2 AtomsAll matter, however solid it appears, is made up of tiny building blocks, which themselves are mostly empty space. These are atoms - which can combine to form molecules, which in turn flock together to form the matter that we see around us.
3 Greeks - AtomsThe idea that matter is composed of atoms goes back to the Greeks in the fifth century BC.
4 Aristotle - AtomsAristotle, the most famous of the early Greek philosophers, didn't agree with the idea of atoms. In the fourth century BC he taught that all matter is composed of different combinations of four elements - earth, air, fire, and water. This view seemed reasonable, for in the world around us matter is seen in only four forms: solids (earth), gases (air), liquids (water), and the state of flames (fire). The Greeks viewed fire as the element of change, since fire was observed to work changes on substances that burned. Aristotle's ideas lasted for more than 2000 years.
5 John Dalton - AtomsThe atomic idea was revived in the early 1800s by an English chemist and school teacher, John Dalton. He explained chemical reactions by supposing all matter is made of atoms.
6 Robert Brown - AtomsThen in 1827 a Scottish botanist, Robert Brown, noticed something very unusual in his microscope. He was studying grains of pollen suspended in water and saw that the grains were continually moving and jumping about. This perpetual jiggling of particles—now called Brownian motion —results from collisions between visible particles and invisible atoms. The atoms are invisible because they're so small. The pollen grains that Brown observed moved because they were constantly being jostled by the atoms (actually the atomic combinations called molecules) that made up the water surrounding them.
7 Albert Einstein - Atoms All this was explained in 1905 by Albert Einstein, the same year that he announced the theory of special relativity. Until Einstein's explanation—which made it possible to find the masses of atoms - many prominent physicists remained skeptical of atoms. So we see that the reality of the atom was not established until the early twentieth century.
8 Richard Feynman - Atoms In 1963 the importance of atoms was emphasized by the physicist Richard Feynman, who stated that if some cataclysm were to destroy all scientific knowledge and only one sentence could be passed on to the next generation of creatures, the statement with the most information in the least words would be: “ All things are made of atoms—little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. ” All matter—shoes, ships, sealing wax, cabbages, and kings - any material we can think of is made of atoms. This is the atomic hypothesis, which now serves as a central foundation of all of science.
9 Characteristics of Atoms The smallest particle of an element that has all of the element’s chemical properties. Atoms are the building blocks of matter. Atoms are incredibly small . Atoms are numerous . There are about 1023 atoms in a gram of water. Atoms get around as they are perpetually moving. In air they move around at speeds up to ten times the speed of sound. Atoms are ageless - they cycle from person to person.
10 Atomic ImageryAtoms are too small to be seen with visible light. because light is made up of waves, and atoms are smaller than the wavelengths of visible light. Atoms are incredibly small.
11 Atomic ImageryThe wavelength of an electron beam is smaller than the wavelength of visible light. So atoms are larger than the tiny wavelengths of an electron beam.Historic 1970 image of chains of individual thorium atoms.The photograph was not made with light but with a thin electron beam in a scanning electron microscope developed by Albert Crewe at the University of Chicago's Enrico Fermi Institute.
12 AtomsAtoms are so small that we can’t see inside an atom, we construct models. Classical model of the atom consists of a tiny nucleus surrounded by electrons that orbit within spherical shells.
13 Atomic StructureThe most familiar model of the atom is akin to that of the solar system. Most of the volume is empty space, and small parts orbit about the center where most of the mass is concentrated. This is the classical model, first proposed by Ernest Rutherford in 1911 and elaborated later by Niels Bohr and others.
14 Atomic StructureGrains of Mystique: Rutherford's Model and Its Drawbacks:
17 Atomic StructureNearly all the mass of an atom is concentrated in the atomic nucleus. The nucleus occupies only a few quadrillionths of its volume. The nucleus, therefore, is extremely dense. If bare atomic nuclei could be packed against each other into a lump 1 centimeter in diameter (about the size of a large pea), the lump would weigh 133,000,000 tons! Huge electrical forces of repulsion prevent such close packing of atomic nuclei because each nucleus is electrically charged and repels all other nuclei.
