The atoms in a solid do not sit rigidly in one place, but rather, vibrate as though they are connected by springs. Liquids are somewhere in between, and act roughly as though the molecules are tied together by loose strings. The atoms of a gas bounce around freely in space like a pack of ping-pong balls in a clothes dryer. Gases have practically no intermolecular attraction at all. Solids have very strong intermolecular bonds, to the extent that they essentially lock each other into place like the mortar between so many bricks. The only difference between solids, liquids, and gases is how strongly their molecules attract each other. (In other words, if it isn't listed in the periodic table of elements, then it is made up of molecules.) Molecules in turn exert attractive forces on each other to form bulk materials: solids, liquids, and gases. A molecule represents the smallest possible unit of everything which is not an element. The only measurable difference between heavy water and ordinary water is their relative weight ratio of 18 versus 20 for a given volume.Ītoms can share electrons to form molecules. The total atomic weight of water is 18 (2 hydrogens at 1 each, and one oxygen atom at a weight of 16), so the atomic weight of heavy water is (2 X 2 + 18) = 20. Because the deuterium has one proton, it acts exactly like hydrogen chemically. For example, the compound "heavy water" is exactly like water except that it contains deuterium (an atom that has one proton and one neutron) rather than hydrogen. Atoms with the same number of protons, but differing numbers of neutrons, are alike in every way except for their weight. The neutrons do not have anything to do with the chemical properties of an element. (This topic will be covered in much more detail when we come to nuclear physics.) Neutrons help to bind the nucleus together, because they contribute attractive nuclear binding forces but do not contribute repulsive electrical forces. ![]() Since protons have positive charges, they strongly tend to fly apart (because like charges repel) and in fact, it is not possible for a nucleus to contain only protons unless you count hydrogen, which only has one proton. The "size" of an atom is determined by the size of the shells of its electrons.Īs for neutrons, they are electrically neutral, almost exactly the same size and mass as a proton, and together with the protons form the nucleus of the atom. In other words, an atom is mostly empty space. Sizewise, if the nucleus of say, an iron atom, were the size of a marble, then it would weigh several billion tons, and its electrons would be orbiting it in "shells" about a mile away. ![]() The table is arranged in such a way that elements in the same column of the table have similar chemical properties. ![]() The collection of all known elements is called the periodic table. These are elements one, two, and three respectively, because the number of the element always indicates the number of protons and electrons it has. Hydrogen has one proton and electron helium has two protons and two electrons lithium has three protons and three electrons, etc. (Actually, this picture isn't terribly accurate either, but it will have to do until we discuss quantum mechanics.)Ĭhemical elements exist because different atoms can have differing numbers of protons and electrons. It is OK (just barely) to think of the electron as orbiting the proton like the Earth orbits the Sun.but it is much more accurate to think of the electron as moving around the proton in a chaotic orbit such that it more-or-less forms a spherical "shell" around the proton. The simplest atom consists of one proton (positively charged) with one electron (negatively charged) moving around it. Simple model of lithium atom (i.e., poor model of lithium atom)Īll the matter you see around you is made up of only three particles: protons, electrons, and neutrons.
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