Democritus • Named the atom from the Greek word “atomos” meaning indivisible. • Wrote that atoms were the smallest unit of matter to keep its identity.
John Dalton • England 1780’s • All matter is made up of atoms. • All atoms of the same element are the same. • Atoms combine in specific ratios. • There is nothing smaller than an atom.
J. J. Thomson • Discovered the electron while running electricity through a gas. • Discovered that electrons were negative. • “Plum pudding” model of the atom.
Marie & Pierre Curie • Provided evidence for the existence of protons and neutrons. • Discovered radium and polonium. • Marie Curie was the first female winner of the Nobel prize and the first double winner.
Ernest Rutherford • Discovered that the nucleus was positively charged and very tiny. • Found that the atom is mostly empty space.
Niels Bohr • Developed the “solar system” model of the atom. • The nucleus in the center of the atom is made of protons (+) and neutrons. • Electrons (-) orbit the nucleus like the planets around the sun.
Electron Cloud Model • The current model of the atom. • Electrons do not orbit the nucleus in orderly paths. • Electrons move around the nucleus at near the speed of light in a general area. • Research continues.
Protons • Found in the nucleus of the atom. • Have a charge of +1 • Have a mass of 1 atomic mass unit (amu)
Neutrons • Found in the nucleus of the atom. • Mass of 1 amu • Neutrons have no charge.
Electrons • Found in orbits (energy levels or shells) around the nucleus. • Travel at near the speed of light. • Charge is –1 • Mass is 1/1800th amu.
What makes one element different from another? • The number of protons is always the same as the atomic number. • The number of electrons is usually the same as the atomic number. • To find the number of neutrons: take the atomic mass, rounded to the nearest whole number, and subtract the atomic number.
Finding the number of neutrons The mass number of this isotope of lithium is 7. Notice that 7 is equal to the total number of protons and neutrons. If you remove the protons (atomic number), the neutrons are left.