P. Sci.Unit 8

Chapter 4Atoms

Atomic Structure – timeline • Ancient Greece - Democritus

proposed the atom – a tiny solid particle that could not be subdivided.

• 1904 – J.J. Thomson – discovered that atoms contained small, negatively charged particles called electrons.

Dalton’s Atomic Theory (#9)• 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 or compounds

• 1911 – Ernest Rutherford – proposed that the atom had two parts – the nucleus in the center (most of the mass) surrounded by the electrons.

• 1913 – 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.

• 1926 – 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.

electron

neutron

proton

Atoms• The smallest particle that has the

properties of an element.

Parts of an Atom• Nucleus – small, dense center of

an atom made up of 2 subatomic particles that are identical in size and mass.–Protons – have a positive charge–Neutrons – have no charge

Parts of an Atom cont.• Electrons – are tiny subatomic

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 and electrons 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 - +2 2 neutrons - 0 2 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• Atomic Mass – atomic number = # of neutrons (rounded to whole #)

Electrons• negative (–) charge • travel in orbitals (or energy levels)

around the nucleus. (electron cloud) • Equals atomic number in neutral atoms• valence electrons - the # of electrons

in the outer shell and relates to the oxidation #

• Unit of measure for atomic particles is Atomic mass unit (amu) protons and neutrons = about 1 amu (electrons are about 1/2000 of the size of protons and neutrons))

Chemical symbols• The one or two letter abbreviation of the element name.• Some are based on Latin name• ALL 1st letter is upper case• ALL 2nd letter is lower case

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 #

Atom Summary• Atomic Number = protons = electrons• Atomic Mass = Protons + Neutrons

• Neutrons = atomic mass – atomic number• Atomic symbols

– First letter is ALWAYS upper case– Second letter is ALWAYS lower case

• Example: Identify the Number of Protons, Neutrons and Electrons in OxygenOxygen element 6 with mass 16P = 8 N = 16-8 = 8E = 8

Atom Summary• Example 2: Identify the Number of Protons,

Neutrons and Electrons in Sulfur and Sodium

Isotopes• Atoms of the same element that have a

different # of neutrons and a different atomic mass. (identified by the element name followed by the mass # )

• ex. C-12, C-14, B-10, B-11)

6 electrons6 protonsCarbon 12 = 6 neutrons

Carbon 14 = 8 neutrons6 protons

6 electrons

Isotope Practice - Find Protons Neutrons and Electrons for following elements

• Silicon– Protons = 14– Electrons = 14– Neutrons = 28-14=14

• Si-30 (Silicon-30)– Protons = 14– Electrons = 14– Neutrons = 30-14=16

• Uranium– Protons = 92– Electrons = 92– Neutrons = 238-92=146

• U-235 (Uranium-235)– Protons = 92– Electrons = 92– Neutrons = 235-92=143

Remember protons ALWAYS equals the atomic number. In neutral atoms electrons equals protons. But neutrons is atomic mass minus atomic number. For isotopes you MUST use the mass given in the name NOT the mass on the periodic table

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.

• This is the number found on the periodic table

Using Bohr’s Atomic Model• Bohr was the 1st person to propose the concept of

electrons having specific energy levels• This explained how electrons could give off light ( by

gain or lose energy) • Bohr’s Model can be used to show how the

electrons will arrange themselves around a nucleus.

BOHR MODEL• Electrons are placed in energy levels surrounding the nucleus

• You fill the lower energy levels first • 1st energy level can hold max of 2 electrons• 2nd energy level can hold max of 8 electrons• 3rd energy level can hold max of 18 electrons• 4th energy level can hold max of 32

Nucleus

(p+ & n0) 2e-

8e-

18e-

ExampleChlorine17 protons17 electrons18 neutrons

Valance Electrons: Electrons found in outer energy level.Chlorine has 7 electrons in outer energy level (ring 3)