8 th 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 letters ex: 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-14 has 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: