The atom Ch 3

aristotle Aristotle: 384 - 322 B.C. Greek philosopher Described composition and behavior of matter in terms of 4 qualities: hot, cold, moist, and dry Said any substance could be changed into another by changing one of those qualities Meaning, we could change lead to gold Alchemy: a field of study concerned primarily with finding potions that would produce gold

democritus Democritus: 460 - 370 B.C. Greek philosopher “matter is composed of a finite number of incredibly small but discrete units we call atoms.” Coined the term atom from the Greek phrase atomos, meaning “not cut” or “that which is indivisible” The texture, mass, and color of a material are a function of the texture, mass, and color of its atoms.

lavoisier Antoine Lavoisier (1743 - 1794) An element is any material made of only one component Identified a compound as any material composed of two or more elements Lavoisier is considered the “father of modern chemistry” and ideas are in line with our present understanding His definitions required experimentation—Greek based their ideas on logic and reason.

Lavoisier Hypothesized that mass is always conserved during a chemical reaction Law of Conservation of Mass: amount of mass present after the reaction is the same as amount present before the reaction If I burn a piece of wood, does the wood weigh the same after as it does before? Why? Lavoisier accounted for gases! When the wood burns, gaseous carbon dioxide and water vapor are released!

dalton John Dalton: 1766 – 1844 self-educated English schoolteacher Each element consists of indivisible, minute particles called atoms Atoms can be neither created nor destroyed in chemical reactions All atoms of a given element are identical Atoms chemically combine in definite whole-number ratios to form compounds Atoms of different elements have different masses

mendeleev Dmitri Mendeleev: 1834-1907 Russian chemistry professor Produced a chart summarizing the properties of known elements Arranged the elements based on their properties Sometimes there were blanks that could not be filled by any known elements; he left these open and others eventually discovered the missing elements

thomson Thomson: 1856-1940 Used cathode ray tube experiments to discover negative part of atom, but was unable to calculate the charge or mass of the particle We know this particle today as the electron The electron determines many of a material’s properties, including reactivity

millikan Robert Millikan: 1868-1953 Calculated the numerical value of a single electric charge He calculated the mass of a cathode ray particle to be considerably less than that of the smallest known atom, hydrogen

rutherford Ernest Rutherford: 1871-1937 It was reasoned that if atoms had negative particles, there must also be a balancing positive force Used the gold foil experiment to show that atoms are mostly empty space with the majority of mass concentrated in a central core called the atomic nucleus

Atomic nucleus Atomic Nucleus: tiny central core of the atom where most of the mass is concentrated (99.99% of mass) Think of a marble inside a football stadium Determined that the nucleus was the balancing positive force in an atom and it was made up of subatomic particles

Subatomic particles Atoms are made up of three subatomic particles Neutron: neutral (no charge) particle in the nucleus Proton: positively charged particle in the nucleus Electron: negatively charged particle orbiting the nucleus in electron cloud In a neutral atom, the number of protons is equal to the number of electrons, meaning the atom is electrically balanced!

Subatomic particles Subatomic particles are extremely small; therefore scientists cannot measure them in grams Instead, they are measured in atomic mass units, or amu 1 amu = 1.61 x 10-24 g 1 g is about the mass of a paperclip

Atomic number Atomic Number (Z): number of protons in a nucleus Used to identify the element, like a fingerprint, because it NEVER CHANGES The modern periodic table lists elements in order of increasing atomic number

Atomic Mass Atomic Mass: total mass of an atom (sum of all components – e-, p+, and n0) Electrons are so much less massive than protons and neutrons, their mass is negligible Mass Number (A): total number of neutrons and protons in an atom subtract atomic number from mass number to find number of neutrons

electrons The trends found in the periodic table are a result of electron arrangement, specifically, the number of valence electrons Valence Electron: electrons that fill the outermost energy level

ions The group number of an element will tell you the number of valence electrons it has Group 1: 1 valence electron Group 2: 2 valence e- ’s Skip 3-12 Group 13: 3 valence e- ’s Groups 14-18: 4, 5, 6, 7, and 8 valence e- ’s respectively. The period number will tell you how many energy levels have electrons

ions Ions: charged particles (atoms that are not neutral) Formed when an atom gives up or gains electrons Cation: positive ion, has given away electrons Anion: negative ion, has gained electrons

ions All atoms want to have 8 valence electrons in the outer shell – making their outer shell full and the atom stable Elements really close to 8 tend to be the most reactive. Elements already “full” are considered inert, they don’t react because they don’t need to gain or lose electrons

isotopes Isotopes: atoms of the same element that contain a different number of neutrons Carbon-12: 6 neutrons Carbon-13: 7 neutrons Carbon-14: 8 neutrons

isotopes Find the number of neutrons by subtracting the atomic number (Z) from the mass number (A) Sometimes Z is not written because the atomic number is always the same for a specific element