18 Atomic StructureOnly under special circumstances are the nuclei of two or more atoms squashed into contact. When this happens, a violent nuclear reaction may take place. Such reactions, thermonuclear fusion, occur in the centers of stars and ultimately makes them shine.
19 AtomsWhen we compare the mass-to-charge ratios of different atomic nuclei, we find that the nucleus is made up of more than protons. The helium nucleus, for example, has twice the charge of the hydrogen nucleus but four times the mass. The added mass is due to another particle, the neutron . A neutron has about the same mass as the proton but has no electrical charge.
20 AtomsWhereas the number of protons in a nucleus exactly matches the number of electrons around the nucleus in a neutral atom, the number of protons in the nucleus need not match the number of neutrons there. For example, all hydrogen nuclei have a single proton but most have no neutrons. A small percentage contain one neutron and a smaller percentage, two neutrons. Similarly, most iron nuclei with 26 protons contain 30 neutrons, while a small percentage contain 29 neutrons.
21 IsotopesAtoms of the same element that contain different numbers of neutrons are isotopes of the element. The various isotopes of an element all have the same number of electrons, and so for the most part they behave identically. The hydrogen atoms in H2O, for example, may or may not contain a neutron. The oxygen doesn't “know the difference.” But if significant amounts of hydrogen have neutrons, then the H2O is slightly heavier, and it's appropriately called “heavy water.”
22 AtomsThe principal building block of the nucleus is the nucleon. Nucleon is composed of quarks. When a nucleon is in an electrically neutral state, it is a neutron; When a nucleon is in an electrically charged state, it is a proton;
23 ElectronsThe number of protons in the nucleus is electrically balanced by an equal number of electrons whirling about the nucleus. Atom itself is electrically neutral. Millikan calculated the mass of an electron to be about 1/2000 the mass of the lightest known atom, hydrogen. This confirmed that the atom was no longer the least massive particle of matter. For his work in physics, Millikan received the 1923 Nobel prize.
24 Electrical repulsionThe fact that electrons repel other electrons has interesting consequences. Electrical repulsion prevent ours hand from passing through the wall when we push. Prevent us from falling through the solid floor. When we touch someone, our atoms don’t meet the atoms of the one you touch. Instead, the atoms get close enough so that we sense electrical repulsion forces. There is still a tiny gap of space between we and the person we are touching.
25 The ElementsSo atoms make up the matter we see about us. We might think that an incredible number of different kinds of atoms exist to account for the rich variety of substances around us. But the number is surprisingly small. The great variety of substances results not from any great variety of atoms, but from the many ways a few types of atoms can be combined - just as combinations of only three colors form almost every conceivable color in a color print. To date (2001) we know of 118 distinct atoms, called the chemical elements . Only 88 elements are found naturally; the others are formed in laboratories with high-energy nuclear accelerators and reactors. These heaviest elements are too unstable (radioactive) to occur naturally inappreciable amounts.
26 The ElementsWhen a substance is composed of atoms of the same kind, we call that substance an element. Hydrogen, the lightest of all elements. It makes up over 90% of the atoms in the known universe. Its principal atomic form consists of a lone electron buzzing about a single central proton. Helium is the second lightest element.
27 AtomsOur sun is predominantly hydrogen, and thermonuclear fusion converts some of it into helium. Nearly all the elements on Earth are remnants of stars that exploded long before the solar system came into existence.
28 AtomsMore than 99% of the material on Earth is formed from only about a dozen of the elements.Living things are composed primarily of five elements:Oxyzen (O)Carbon (C)Hydrogen (H)Nitrogen (N) andCalcium (Ca)
29 Periodic table of the elements Elements are classified by the number of protons their atoms contain, which is their atomic number. The arrangements of elements by their atomic numbers makes up the periodic table of the elements.
