in atoms of all elements Two additional pieces of information have developed from experimentation on atoms Because atoms are electrically neutral, they must contain positive charges to balance our the charges of the electrons Since electrons are so much smaller in mass than atoms, the mass of the atom is due to the nuclear mass and not that of the electrons The size of the atom is based on the electron cloud The atomic radius is the distance from the center of the atom to the outermost electron Atomic radii can be measured in nanometers (1 X 10-7 cm) Most atoms range in size form 10 to 50 nm
Problem Solving Using Moles and Avogadro’s Number
Avogadro’s number = 6.02 X 1023 particles/mole Atomic weights and molecular weights are determined from masses of atoms on the periodic chart Moles to grams: Find atomic or molecular weight Multiply moles times the atomic or molecular weight giving an answer in grams Grams to moles: Find atomic or molecular weight Divide the number of grams given in the problem by the atomic or molecular weight giving an answer in moles Moles to particles (Usually atoms or molecules): Multiple the number of moles given by Avogadro’s number giving an answer in particles Molecules or atoms to moles: Divide the number of molecules or atoms by Avogadro’s number giving an answer in moles Grams to particles: Find the atomic or molecular weight Divide grams given by the atomic or molecular weight giving an answer in moles Multiply this number of moles time Avogadro’s number giving a final answer in particles Molecules or atoms to grams: Take the number of molecules or atoms given and divide by Avogadro’s number giving an answer in moles Find the atomic or molecular weight Multiply the number of moles times the atomic or molecular weight
Electromagnetic Radiations
Electromagnetic radiation is a form of energy that travels in waves as it goes through space Some examples include visible light, UV light, infrared radiation, microwaves, x-rays, and radio waves These radiations move through a vacuum at the speed of light
2. Wavelength ( ) is the distance between corresponding points on adjacent waves
Frequency (v) is defined as the number of waves that pass a given point in a specific time, usually a second Frequency is measured in hertz The relationship between frequency and wavelength is given by the following equation where c represents the speed of light c = v Visible light makes up only a small portion of the entire electromagnetic spectrum Einstein proposed the photoelectric effect The photoelectric effect is the emission of electrons by certain metals when light shines on them Planck’s description of the particle-like properties of light helped Einstein explain how electrons could be emitted from metals Planck’s Theory A quantum is a finite quantity of energy that can be gained or lost from an atom A photon is a quantum of light, which travels in a wave-like motion, thus it is a type of electromagnetic radiation Photons are thought of as particles of radiation Radiation is emitted and absorbed in only whole numbers of photons The energy of the photon is calculated by the equation E = hv, where E is the energy of the photon, v is the frequency of the radiation emitted, and h is Planck’s constant (6.626 X 10-34 J sec) When atoms in their gaseous state are heated, their potential energy increases. Then, almost immediately, the atoms return to their original energy state, and give off the added energy in the form of electromagnetic radiations The ground state of an atom is the lowest possible energy state of an atom A state in which an atom has a higher potential energy than the ground state is an excited state of the atom Light of a particular wavelength has a definite frequency (based on Einstein’s equation) and also a definite energy (based on Planck’s equation) Whenever an electron of an atom in the excited state drops to a less excited state or the ground state, it emits a photon The energy of this photon is determined from the wavelength of of light emitted. These wavelengths can be measured with a spectroscope, and produce distinct spectral lines for excited states of atoms (P. 101-105) Spectral data helped determine that electrons travel in orbitals of specific energies An orbital is a three-dimensional region about the nucleus in which an electron can be located Orbitals can be thought of as clouds where the chances of finding electrons is greatest The closer an orbital is to the nucleus, the lower the total energy of an electron in that orbital To jump from a lower energy orbital to a higher energy orbital, an electron must absorb a quantity of energy exactly equal to the energy difference between the two orbitals When an electron drops from this higher energy orbital to a lower energy orbital it emits photons of a precise energy. The difference in the two energy levels may be calculated using spectral data Example: The amount of energy emitted from an electron in hydrogen emitting photons giving a spectral line at 364.6 nm
Quantum Numbers
Electrons in atoms are located in energy levels called orbitals Each principle energy level in an atom can contain a maximum of 2n2 electrons, where n is the principle quantum number (shell number) Spectral lines indicate that electrons are distributed into sublevels within the principle energy level The energy sublevels are designated by s, p, d, and f The number of energy sublevels in a principle energy level is equal to the principle quantum number For n = 1, there is one sublevel designated by the letter s. For n = 2, there are two sublevels designated by the letters s and p. For n = 3, the sublevels are designated by the letters s, p, and d. For n = 4, the sublevels are designated s, p, d, and f In the presence of a magnetic field additional spectral lines are observed which indicate that each energy sublevel (s, p, d, f) consists of one or more orbitals with distinct shapes There can be one s orbital, three p orbitals, five d orbitals, and seven f orbitals Diagrams of the shapes of s, p, and d orbitals (P. 109-110) The principle quantum number is designated by the letter n This quantum number determines to a large extent the relative energy of an electron, and is related to the average distance of an electron from the nucleus As the distance from the nucleus increases, the electrons have more energy The orbital quantum number is indicated by the letter l l is related to the angular momentum of the electrons and the shape of the suborbitals (s, p, d, f) The magnetic quantum number is designated as ml As a result of an electron’s angular momentum, a magnetic field is produced which can interact with an external electrical or magnetic field The value of the magnetic quantum number is related to the spatial orientation within an orbital sublevel. For example, the three p orbitals align themselves spatially along the x, y and z axis Each arrangement creates a different magnetic field The spin quantum number is designated as ms An electon spinning on its own axis has a certain angular momentum and behaves like a small magnet A small magnetic field is generated by the electron as it spins on its own axis No two electrons in the atom have the exact same set of quantum numbers
Electron Configurations
An electron configuration is the arrangement of electrons in an atom The Aufbau Principle states that electrons will occupy the lowest energy orbital that can receive the electron. The periodic chart is arranged on this principle 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 5d1 4f 5d2-10 6p 7s 6d1 5f 6d2-10 7p
Hund’s Rule says that orbitals of equal energy are each occupied by one electron before any is occupied by a second electron, for example, one electron must go into each of the three p orbitals before a second electron is put into any of the p orbitals The Pauli Exclusion Principle says that no two electrons in the same atom can have the same four quantum numbers If a second electron is placed in an orbital with another electron, they are not exactly the same because they will spin in opposite directions on their axis Orbital Notation In orbital notation a blank line is used to represent an unoccupied orbital. This blank line should be labeled underneath the line to indicate the orbital represented An orbital containing one electron is indicated by an arrow on the line pointing in either direction An orbital containing two electrons is indicated by two arrows on the line pointing in opposite directions Electron Configuration Notation Electron configuration notation eliminates the use of lines and arrows The number of electrons in a sublevel is shown by adding superscripts to the sublevel designation Electron Dot Notation Electron dot notation shows only electrons in the highest or outermost, main energy level The highest occupied energy level is the energy level with the highest principle quantum number This would include only s and p orbital electrons Diagram of dot location Atoms that become involved in compounds try to obtain an octet arrangement or the arrangement of a noble gas. This tends to give atoms stability The arrangement of elements on the periodic chart puts elements of atoms of similar energies in the same period and similar electron configurations in the same group The chart can be broken into the s block, p block, d block, and f block Many charts also give electron configuration notation for atoms of that element in an abbreviated form