Atoms: the building blocks of matter Chapter 3 Chemistry chapter 3

The atom The atom – smallest piece of matter that has the properties of an element. Made of Protons Neutrons Electrons Each specimen of a specific subatomic particle is the same  If we split an atom, we no longer have a specific element Chemistry chapter 3

Early atomic theory - Democritus Greek philosopher about 400 B.C. Gave us the word atom Atomos - indivisible. Thought The world was made of empty space and particles called atoms. There were different types of atoms for different types of materials. Theory was not supported by experimental evidence. Chemistry chapter 3

Early atomic theory – Aristotle Aristotle did not believe in atoms thought matter was continuous He was very influential, so Democritus’s theory was not accepted for many centuries. Chemistry chapter 3

17th century People began to express doubts in Aristotle’s theory. Isaac Newton and Robert Boyle published articles stating their belief in the atomic nature of elements, but they had no proof. Their theory also had no ability to predict the unknown. Chemistry chapter 3

Antoine Lavoisier – late 1700s Law of conservation of mass during a chemical change in a closed system, no mass is lost Chemistry chapter 3

Joseph Proust – late 1700s to early 1800s Law of definite proportions specific substances always contain elements in the same ratio by mass Chemistry chapter 3

Law of multiple proportions Some elements form more than one compound with each other. If two or more different compounds are composed of the same two elements, then the ratio of their masses always contains small whole numbers Chemistry chapter 3

John Dalton – early 1800s Studied experimental observations of chemical reactions Proposed explanation of these three laws Chemistry chapter 3

Dalton’s Hypothesis All matter is composed of very small particles called atoms. All atoms of an element are exactly alike; atoms of different elements are very different. Atoms cannot be subdivided, created, or destroyed. Atoms unite with other atoms in simple ratios to form compounds In chemical reactions, atoms are combined, separated, or rearranged. Chemistry chapter 3

Did Dalton’s theory work? Conservation of mass the atoms are simply rearranged because they cannot be created or destroyed Laws of definite and multiple proportions Only whole atoms can combine, giving small whole numbers in ratios Chemistry chapter 3

Gas research J.L. Gay-Lussac Under constant temperature and pressure Volumes of reacting gases and gaseous products are in a ratio of small whole numbers. Amadeo Avogadro explained Gay-Lussac’s work with Dalton’s theory. Equal volumes of gases, under the same temperature and pressure, have the same number of molecules. Helped Dalton’s theory get accepted Chemistry chapter 3

Dalton’s theories Atomic theory and law of multiple proportions have been tested and accepted as correct. However, there some major exceptions to the rules. Splitting atoms Different atoms of the same element Chemistry chapter 3

Discussion Section review on page 69 Chemistry chapter 3

Cathode tubes Anode – positive electrode Cathode – negative electrode When the tube is on, cathode rays appear that begin at the cathode and travel to the anode. Chemistry chapter 3

Cathode rays and electrons 1897 – J.J Thomson tested cathode rays and discovered that they were electrons. Rays turned a paddlewheel – they had mass Rays deflected by a magnet just like current-carrying wire – they were negatively charged He determined the ratio of the electron’s charge to its mass. Chemistry chapter 3

Charge on an electron Robert Millikan’s famous oil drop experiment. Tiny oil drops fell through a chamber gravitational force offset by applying an opposing electrical force. Charge on oil drops determined This charge was always a whole number multiple of one small charge Chemistry chapter 3

Charge on an electron This small charge was the charge on one electron. This is now the standard unit of negative charge (1-). It can be written e-. e- can also represent an electron Chemistry chapter 3

Mass of an electron Using Thomson’s ratio and Millikan’s charge, determined to be 9.1 x 10-31 kg It was found that it’s mass is only 1/1837 the mass of the lightest atom known – the hydrogen atom. Most of the mass must be somewhere else Since atoms are neutral, there must be some positive charge Chemistry chapter 3

Thomson’s plum pudding model In this model, the raisins were the electrons and the pudding was the positive charge. Sort of like chocolate chip cookie dough. The chips are the electrons and the dough is the positive charge. Explained the experiments that had been done so far. Chemistry chapter 3

Testing the plum pudding model See page 72 fired alpha particles at a very thin (a few atoms thick) sheet of gold foil. They expected the particles to go right through because the spread out positive charge in the “pudding” wouldn’t be strong enough to deflect them. Chemistry chapter 3

