Introduction to Atoms

Introduction to Atoms Chapter 10 – Section 1  Atom: the smallest unit of an element that maintains the chemical properties of that element. The idea of an atom was developed by Democritus. All matter is made of atoms. 440 B.C.  Aristotle disagreed!

Section 1  Scientists had discovered that all things combine in set ratios! John Dalton (school teacher/chemist) wanted to know why????  Dalton’s Atomic Theory: 1803  All substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed.  Atoms of the same element are exactly alike, and atoms of different elements are different.  Atoms join with other elements to make new substances.  This was NOT ALL CORRECT!

Section 1 Pg. 262: Cathode-ray tube There are small particles inside an atom. Atoms can be divided into smaller parts. Electrons = negative charge. Plum-pudding model: proposed by J.J. Thompson….thought that electrons were mixed throughout an atom like plums in pudding. Ernest Rutherford further tested this…pg. 263.

Section Rutherford (former student of Thompson) revised the atomic theory. He developed the idea of a nucleus. He had electrons as free floating. Niels Bohr: Bohr’s electron levels (1913): electrons move around in certain paths or energy levels. There are no paths between levels but electrons can jump between levels if energy is added (heat). According to Bohr, electrons will remain at lowest energy level until enough energy is added. Bohr did a lot of experiments with light.

Section 1 Current atomic theory: Schrodinger and Heisenberg: electrons do not travel in definite paths as Bohr suggested. Their path cannot be predicted. There are regions where they are likely to be found. This is called an electron cloud.

Section 2  Atoms are composed of electrons, protons and neutrons---protons and neutrons are found in the nucleus.  Electron cloud: a region around the nucleus of an atom where electrons are likely to be found

Section 2  Electron: a subatomic particle that has a negative charge. Much smaller than neutrons and protons. Takes 1800 to equal the mass of 1 proton.  Neutron: a subatomic particle that has no charge and that is found in the nucleus of an atom  Proton: a subatomic particle that has a positive charge and that is found in the nucleus of an atom.  A single penny contains 20 thousand billion billion atoms.

Section 2 Nucleus: in physical science, an atom’s central region, which is made up of protons and neutrons. Protons and Neutrons are the most massive particles in an atom. They are about the same size but neutrons have the slightest bit more mass. Protons and electrons may differ in size but their charges cancel each other out. If charges are unequal then you will have an ion. -1 electron then positive ion. +1 electron then negative ion. The diameter of the nucleus is 1/100,000 the diameter of the atom.

Introduction to Atoms Chapter 10 – Section 2  Atomic mass unit: a unit of mass that describes the mass of an atom or molecule. Scientist made this specifically for atoms because they are so small. Each proton has a mass of 1 amu. Neutrons are larger (mass) but not by much. So their mass can also be estimated to 1 amu. Nucleus is small and very dense. If a nucleus were the size of a grape it would weigh 9 million metric tons.  Atomic number: the number of protons in the nucleus of an atom; the atomic number is the same for all atoms of an element

Introduction to Atoms Chapter 10 – Section 2 Isotope: an atom that has the same number of protons (or the same atomic number) as other atoms of the same element but has a different number of neutrons (and thus a different atomic mass). Unstable isotopes tend to be radioactive. They will change over time. This is half-life. Mass number: the sum of the number of protons and neutrons in the nucleus of an atom. This is how you can tell isotopes apart. Electrons have very little effect on the total mass of an element.

Section 2 Naming isotopes: name of element and hyphen, then mass number. Ex. Carbon-12. Atomic mass: the mass of an atom expressed in atomic mass units. Why are mass numbers not all whole numbers???

Section 2 Forces: pg. 272  Gravitational Force: Force acts between all objects all the time. Pulls objects toward one another. Us toward the earth.  Electromagnetic Force: Objects with opposite forces attract while like forces repel.  Strong Force: what keeps protons in the nucleus. The strong force is stronger than the electromagnetic force. Protons want to push away from each other (positive charges repel).  Weak Force: assists in radioactive change----neutrons changing to protons and electrons. It plays a role in radioactive decay.