P. Sci. Unit 9 Chapter 4 Atoms

Atomic Structure – timeline Ancient Greece - Democritus proposed the atom – a tiny solid particle that could not be subdivided – J.J. Thomson – discovered that atoms contained small, negatively charged particles called electrons.

1911 – Ernest Rutherford – proposed that the atom had two parts – the nucleus in the center (most of the mass) surrounded by the electrons – Niels Bohr – hypothesized that electrons traveled in fixed orbits around the atom’s nucleus.

1913 – James Chadwick – concluded that the nucleus contained positive protons and neutral neutrons – Erwin Schrodinger – developed the quantum mechanical model – which is based on the wavelike properties of the electron. (not a particle – leads to quantum physics)

1927 – Werner Heisenberg – (the Heisenberg uncertainty Principle) described that it is impossible to know precisely both an electron’s position and path at a given time. Led to the electron cloud theory.

Atoms The smallest particle that has the properties of an element.

Dalton’s Atomic Theory Every element is made of tiny, unique, particles called atoms that cannot be subdivided. Atoms of the same element are exactly alike. Atoms of different elements can join to form molecules

Parts of an Atom Nucleus – small, dense center of an atom made up of 2 subatomic particles that are almost identical in size and mass. –Protons – have a positive charge –Neutrons – have no charge

Parts of an Atom cont. Electrons – are tiny particles that have very little mass that moves around the outside of the nucleus. These particles are negatively charged and form a “cloud” around the nucleus.

The number of protons an atom has is unique for each element.

Atomic Charge Atoms have no overall charge because the protons (+) cancel out the electrons (-). Helium 2 protons neutrons electrons - -2 total charge 0

Protons positive (+) charge Found in the nucleus # of protons = atomic # The number of protons identify the element (atomic #)

Neutrons no charge Found in the nucleus Along with protons makes up atomic mass protons and neutrons are made up of quarks – which three quarks – determines whether it becomes a proton or a neutron.

Electrons negative (–) charge travel in orbitals (or energy levels) around the nucleus. (electron cloud) valence electrons - the # of electrons in the outer shell = oxidation #

Unit of measure for atomic particles is Atomic mass unit (amu) protons and neutrons = about 1 amu (electrons are about the size of 1 quark)

Chemical symbols The one or two letter abbreviation of the element name.

Mass Number or Atomic Mass the sum of the number of protons and the number of neutrons in the nucleus of an atom. # of neutrons = mass # - atomic # Protons + Neutrons

Average atomic mass the weighted - average mass of the mixture of all an atoms isotopes. The average atomic mass is close to the mass of its most abundant isotope.

Isotopes Atoms of the same element that have a different # of neutrons. (identified by the element name followed by the mass # ) ex. C-12, C-14, B-10, B-11) Carbon 12 = 6 neutrons Carbon 14 = 8 neutrons

For now