Atoms Chapter 4

Let’s Review! Matter is… Anything that has mass and takes up space All matter is made of elements – substances that cannot be broken down And elements are made of – ATOMS! The atom is the SMALLEST unit of matter

ATOMS ARE EVERYWHERE! They are in the air you breathe, the chair you’re sitting on, and the clothes you’re wearing.

Atomic Theory Section 1

Democritus (400 B.C.) Ancient Greece Theorized that matter could not be divided infinitely, you had to reach a smallest piece Atomos: indivisible or can’t be cut (becomes atom) Since this was ancient Greece, he had no proof and few people believed it.

John Dalton (1808) People started to accept his idea of atoms because of his experiments He thought: all elements are made of atoms (indivisible and indestructible) atoms of the same element are exactly alike atoms of different elements are different

Law of Definite Proportions A chemical compound is made up of the same percentage of elements. For example, water is always made up of 2 parts hydrogen to 1 part oxygen. Therefore we have the equation H2O This is true for all chemical compounds! This law supported Dalton’s theory of atoms.

Sir J. J. Thomson (1897) Conducted an experiment that showed atoms could be divided Used cathode ray experiments to show negatively charged particles that came from inside atoms These are electrons!

Cathode Rays There are two metal plates at the end of a vacuum tube Cathode, which has a negative charge Anode, which has a positive charge When voltage is applied across the plates, a glowing beam comes from the cathode and strikes the anode Since it was a vacuum tube (a tube with all the air vacuumed out), he could see that the beam came from the negative end of the cathode

Plum Pudding He proposed that electrons are spread throughout the atom just like blueberries are in a muffin Plum Pudding Model

Thomson Found negative particles could come from neutral elements Atom is made of smaller things (+ & -), and is divisible Successfully separated negative particles (electrons) but could not separate the positive particle

Ernest Rutherford (1911) Fired positively charged particles at a sheet of gold foil Most went through unaffected, some bounced away His experiments suggested that an atom’s positive charge was concentrated at the center of the atom This is the NUCLEUS!

Ernest Rutherford Electrons are scattered near the outside of the atom with mostly empty space between the nucleus and the electrons Compared to the atom, the nucleus is very small Rutherford’s experiments led to a new model of the atom

Niels Bohr (1913) Rutherford proposed that electrons orbited the nucleus Similar to planets orbiting the sun Bohr discovered the electrons are actually in energy levels (orbitals) Their path around the nucleus is limited to their energy Only certain electrons can be in certain levels

Modern Atomic Theory Today we know that atoms have A nucleus with Protons Neutrons Electrons that orbit the nucleus

TUESDAY, March 8th AFTER THE QUIZ: You need: Guided Reading Notes Calculator Periodic Table

Atomic Structure Section 2

Subatomic Particles Subatomic: lower (or smaller) than an atom Protons: positive charge Neutrons: no charge (neutral) Electrons: negative charge Protons and neutrons are both in the nucleus of the atom. Electrons are buzzing around the outside in the electron cloud

Mass and Volume The nucleus makes up 99.99% of the mass of the atom. However, the nucleus is 1/100,000 of the volume of an atom. The volume is determined by the electron cloud.

Mass of Particles Since subatomic particles are so small they cannot be measured in grams They are measured in atomic mass units or amu 1 amu = 1.61x10-24 g 0.00000000000000000000000161! 1 g is about the mass of a paper clip!

Mass of Particles protons p+ positive nucleus 1 amu neutrons no name symbol charge location mass protons p+ positive nucleus 1 amu neutrons no neutral electrons e- negative electron shell .0006 amu

How do we know how many protons, neutrons, and electrons are in an atom?

Atomic Number & Atomic Mass number of protons found in the nucleus (therefore, it is also the number of electrons orbiting the nucleus) Atomic Mass: weight of the protons and the neutrons combined.

Let’s Try! Atomic # Mass Protons Neutrons Electrons 14 28 Atomic # 3 7 4 Atomic # Mass Protons Neutrons Electrons 9 19 10 Atomic # Mass Protons Neutrons Electrons 90 232 142

Charges The # of protons in an atom is unique to each element (this is how they are identified) If an atom is stable, there will be an equal number of protons and electrons This means there will be NO overall charge on the atom The number of neutrons in an atom can vary also!

Electrons Electrons fill orbitals around the nucleus Each orbital can only hold so many electrons The number of levels filled depends on how many electrons the atom has The electrons that fill the outermost orbital are called valence electrons

Isotopes The atomic number, or number of protons, will never change, but the number of neutrons CAN change Isotope: Any two atoms of the same element with a different number of neutrons. Isotopes have the SAME chemical and physical properties. There are normally one or two stable isotopes for an element All others are unstable (they fall apart) radioactive decay

Representing Isotopes Isotopes are shown in isotope notation Since isotopes have a different atomic mass, you must write the symbol of an element AND write its atomic mass

MONDAY, March 14th PICK a PARTNER that you are SURE you will succeed with in lab today. You will walk around the classroom together and compile information about atoms for your data table. THEN return to your seat and FINISH the data table (Atomic #, MASS, ELEMENT, SYMBOL, ISOTOPE?) ANSWER Analysis Questions. SUBMIT BEFORE end of PERIOD.

RED- PROTONS BLUE-NEUTRONS YELLOW- ELECTRONS