8th Grade Science-Atoms Unit

Properties of Atoms and the Periodic Table

Structure of the Atom

• Chemical Symbols-consist of one capital letter or a capital letter plus one or two small lettersex: table 1—pg. 544

Atomic Components

• Atoms—the smallest piece of matter that still retains the property of the element

• Atoms: Contain protons and neutrons in the nucleus

• Electrons—contained in electron cloud

• Quarks—smaller particles that comprise protons and neutrons

• Scientists have confirmed the existence of six uniquely different quarks

• The search for the composition of protons and neutrons is an ongoing effort

Atomic Theory-Past Models of the Atom• Democritus—”uncuttable”

composed of tiny, solid particles that could not be subdivided—(atomos)

**Dalton—solid sphere**Thomson Model—”cookie

dough”charged particles were evenly embedded throughout a positively charged sphere

(more models)

• Rutherford Model—proposed almost all the mass of an atom (& + charges) were concentrated in a central nucleus surrounded by electrons

• Bohr Model—– Hypothesized electrons

traveled in fixed orbits

Electron Cloud Model (Quantum Mechanical)• 1926—current model• Electron cloud—area

around the nucleus of an atom where its electrons are most likely found

• Energy levels are areas of the cloud where electrons are more likely 2 B found

Masses of Atoms• AMU—unit of measurement

used for atomic particles• Mass of 1 proton or 1

neutron is almost 1 amu.• Atomic number—the

number of protons in an atom

• Mass number—the sum of the number of protons and the number of neutrons in the nucleus

• Atomic mass = decimal number under the symbol on the periodic table

• NOT THE SAME THING AS ATOMIC NUMBER or MASS NUMBER

• It is the average weight of all atoms of the element (including isotopes)

Calculating neutron number

• If you know the mass number and atomic number of an atom, you can find the number of neutrons as well,.

• Neutron # = mass # - atomic #

Isotopes

• Carbon-12 is the most common form of carbon

• However, Carbon-14 is a radioactive form of carbon

• WHY?• Not all the atoms of an

element have the same number of neutrons

ISOTOPES

• Isotopes—atoms of the same element that have different numbers of neutrons

• Average atomic mass—the weighted average mass of an element’s mixture of isotopes (used because most elements have more than one isotope)

The Periodic Table

• Dimitri Mendeleev-arranged all the elements known in order of increasing atomic masses and discovered a pattern

• Today’s Periodic Table—elements are arranged by increasing atomic number and by changes in physical and chemical properties

• Mendeleev-left blank spaces to keep elements in line according to chemical properties

• He predicted the existence of two elements not yet discovered

Groups

• The vertical columns in the periodic table-also called families

• Periodic Table—pg. 556 and 557

Electron Cloud Structure

• In neutral atoms, the electron number = the proton number

• SO: Atomic number tells how many protons AND how many electrons the element has

• Elements in the same group have the same number of electrons in their outer energy level-called VALENCE ELECTRONS

• This number determines properties of the elements

Number of Atoms in Energy Levels• Energy Level 1—holds 2 e• Energy Level 2—holds 8 e• Energy Level 3—holds 18

e• Energy Level 4—holds 32

e

• Octet Rule—explains that atoms are most stable with an outer valence holding 8 electrons

Periods/Rows• Horizontal groups

across the periodic table

• Each row ends with an element with a full outer valence (8 electrons)

• Periods increase by one proton and one electron going L to R

Metals

• Metals are on the left-hand side of the periodic table

• Most are shiny, ductile, malleable and are good conductors

• Ductile—drawn into wire• Malleable-can be

hammered into sheets

Non-metals

• Non-metals are on the right side of the periodic table

• Most are gases, brittle, and poor conductors

Metalloids

• Metalloids-run along the middle zig-zag line

• Metalloids have some of the properties of both metals and non-metals

• Carbon-14has 6 protons and 8 neutrons

Isotopes—atoms of the same element that have different numbers of neutrons

Radioactive isotopes—those isotopes that are unstable and become radioactive

Identifying Isotopes

• Average atomic mass—the weighted-average mass of the mixture of its isotopes

• Ex: 4 out of 5 atoms of B are boron-11 and 1 out of five is boron-10

• Weighted average = 4/5 (11) + 1/5(10) = 10.8

AMU

Periodic Table websites:

• www.chemicool.com• www.Ptable.com• www.periodictable.com• www.webelement.com