Download 8th Grade Science Notes Chapter 2

8th Grade Science - Chapter 2 Notes
Matter - anything that has mass and volume
Atoms - the smallest piece of an element that still represents that element. It is the ‘stuff’
that makes up matter. 99% of an atom is empty space.
Democritus - ancient Greek philosopher who believed everything was made of small,
indivisible objects, which he called atomos.
Aristotle - ancient Greek philosopher who believed everything was made up of a
combination of four elements: fire, water, air, earth.
John Dalton - came up with the atomic theory of matter.
JJ Thompson - discovered the electron. His atomic model had electrons orbiting the center
of the atom.
Electron - a particle that orbits the nucleus of an atom. It has one negative electrical charge
(1-).
Ernest Rutherford - discovered the nucleus of an atom.
Nucleus - the center of an atom where most of its mass is located. Protons and Neutron
make of an atom’s nucleus.
Proton - an atomic particle that has one positive charge (1+).
Neutron - an atomic particle with no electrical charge.
Protons & Neutrons have the same relative mass
Niels Bohr - proposed an atomic model that placed electrons in circular orbits called
energy levels.
Electron Cloud - the modern atomic model. Electrons move in an area represented as a
cloud around the nucleus.
Quarks - smaller particles that make up protons and neutrons. There are six: up, down,
charm, strange, top, bottom.
Atomic Number - the number of protons in an atom of an element. For example: carbon
atoms have 6 protons each, oxygen atoms have 8 protons each, etc.
Neutral Atoms - most atoms contain the same number of electrons as protons making
them electrically neutral.
Isotopes - atoms of the same element that have different numbers of neutrons.
Mass Number - is the sum of the protons and neutrons in an atom
Average Atomic Mass - is the average mass of the element’s isotopes
Radioactive Elements - elements that spontaneously emit (give off) radiation
Radioactivity - energy given off spontaneously by certain elements. E.g. uranium
Spontaneous - without external force or cause
Nuclear Decay - a process that occurs when an unstable atomic nucleus changes into
another more stable nucleus by emitting radiation.
Alpha Particle - a particle formed of two protons and two neutrons
Alpha Decay - when an atom releases an alpha particle. This forms a new element.
Beta Particle - a high-energy electron (x-rays)
Beta Decay - when a neutron changes into a proton by releasing a beta particle.
This forms a new element.
Gamma Rays - high energy rays that do not contain particles (The Hulk)
Ion - an atom that is no longer neutral because it has gained or lost electrons. Positive ions
contain more protons than electrons, negative ions contain more electrons than protons
Periodic Table - a chart of the elements arranged into rows and columns according to
their physical and chemical properties.
Groups - a column in the periodic table
Periods - a row in the periodic table
Metals - shiny, conduct heat & electricity. Nearly ¾ of periodic table
Nonmetals - most are gases. Do not conduct heat or electricity
Metalloids - have properties of both metals and nonmetals
Compounds - formed when atoms of different elements chemically combine forming a
chemical bond.
Valence Electron - an outermost electron of an atom that participates in chemical bonding.
Electron Dot Diagram - a model that represents valence electrons in an atom as dots
around the element’s chemical symbol.
Noble Gases - have 8 valence electrons and are chemically stable.
Atoms with unpaired dots in their electron dot diagrams are reactive or chemically
unstable.