Atomic Theory PDF

# Atomic Theory ### Democritus (440 BCE) - Realized that if you continued to cut something, eventually you would end up with something that couldn’t be cut anymore, “atomos” – meaning not able to divide. - **Atoms** - smallest particle that an element can be divided & still be the same substance....

# Atomic Theory ### Democritus (440 BCE) - Realized that if you continued to cut something, eventually you would end up with something that couldn’t be cut anymore, “atomos” – meaning not able to divide. - **Atoms** - smallest particle that an element can be divided & still be the same substance. - All matter is made of atoms. ### Model of Atom (Democritus (400 BC)) - He found that all matter is made of atoms. - Atoms - invisible and indestructible. - Didn’t experiment but based theories on observations. - Said that compounds are made of two or more different kinds of atoms. - Chemical reaction is a rearrangement of atoms ### John Dalton (1803) - Realized that atoms combine in very specific proportions (ratios) based on mass. - All substances are made of atoms and they cannot be created, divided or destroyed (made of a single substance). - All atoms of the same element are exactly alike and different from other elements, (unique). - Atoms join with other atoms to form new substances. ### Dalton’s Model - Solid Sphere Model or Bowling Ball Model - Proposed by John Dalton ### J. J. Thomson (1897) - Discovered that there were small particles inside the atom, meaning that atoms can be divided into smaller substances. - **Electrons** - negatively charged particles attracted to positively charged particles - **Plum Pudding Model** - electrons are mixed throughout the atom, soft blobs of matter. ### Thomson’s Atomic Model - Depiction of Thomson’s atomic model with positively charged particles and electrons embedded in a sphere. - A picture of a watermelon with an electron on the outside and red part of the watermelon labeled as “Positive Charge”. ### Ernest Rutherford (1909) - Discovered that an atom contains a nucleus with positively charged particles and that the electrons must be “floating” around the nucleus. - Most of an atom is empty space. - Diagram of experiment showing how positively charged particles are shot at a thin sheet of gold foil. Most pass through, but some are deflected at large angles, suggesting a concentrated positive charge in the core. - Diagram of a positively charged nucleus with orbiting electrons in a sphere like structure. ### Niels Bohr (1913) - Proposed that electron moved around the nucleus in energy levels (shells), but no electrons between the energy level (think ladder). - Electrons can jump from one level to another. - Travel in a definite path. - Diagram of an atom with a nucleus and several electrons on orbits around it. ### Modern Atomic Theory - Erwin Schrödinger & Werner Heisenberg - Electrons have no predictable pattern and move in a region where electrons are likely to be found called the **electron cloud**. - Diagram of an atom with a nucleus containing neutrons, protons and a series of electrons in orbitals around it. - Another Diagram with a nucleus and orbiting electrons. ### Atom Timeline - A timeline showing the contributions of Democritus, Dalton, Thomson, Rutherford, Bohr and modern quantum Mechanics. ### Atoms - All atoms have a nucleus - Protons (+) - Neutrons (no charge) - Electrons (-) - Same number of protons and electrons → an atom has no charge. - More protons (+) than electrons (-) → the atom has a positive ion form (more positives than negatives). - More electrons (-) than protons (+) a negative ion is formed (more negatives than positives). - Diagram of an atom with a nucleus, protons, neutrons, and electrons. ### 117 different elements that are unique and all things known to exist come from a combination of these elements in specific mass ratios ### Simplest atom is made of one proton, and 1 electron – hydrogen (has no neutrons.) - Diagram of the periodic table of elements ### All additional element will have protons, neutrons and electrons ### The atomic number of an element is determined by the number of protons - 1 is hydrogen, 6 is carbon, hydrogen has 1 proton, carbon has 6 protons **(you cannot change the number of protons)** ### To find neutrons take the mass number (rounded) and subtract the protons. ### Isotopes - Isotopes have the same number of protons but additional neutrons which causes the atomic mass to be different. - Isotopes can be stable (maintain their structure) and unstable (fall apart over time). - Unstable isotopes are radioactive and will decay over time giving off particles and energy (radioactive). - Diagram of three isotopes of carbon with different numbers of neutrons in their nuclei. ### Mass number determines the isotope, the number of protons and neutrons added together - Most elements have isotopes - All isotopes of an element have the exact properties of the element. - Diagram of stable and unstable carbon isotopes with different proton and neutron counts. ### Forces in atoms - **Gravitational force** - Pulls objects toward a=each other-depends on mass and distances between the objects-very small force in atoms - **Electromagnetic force** - Proton (+) and electrons (-) have strong attraction which keeps the electrons in motion around the nucleus of atoms. - Diagram of an atom with the nucleus and electrons being held together by electromagnetic forces. ### Strong force - force which keeps protons from flying apart due to close distance between protons and neutrons ### Weak force - relevant to radioactive atoms-allows neutrons to change into proton and electron - Diagram showing strong and weak forces in atoms and their relationship to the electromagnetic force. ### A History of the Atom: Theories and Models - This diagram illustrates the evolution of atomic models through the contributions of Dalton, Thomson, Rutherford, Bohr, and Schrödinger. - **Solid sphere model** - John Dalton (1803) - Dalton drew upon the Ancient Greek idea of atoms (the word 'atom' comes from the Greek 'atomos' meaning indivisible). His theory stated that atoms are indivisible, those of a given element are identical, and compounds are combinations of different types of atoms. - **Plum pudding model** - J.J. Thomson (1904) - Thomson discovered electrons (which he called 'corpuscles') in atoms in 1897, for which he won a Nobel Prize. He subsequently produced the 'plum pudding' model of the atom. It shows the atom as composed of electrons scattered throughout a spherical cloud of positive charge. - **Nuclear model** - Ernest Rutherford (1911) - Rutherford fired positively charged alpha particles at a thin sheet of gold foil. Most passed through with little deflection, but some deflected at large angles. This was only possible if the atom was mostly empty space, with the positive charge concentrated in the centre: the nucleus. - **Planetary model** - Niels Bohr (1913) - Bohr modified Rutherford's model of the atom by stating that electrons moved around the nucleus in orbits of fixed sizes and energies. Electron energy in this model was quantised; electrons could not occupy values of energy between the fixed energy levels. - **Quantum model** - Erwin Schrödinger (1926) - Schrödinger stated that electrons do not move in set paths around the nucleus, but in waves. It is impossible to know the exact location of the electrons; instead, we have 'clouds of probability' called orbitals, in which we are more likely to find an electron.