31 AtomsEach element, from left to right, has one more proton and electron.Size decreases to the rightReading down the table, each element has one more shell.Size increases to the bottomUpto seven shells.
33 IsotopesWhereas the number of protons in a nucleus exactly matches the number of electrons around the nucleus in a neutral atom, the number of protons in the nucleus need not match the number of neutrons there. For example, all hydrogen nuclei have a single proton but most have no neutrons. A small percentage contain one neutron and a smaller percentage, two neutrons. Similarly, most iron nuclei with 26 protons contain 30 neutrons, while a small percentage contain 29 neutrons.
34 IsotopesAtoms of the same element that contain different numbers of neutrons are isotopes of the element. The various isotopes of an element all have the same number of electrons, and so for the most part they behave identically. The hydrogen atoms in H2O, for example, may or may not contain a neutron. The oxygen doesn't “know the difference.” But if significant amounts of hydrogen have neutrons, then the H2O is slightly heavier, and it's appropriately called “heavy water.”
35 Mass of an atomThe total mass of an atom is the sum of the masses of all its components (protons, neutrons, and electrons), minus a negligible amount of mass that was converted to energy when the components came together to form the atom.The mass of an atom expressed in grams or kilograms, however, is a very small number and working with it is difficult.Chemists and physicists have devised a unit called the atomic mass unit , or amu , that makes comparing the masses of different atoms quite easy.The approximate mass of a single proton or neutron is 1 amu, so the mass of an atom in atomic mass units is simply the sum of its protons and neutrons and is known as the atomic mass number . (The mass of an electron is so small that it's disregarded.)
36 Atomic Mass UnitA neucleon has a mass of about 1 amu. Most elements have a variety of isotopes. The atomic mass number for each element listed in the periodic table is the weighted average of the masses of these isotopes based on the occurrence of each isotope on Earth. For example, carbon with six protons and six neutrons has an atomic mass of amu. About 1 percent of all carbon atoms, however, contain seven neutrons. The heavier isotope raises the average atomic mass of carbon from amu to amu.
37 Atoms Which contributes more If two atoms are isotopes of each other, to an atom's mass, electrons or protons?To an atom's volume (its size)?If two atoms are isotopes of each other,do they have the same atomic number ?The same atomic mass number ?
38 AtomsProtons contribute more to an atom's mass; electrons to its size.Both atoms have the same atomic number but different atomic mass numbers (because they have the same number of protons in the nucleus, but different numbers of neutrons).
39 QuarksQuarks and electrons, are now regarded as fundamental particles.Quarks were first proposed in 1963 by theoretical physicist Murray Gell-Mann.Two kinds of quarks are the fundamental building blocks of all protons and neutrons.One kind is whimsically called the up quark, andthe other the down quark.A proton is composed of three quarks, two ups and one down.The neutron is composed of one up and two downs.
40 Elements, Compounds, and Mixtures Certain solids such as gold, liquids such as mercury, and gases such as neon are composed of a single kind of atom. These substances are called elements . Certain other solids such as crystals of common table salt, liquids such as water, and gases such as methane are made up of elements that are chemically combined. These are called compounds.
41 MixturesSubstances that are mixed together without chemically combining are called mixtures . Sand combined with salt is a mixture. Hydrogen and oxygen gas form a mixture until ignited, where upon they form the compound water. A common mixture on which we all depend is nitrogen and oxygen together with a little argon and small amounts of carbon dioxide and other gases. It is the air we breathe.
42 AtomsCompounds are formed only when elements react chemically and bond with one another.But not all substances react with one another chemically when they are brought close together.
