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Atoms Atoms are everywhere and they determine all the properties of the matter they compose. The Atomic Theory: Today we know that matter is made up of atoms, but it has taken a long time to realize that. The beginning of the Atomic Theory: Democritus: (400 BC) -Greek philosopher who suggested the universe is made of indivisible units. •He called the units atoms. •”atom” comes from the Greek word atomos meaning unable to be cut or divided. -He thought movements of atoms caused the changes in matter that could be observed. Although Democritus’ theory explained some observations, he didn’t have the evidence needed to convince people that atoms existed. John Dalton: -English schoolteacher who revised the Atomic Theory in 1808. -Dalton agreed with Democritus that atoms couldn’t be divided. According to Dalton, all atoms of a given element were exactly alike, the atoms of different elements could join to form compounds. - Unlike Democritus, Dalton based his theory on experimental evidence by using the Law of Definite Proportions. Law of Definite Proportions: states a chemical compound always contains the same elements in exactly the same proportions by weight or mass. - Dalton’s theory is considered to be the foundations for the modern atomic theory. • Some parts were correct • His theory could not explain all experimental evidence. - Over time and experiments, Dalton’s theory changed slightly. Thomson’s Model of the Atom: 1897 – J.J. Thomson (British scientist) suggested that atoms were NOT indivisible. - He wasn’t experimenting on atoms, he was experimenting on electricity. (Studying cathode rays – “mysterious” rays in vacuum tubes.) Thomson’s cathode-ray tube experiment suggested that cathode rays were made of negatively charged particles that came from inside atoms. - This revealed atoms could be divided into smaller parts. - Thomson discovered electrons. - Thomson proposed a new model of the atom based on his discovery. • in his model, electrons were spread throughout the atom. (like blueberries in a muffin) Rutherford’s Model of the Atom: Shortly after Thomson proposed his model, Ernest Rutherford (British Scientist) designed an experiment to test Thomson’s model. - Rutherford found that Thomson’s model needed to be revised. - Rutherford proposed that most of the mass of the atom was concentrated in the atom’s center! • This conclusion was based on the results of his experiment. - Rutherford discovered the nucleus. o His experiment suggested that an atom’s positive charge is concentrated in the center of the atom. o This positively charged, dense core is called the nucleus! In Rutherford’s model, negative electrons orbit the positively charged nucleus in the same way the planets orbit the sun. **See Figure 7 Structure of Atoms: What is an atom? Atoms are made up of various subatomic particles. The three main subatomic particles are distinguished by mass, charge, and location in the atom. ** See Figure 1 - At the center of the atom is a small, dense nucleus. Inside the nucleus: Protons- Positive Charge (p+) = 1 amu Neutrons- No Charge (n0) = 1 amu • p+ and n0 are almost identical in size and mass. Mass is measured in Atomic Mass Units (amu) - Moving around outside the nucleus is a cloud of very tiny, negatively charged electrons (e-) • The mass of an e- is much smaller than that of a p+ or n0 Characteristics of Atoms: 1. Each element has a unique number of protons. - An element is defined by the number of p+ is in an atom. - As you move numerically through the Periodic Table, the protons increase by 1 for each element. 2. Unreacted atoms have no overall charge. (also called…Neutral Atoms) - Even though protons and electrons in atoms have electric charges, most atoms do not have an overall charge. o This is because most atoms have the same number of p+ and e- ….therefore the charges cancel out. - If an atom gains or loses e- it becomes charged. o A charged particle is called an ion. 3. Positive and Negative charges attract each other inside the atom. -This force is called Electric Force. -p+ and e- attract each other by electric force. - Electric force holds the atom together. •this is the force that holds solids and liquids together. Atomic Number and Mass Number Atoms of different elements have their own unique structures. Because atoms have different structures they have different properties. Atoms of each element have the same number of protons, but they can have different numbers of neutrons. The atomic number of an element tells you how many protons are in an atom of an element. - In neutral atoms, the atomic number equals the number of electrons. - Each element has a unique number of protons, therefore each has a unique atomic number. Mass Number equals the total number of subatomic particles in the nucleus. -Mass number equals the number of protons plus the number