Chapter 3: Atoms and Moles Mr. Hua

Warm-Up (09/02/14) Draw an atom. Label all parts, if any. What is a compound? (hint: related to atoms)

Precision vs. Accuracy Scientists need to be precise and accurate when making measurements Precision – the exactness of a measurement; measures how close multiple measurements are Accuracy – description of how close a measurement is to the true value

Precision You are precise when you are consistent. You weigh out something 3 times and get 3.9g, 4.0g, and 4.1g. All these numbers are similar and agree with each other The actual value was 4.5 so these ARE NOT accurate.

Precision vs. Accuracy

Accuracy How close your measurement is compared to the true or actual value I ask you to weigh 5g of a substance. You weigh 5.3g, 5.7g, and 4.5g. Which number is the most accurate?

Significant Figures Tells your how many digits to record There are many rules In a problem, use the same number of figures given Ex. What is 10.5 x 9.45? (Answer is , but your answer should be rounded to 99.2) [3 sig figs] Write out all measurements as given by devices

History of the Atom What are the building blocks of all matter? 400 BC : Democritus came up with the idea that there’s a limit to how much you can divide something; There is a unit that is indivisible, the atom (atomic theory)

Warm-Up (09/03/14) Atoms can be divided into three parts. What are these parts? What charge (positive, negative, neutral) do these parts have?

Please come in and sit quietly. Take out 1 sheet of paper.

POP QUIZ Period 6 Only 1. When two or more atoms of different elements combine, they form _____________. 2. Explain the difference between precision and accuracy. You may use pictures/diagrams.

History of the Atom What are the building blocks of all matter? 400 BC : Democritus came up with the idea that there’s a limit to how much you can divide something; There is a unit that is indivisible, the atom (atomic theory)

Law of Definite Proportions A chemical compound always contains the same elements in exactly the same proportions by weight/mass

Law of Definite Proportions Ex. Table salt, or NaCl is always made of sodium and chloride in the same proportions By mass: 60.66% Chlorine and 39.34% Sodium

Law of conservation of mass Mass cannot be created or destroyed

Dalton’s Atomic Theory 1. Atoms cannot be subdivided, created or destroyed 2. All atoms of an element are identical Are these true or false? FALSE!!!!!!!

Dalton’s Atomic Theory 1. Has been modified 2. Has been modified 3. Atoms of different elements differ in their __________ and ___________ properties 4. Atoms of different elements combine in simple, whole number ratios to form _____________. 5. In ___________ reactions, atoms combine, separate, and rearrange, but are never created, destroyed or changed.

Cathode Ray Experiment (J.J. Thomson)

Exit Slip (don’t write ?s) Why is it important to study the atom before other chemistry topics? How did Thomson determine electrons were negatively charged? One question or thing you’d like to know more about atoms

Warm-Up (September 4 th ) I ask you to weigh something that is known to be 30g. Give me 3 measurements that are precise. What are 2 accurate measurements?

Thomson’s Atom Model Plum-pudding model: negative electrons in a ball of positive charge

Protons + Neutrons = Nucleus Nucleus: atom’s central region; contains most of the atom’s mass The nucleus is much smaller than the entire atom Radius of an atom is 10,000x larger

Atomic Number Atomic number = number of protons Protons and electrons have equal and opposite charge, so for neutral atoms, # protons = # electrons

Atomic Number Atomic number = number of protons Protons and electrons have equal and opposite charge, so for neutral atoms, # protons = # electrons Element name Atomic number (#protons) Atomic symbol Mass number

Mass Number Mass Number – Atomic Number = Number of Neutrons Mass number Atomic number (#protons) Calculate the number of neutrons in Aluminum

Isotopes Isotope: An atom that has the same number of protons but different number of neutrons than other atoms of the same element Mass number Atomic Number (#protons) How many neutrons?

Class Practice How many protons? How many electrons? How many neutrons? Atomic mass?

Isotopes Practice How many protons and neutrons are in a regular Uranium atom? What is it’s mass number? How many protons and neutrons are in Uranium-235? What is the difference between them?

Warm-Up (Sep 9,2014) Draw the model of an atom and include protons, neutrons, and electrons. Why do scientists use models?

Rutherford’s Atomic Model Rutherford proposed electron orbits What is wrong with his model?

Bohr’s Atomic Model Electrons are confined to assigned energy levels Closer to nucleus  lowest energy level

Electrons Act Like Both Particles and Waves Electrons have mass  particle Electrons are confined to the space around a nucleus Electrons  waves with specific frequencies (energy levels) Electrons are located in orbitals (electron clouds)

Electrons, energy, and Light Normally, electrons will be in the ground state – the lowest possible energy If an electron gains energy, it moves to an excited state (more energy)

What happened? (Think of electrons, energy, light) Energy is emitted as wavelengths of light

Counting Atoms Atomic mass – mass of an atom expressed in atomic mass units (AMU) [Same number as mass number] Mole – SI unit used to measure amount of substance Carbon-12 used as the standard for the atomic mass scale Molar mass – the mass in grams of 1 mol of a substance (g/mol) Avogadro’s number – x10 23 ; the number of atoms/molecules in 1 mol

Practice Problem What is the mass (in grams) of 1.0 mol of aluminum? What is the mass of 2.0 mol of Al? How many moles are in 27g of Aluminum?

Practice Problem #2 How many moles of Nitrogen are represented by 35g of Nitrogen? What is the mass of 4.5 mol of Nitrogen?