Properties of Atoms and the Periodic Table Chapter 19 Sections 1 and 2

Section 1 Structure of the Atom

Structure of the Atom An element of matter is made up of one type of atom Atoms are composed of particles called protons, neutrons, and electrons Protons-positive charge Electrons-negative charge Neutrons-no charge Protons and neutrons are in centrally-located nucleus surrounded by a cloud containing electrons

Quarks Scientists hypothesize that electrons are not composed of smaller particles Protons and neutrons are made up of smaller particles called quarks The search for the exact composition of protons and neutrons is ongoing

Atomic Models Many different models of the atom were proposed throughout the years Aristotle argued that atoms didn’t even exist John Dalton’s provided evidence that atoms existed His model was a simple sphere Joseph Thomson proposed the “plum-pudding” model Ernest Rutherford’s model included a nucleus Niels Bohr suggested that electrons travel in fixed orbits around the nucleus Dalton’s Model Thomson Model Rutherford Model Bohr Model

The Electron Cloud Model By 1926, scientists developed the electron cloud model of an atom This is the currently accepted model The electron cloud is the area around the nucleus where its electrons are most likely to be found The cloud is 100,000 times larger than the diameter of the nucleus

Section 2 Masses of Atoms

Atomic Symbols All elements are represented by a chemical symbol The symbols consist of one capital letter or a capital letter and one or two lowercase letters Some symbols are the first letter of the elements name Oxygen O Some are derived from Latin Silver Argentum Ag Some are name in honor of scientists Some are named for where they were discovered The symbols are recognized worldwide

Atomic Number The number of protons in an atoms is known as the atomic number The number of protons in an atom is what gives an atom its identity Carbon will always have 6 protons Atomic number = 6 Neon will always have 10 protons Atomic number = 10 The atomic number is on the periodic table and is used to identify each element

Atomic Mass The nucleus contains most of the mass of an atom The mass of a proton and a neutron are very similar 1.67 x 10-24 g The mass of a proton (or a neutron) is 1,836 times greater than that of an electron Because the mass of proton or neutron is awkward to use, a standard measurement is used. Each proton or neutron has a mass of approximately 1 atomic mass unit (1 amu) Carbon has 6 protons and 6 neutrons 6(1) + 6(1)= 12 amu (mass of a carbon atom) Why didn’t we use the mass of an electrons?

Mass Number The mass number of an atom is the sum of the number of protons and neutrons Remember: This is because each proton or neutron has a mass of 1 amu If you know the mass number and the atomic number of an atom, you can determine the number of neutrons # of Neutrons = Mass Number – Atomic number Example: How many neutrons are in an Carbon atom that has a mass number of 14? What is the atomic number of Carbon? 6 14 – 6 = 8 There are 8 neutrons in the atom.

Isotopes All atoms of an element will have the same number of protons But not all the atoms will always have the same number of neutrons Atoms of the same element that have different numbers of neutrons are called isotopes Isotopes are usually identified by their mass number Carbon-12 (6 protons, 6 neutrons) Carbon-14 (6 protons, 8 neutrons) Boron-10 (5 protons, 5 neutrons) Boron-11 (5 protons, 7 neutrons) How many neutrons does Oxygen-18 have?

Average Atomic Mass Because not all the atoms of an element have the same mass, an average mass is used To figure out the average mass, we need to know how many of each isotope is present in a sample Scientists do this by using weighted averages For example: 80% of all Boron atoms are Boron-11 20% of all Boron atoms are Boron-10 .80(11) + .20(10) = 10.8 amu