9/13/2013. However, Dalton thought that an atom was just a tiny sphere with no internal parts. This is sometimes referred to as the cannonball model.

Transcription

1 John Dalton was an English scientist who lived in the early 1800s. Dalton s atomic theory served as a model for how matter worked. The principles of Dalton s atomic theory are: 1. Elements are made of tiny particles called atoms. 2. All atoms of a given element are identical. 3. The atoms of a given element are different from those of any other element. 4. Atoms of one element can combine with atoms of other elements to form compounds. A given compound always has the same relative numbers and types of atoms. 5. Atoms are indivisible in chemical processes. This means that atoms cannot be created or destroyed in chemical reactions. This is the law of conservation of mass. Dalton s atomic theory helped explain the law of constant composition. The law of constant composition states that a compound always contains the same proportion of elements. This makes sense because a compound must always contain the same number of each element. However, Dalton thought that an atom was just a tiny sphere with no internal parts. This is sometimes referred to as the cannonball model. 1

2 The structure of the atom was the result of research by many scientists over many years. Most of this research was carried out in the late 1800s and early 1900s. Some important scientists were Thomson, Rutherford, and Bohr, J. J. Thomson was a physicist who studied the atom in the 1890s. Thomson used a cathode ray tube to show that atoms can be made to emit (release) tiny negative particles. These particles were called electrons. He knew these electrons were negative because they were repelled by negative electric fields. Thomson s Cathode Ray Tube Experiment Thomson knew that whole atoms are neutral. This means that something positive is cancelling the negative electrons. To explain how the atom could be neutral, Thomson proposed the plum pudding model. According to the plum pudding model, the atom was a sphere of positive charge with negative electrons randomly scattered on the inside. 2

3 The Plum Pudding Model This contradicts Dalton s statement that atoms are indivisible. In fact, atoms are made of small subatomic particles and are divisible. In 1911, Ernest Rutherford wanted to prove that the plum pudding model was correct. He set up an experiment where tiny positive particles called alpha (α) particles were shot at a thin piece of gold foil. The gold foil was surrounded by a detector that produced flashes of light every time it was hit by an alpha particle. If the plum pudding model was correct, the alpha particles would have passed right through the gold foil. However, while most alpha particles did pass through, a few were deflected. 3

4 Rutherford expected alpha particles to pass right through the atom. Rutherford s Gold Foil Experiment Results Actual results of the gold foil experiment, showing that a few alpha particles were deflected Rutherford concluded that the deflected alpha particles must have bounced off of something inside the atom that was positive. He also knew that the positive part of the atom must be small because only a few alpha particles were deflected. Rutherford called this positive part the nucleus. The rest of the atom must be mostly empty because so many alpha particles passed through. 4

5 The two formal conclusions of the gold foil experiment are: 1. The atom contains a central nucleus that is small, dense, and positively-charged, 2. Most of the atom is empty space. An atom is made of three subatomic particles. The nucleus contains protons and neutrons. Protons are positively-charged. Neutrons have no charge (they are neutral). Electrons are found outside the nucleus and have a negative charge. Bohr s Model of the Atom Neils Bohr proposed the planetary model (also called the Bohr model), which placed the electrons on fixed paths called orbits. This model looks like the solar system. The Bohr model worked for the hydrogen atom, but did not work for any other atom. 5

6 The Wave Mechanical Model Today, we know that Bohr s model is too simplistic. Electrons actually move around the nucleus in waves, making it difficult to predict their exact locations at any given time. The wave mechanical model places electrons in orbitals. An orbital is an area where an electron is most likely to be found. Electrons do not follow fixed paths around the nucleus. The darker the color, the more likely it is that an electron will be found there. Since subatomic particles are so small, their sizes are measured in tiny units called atomic mass units (amu). Protons and neutrons are about the same size (1 amu). Electrons are much smaller than protons and neutrons, and have a size of 1 / 1836 amu. Protons and neutrons are responsible for most of the mass of an atom. Electrons are responsible for the chemical properties of an atom. Subatomic Symbol Charge Size Location Particle Proton p amu Nucleus Neutron n amu Nucleus Electron e 1 1 / 1836 amu Orbitals 6

7 Atomic Number on the Periodic Table An atom s atomic number is the number of protons in its nucleus. Each element has its own unique atomic number. You can find an element s atomic number on the Periodic Table. Atomic number This means that sodium has 11 protons in its nucleus. Since all elements on the Periodic Table are neutral, they must all have equal numbers of protons and electrons. So, if sodium has 11 protons, it must also have 11 electrons. An element s mass number is the number of protons plus neutrons in its nucleus. So, if a sodium atom has 11 protons and 12 neutrons in its nucleus, its mass number is = 23 Chemists use a special notation to show the atomic number and mass number of an element. Mass number Atomic number 23 11Na or Mass number Na-23 (You can look up the atomic number on the Periodic Table) 7