43 CompoundsMost compounds have properties quite different from those of the elements of which they are composed.Sodium is a metal that reacts violently with water.Chlorine is a poisonous greenish gas.Yet the compound of these two elements is the harmless white crystal (NaCl) that you sprinkle on your potatoes.At ordinary temperatures water (H2O) is a liquid, yet at these temperatures hydrogen and oxygen are both gases - quite different.
44 CompoundsTable salt (NaCl) is a crystalline compound that is not made of molecules.The sodium and chlorine atoms are arranged in a repeating pattern where each atom is surrounded by six atoms of the other kind.
45 MoleculesMany compounds, but not all, are composed of molecules . A molecule is the smallest unit of a substance consisting of two or more atoms held together by the mutual sharing of electrons. (We say such atoms are covalently bonded .) A molecule may be as simple as the two-atom combination of oxygen (O2 ), or nitrogen (N2 ), which compose most of the air we breathe. Two atoms of hydrogen combine with a single atom of oxygen to produce a water molecule (H2O).
46 Molecules vs. Compounds Note that there is a very important distinction between compounds and molecules.You may at times hear that elements are made up of atoms and compounds are made up of molecules, but it turns out that is not true on several counts. In water, two hydrogen atoms are hooked to an oxygen atom to form both a molecule and a compound. However, when two hydrogen atoms are hooked together they form a molecule, but they do not form a compound. Both atoms are the same element, not two different elements. The same is true with oxygen. Two oxygen atoms hooked together form a molecule but they do not form a compound. Thus, it is possible to have molecules of an element.Sodium chloride brings up a related issue. Although it is a compound, it is not a molecular compound. The way that the sodium and chlorine are hooked together involves a different mechanism and one that does not result in the formation of sodium chloride molecules. Instead, they make a crystalline network. Sodium chloride is a network compound rather than a molecular compound. That becomes important later in the course when we deal with bonding.Although it might be convenient to equate molecules and compounds, they are not the same thing. There are materials which are molecular but are not compounds, like hydrogen and oxygen. There are materials which are compounds but are not molecular, like sodium chloride. There are materials which are neither compounds nor molecular like magnesium. There are also materials, like water, which are both molecular and compounds. So you must treat those two concepts--molecules and compounds--separately. What they do have in common with one another is that they both represent combinations of atoms, but they are different kinds of combinations.
47 AntimatterWhereas matter is composed of atoms with positively charged nuclei and negatively charged electrons, antimatter is composed of atoms with negative nuclei and positive electrons, or positrons .
48 AntimatterPositrons were first discovered in 1932, in cosmic rays bombarding the Earth's atmosphere. Today, antiparticles of all types are regularly produced in laboratories using large nuclear accelerators. A positron has the same mass as an electron and the same magnitude of charge but the opposite sign. Antiprotons have the same mass as protons but are negatively charged. The first complete anti-atom, a positron orbiting an antiproton, was constructed in Every charged particle has an antiparticle of the same mass and opposite charge. Neutral particles (such as the neutron) also have antiparticles, alike in mass and in some other properties, but opposite in some properties. For every particle there is an antiparticle. There are even antiquarks.
49 Dark MatterWe know that the elements in the periodic table are not confined to planet Earth. From studies of radiation coming from other parts of the universe, we find that stars and other objects “out there” are composed of the same particles we have on Earth. Stars emit light that produces the same “atomic spectra” as the elements in the periodic table. How wonderful to find that the laws that govern matter on Earth extend throughout the observable universe. Yet there remains one troubling detail. There is a lot more mass out there than we can directly see.
50 Dark MatterAstrophysicists talk of the dark matter - matter we can't see that tugs on stars and galaxies that we can see. Gravitational forces within galaxies are measured to be far greater than visible matter can account for. Dark matter is estimated to make up some 90 percent of the mass of the universe. Whatever it is, some, most, or all of it is likely to be “exotic” matter - very different from the elements that make up the periodic table, and different from any extension of the present list of elements. It seems to be different stuff. At this writing, the dark matter is not yet identified. Speculations abound, but we don't know what it is.