of neutrons in an atom of the element. Ex. Fluorine Atomic Number = 9 Mass Number = 19 ** Number of Neutrons = MN – AN Protons = 9 Neutrons = 10 - the mass number represents the number of protons and the neutrons…NOT ELECTRONS! Because protons and neutrons provide most of the mass of the atom. Isotopes Atoms of an element always have the same Atomic number, but not always the same mass number How can that be? If protons change…you change the element. However, the number of electrons can change and the number of neutrons can change. If neutrons change…then an isotope is formed. Isotope: elements that vary in mass numbers because their number of neutrons differ. -isotopes have the same number of protons, but a different number of neutrons relative to other atoms of that element. Some isotopes are more common than others. Ex. Isotopes of Hydrogen 1. protiummost common (no neutrons) 2. deuterium- (one neutron) 3. tritiumleast common (two neutrons) -to represent different isotopeswrite the mass number and atomic number of the isotope before the symbol of the element. Ex: Mass Number 35 17 17 protons 17 Electrons 35-17 = 18 neutrons Cl Atomic Number • if you know the mass number and the atomic number you can calculate the neutrons. Mass Number – Atomic Number = Neutrons ***An element’s average atomic mass refers to the weighted average of all the masses of all the naturally occurring isotopes of that element! *** Atomic Masses The mass of a single atom is very small. -because working with such tiny numbers is so difficult, atomic masses are usually expressed in atomic mass units. (AMU) -since there are so many isotopes of each element, the mass number listed in the periodic table is an average of all the isotopes in nature. How do we measure/ count atoms since they are so small? WE USE MOLES! Mole is the unit used to count particles. - a mole is a collection of a very large number of particles. 1 mole = 602 213 670 000 000 000 000 000 particles (6.02 x 1023) • this is called Avogadro’s Number • named for the Italian scientist Amedeo Avogadro - moles and grams can be related. • 1 mole of a substance is equal to the Atomic mass from the periodic table. Converting Moles to Grams to Particles: 1 mole 6.02 x 1023 Atomic Mass particles (g) (Avogadro’s Number) Compounds also have molar mass. - to calculate the molar mass of a compound add up the mass number of all the atoms in the compound. Modern Atomic Theory Dalton’s theory that atoms are indivisible had to be revised after the discovery that atoms are make of protons, neutrons, and electrons. Modern Models of the Atom: The modern model of the atom is very different from Rutherford’s model. - in modern atomic models, electrons can be found only in certain energy levels, not between levels. - Also, the location of electrons cannot be precisely predicted. 1913- Niels Bohr -Danish physicist - suggested that the energy of each electron was related to the path it follows around the nucleus. • electrons can only be found in certain levels. • electrons must gain energy to move to a higher level • electrons must lose energy to move to a lower level. Ex. If the atom is an apartment building with no stairwells, if the nucleus is in the basement then electrons would gain energy by riding up the elevator and lose energy by riding down the elevator. -Electrons can be on any floor but not in between. By 1925, Bohr’s model of the atom no longer explained all aspects of electron behavior. A new model was devised- which no longer assumed that electrons orbited the nucleus like planets orbiting the Sun. - according to the new model, electrons behave more like waves. The exact location of an electron can’t be determined. - the best scientists can do is to calculate the chance of finding an electron in a certain place within an atom. - best way to show them is in “orbitals” (levels) Energy Levels: Within the atom, electrons have various amounts of energy and exist on different energy levels. - there are many possible energy levels that electrons can occupy. - Each energy level holds a certain number of electrons. - The number of energy levels that are filled in an atom depends on the number of electrons. o See figure 3 - the electrons in the outer energy level are called valence electrons valence electrons determine the chemical properties of an atom. Electron Transitions: As we know, the modern model of the atom limits the locations of electrons to specific energy levels. They are never found between levels. - instead, it “jumps” from one level to the next. Why do they move? - electrons jump between levels when an atom gains or loses energy. - The lowest state of energy of an electron is called the ground state. - At normal temperatures, most electrons are in the ground state. - If an electron gains energy, it moves to an “excited state” o It gains energy by absorbing a particle of light called a photon.