8 12 6 C Mass # = 12 Atomic # = 6 C-12 If you need to find the number of neutrons in an atom, just subtract the atomic number from the mass number. For example, how many neutrons are in an atom of Be-9? Answer: According to the Periodic Table, the atomic number of Be is 4. So, 9 4 = 5 neutrons Na Li Fe Mass # = 23 Atomic # = 11 Mass # = 7 Atomic # = 3 Mass # = 56 Atomic # = 26 Na-23 Li-7 Fe-56 How many neutrons are in: Be-9 Cr-52 Ca-41 5 neutrons 28 neutrons 21 neutrons All atoms of the same element must have the same number of protons, but they can have different numbers of neutrons. Atoms of the same element that have different numbers of neutrons are called isotopes. This contradicts Dalton s statement that all atoms of a given element are identical. S neutrons 8

9 Two Isotopes of Sodium The masses for the elements on the Periodic Table are average atomic masses. An element s average atomic mass is calculated by taking the weighted average of the masses of that elements naturally occurring isotopes. Notice that the only differences between these two atoms are the number of neutrons and the mass numbers. Sample Problem: Average Atomic Mass The element lithium has two naturally occurring isotopes. 7.59% of these isotopes have a mass of amu, and 92.41% have a mass of amu. Calculate the average atomic mass of lithium. Step 1: Change the percentages to decimals by shifting the decimal point two places to the left. 7.59% becomes % becomes Step 2: Multiply the new decimals by their masses and add the results = = amu Normally, atoms are neutral. This is because they have an equal number of protons and electrons. Atoms become charged ions when they either gain or lose electrons. Cations are positive ions formed when an atom loses one or more electrons. Losing 1 electron results in a +1 ion. Losing 2 electrons results in a +2 ion. Losing 3 electrons results in a +3 ion. 9

10 11 electrons 10 electrons 12 electrons 10 electrons 1 electron lost 2 electrons lost Neutral sodium atom (Na) Sodium ion (Na + ) Neutral magnesium atom (Mg) Magnesium ion (Mg 2+ ) 13 electrons 10 electrons electrons lost 13+ Notice that the more electrons the atom loses, the smaller its cation is. A cation is named using the name of its parent atom. You can find the names of the elements on Table S in your reference tables. Neutral aluminum atom (Al) Aluminum ion (Al 3+ ) 10

11 17 electrons 18 electrons Anions are negative ions made when an atom gains one or more electrons. Gaining 1 electron results in a 1 ion. Gaining 2 electrons results in a 2 ion. Gaining 3 electrons results in a 3 ion electron 17+ Neutral chlorine atom (Cl) Chloride ion (Cl ) Charges on the Periodic Table When an atom gains electrons, its anion becomes larger. To name an anion, drop the end of the element s name and replace it with ide. For example, fluorine becomes fluoride, chlorine becomes chloride, and oxygen becomes oxide. You can find out the charges of an element s ions by checking with the Periodic Table. Ionic charge Some elements have more than one charge. For now, just use the first charge listed. 11

12 Remember! Orbitals are the areas in an atom where electrons are most likely to be found. These orbitals can be thought of as principal energy levels, each with its own amount of energy. Each principal energy level can be divided into sublevels. Electrons can only occupy an energy level or sublevel that has the same amount of energy as that electron. Electrons can gain energy and move to a higher energy orbital. If the electron loses its gained energy, it drops back to its original orbital. When the electrons are in their lowest possible orbital, the atom is in the ground state. When energy is absorbed by the electrons, they move into a higher orbital and the atom is in the excited state. Visible light produced by electrons can be seen as thin lines of color called bright line spectra. The light is caused by excited electrons dropping back down to the ground state. The electrons of every element occupy orbitals. Each orbital holds a specific number of electrons. 1st orbital holds 2 e 2nd orbital holds 8 e 3rd orbital holds 18 e 4th orbital holds 32 e Each orbital must be filled before electrons can be placed in the next highest orbital. 12

13 The electrons that end up in the last orbital are called valence electrons. All of the other electrons are called the kernel. Valence electrons are important in determining the chemical properties of an atom. Kr The simplified electron configuration of each element can be found on the periodic table. Quantum numbers describe a more complex electron configuration. The principal quantum number is the same as the energy level of the orbital. Orbitals can have sublevels called s, p, d, and f s can hold 2 electrons, p holds 6, d holds 10, and f holds 14. Orbitals are drawn as boxes, with each box holding two electrons. 13

14 The two electrons are written as one up arrow and one down arrow. The order of orbital fill is: 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d 7s 7p Ar Atomic # 18 Electron Configuration: s 2 2s 2 2p 6 3s 2 3p 6 Level 3 is not complete, but it has 8 electrons K Atomic # 19 Electron Configuration: s 2 2s 2 2p 6 3s 2 3p 6 4s 1 Notice 3d is skipped ***This is why you need to know the diagonal rule*** Remember that excited electrons gain energy. When an electron gains energy, it jumps to a higher orbital. In this case, the lower orbital will not be filled up in the configuration. 14