What happened Most of the particles did go right through without being deflected at all. Some were deflected at large angles. Ernest Rutherford explained it: the positive charge on the atom was concentrated at a small core – now called the nucleus. Chemistry chapter 3

The atom as we now “know” it The nucleus contains all of the positive charge and most of the mass. The negatively charged electrons have very small mass and are located around the nucleus in the electron cloud. Most of an atom is empty space. Chemistry chapter 3

Protons same charge as an electron; opposite sign. standard unit of positive charge (1+) Much larger mass than the electron: 1.67 x 10-27 kg The number of protons determines the atom’s identity. Chemistry chapter 3

Neutrons Weren’t discovered until the 1930s. Neutral – no charge – harder to detect Slightly more mass than a proton: 1.68 x 10-27 kg Chemistry chapter 3

Nuclear or Strong Force The force that holds protons and neutrons together. It is effective only for very short distances – about 10-15 m. Chemistry chapter 3

Dalton’s theory Dalton thought that atoms were indivisible discovery of electrons, protons, and neutrons did not fit with his theory. Led to major revisions in atomic theory Chemistry chapter 3

Isotopes Thomson discovered what seemed to be two kinds of neon atoms. Same chemical properties; different masses. Atoms of the same element that differ in mass are called isotopes. Have the same number of electrons and protons but different number of neutrons. Chemistry chapter 3

Atomic number Number of protons in an atom Atoms are electrically neutral,  the number of electrons must equal the number of protons. The number of protons determines the identity of the atom and the number of neutrons determines the isotope. Chemistry chapter 3

Modification of Dalton’s theory All atoms of an element contain the same number of protons but can contain different numbers of neutrons. So we have to use average mass of an atom. Chemistry chapter 3

Nucleons Particles in the nucleus – protons and neutrons Chemistry chapter 3

Mass number Total number of nucleons : protons plus neutrons Number of neutrons = mass number minus atomic number Chemistry chapter 3

Designating Isotopes Hyphen notation Uranium-235 Carbon-14 Carbon-12 The number refers to the mass number Chemistry chapter 3

Nuclide General term for any isotope of any element Chemistry chapter 3

Atomic mass units There must be a standard for all units of measurement. A Carbon-12 atom with 6 protons and 6 neutrons was chosen as the standard Chemistry chapter 3

Atomic mass unit Defined as 1/12 the mass of that carbon atom. Chemistry chapter 3

Average atomic masses Many elements have an average atomic mass close to the number of nucleons in their nuclei – near whole numbers. Some don’t – look at Chlorine The periodic table shows average atomic masses. Chemistry chapter 3

Weighted averages We then use a weighted average to find the average mass of an atom of a given element. This is called the average atomic mass or just atomic mass. Chemistry chapter 3

Finding a weighted average A class of 25 students took a test. 10 of them got 80%. 12 got 90%. 3 got 100%. What was the average score? Not 90% - probably less than that. Chemistry chapter 3

You try Neon has two isotopes. Neon-20 has a mass of 19.992 amu and neon-22 has a mass of 21.991 amu. In any sample of 100 neon atoms, 90 will be neon-20 and 10 will be neon-22. Calculate the average atomic mass of neon. 20.192 amu Chemistry chapter 3

You try Compute the average atomic mass of silver, if 51.83% of the silver atoms occurring in nature have mass 106.905 amu and 48.17% of the atoms have mass 108.905 amu. 107.9 amu Chemistry chapter 3

The Mole SI unit for amount of substance Abbreviated mol A counting unit 6.022 x 1023 particles Avogadro’s number Based on carbon-12, 12 g of C-12 contains 1 mol of atoms Chemistry chapter 3

Molar mass The mass of 1 mol of a pure substance g/mol Numerically equal to the atomic mass in amu On the periodic table the number with a decimal is the atomic mass in amu AND the molar mass in g/mol Chemistry chapter 3

conversions Grams to moles or moles to grams Use the molar mass Chemistry chapter 3

Example What is the mass in grams of 5.60 mol of sulfur? Chemistry chapter 3

Example How many moles of carbon are in a sample with a mass of 567 g? Chemistry chapter 3

Example How many atoms of lithium are in a sample with a mass of 76.2 g? Chemistry chapter 3

You try How many moles of rubidium are in 3.01 x 1023 atoms of rubidium? Chemistry chapter 3

You try How many moles are in 0.255 g of zinc? Chemistry chapter 3

You try What is the mass of 1.20 x 1025 atoms of helium? Chemistry chapter 3