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The Structure of the Atom for AP Chemistry

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By — McGraw-Hill Professional
Updated on Feb 8, 2011

Practice problems for these concepts can be found at:

The States of Matter

Matter can exist in one of three states: solid, liquid, or gas. A solid has both a definite shape and a definite volume. At the molecular level, the particles that make up a solid are close together and many times are locked into a very regular framework called a crystal lattice. Molecular motion exists, but it is slight.

A liquid has a definite volume but no definite shape. It conforms to the container in which it is placed. The particles are moving much more than in the solid. There are usually clumps of particles moving relatively freely among other clumps.

A gas has neither definite shape nor volume. It expands to fill the container in which it is placed. The particles move rapidly with respect to each other and act basically independently of each other.

We will indicate the state of matter that a particular substance is in by a parenthetical s, l, or g. Thus, H2O(s) would represent solid water (ice), while H2O(g) would represent gaseous water (steam).

The Structure of the Atom

Historical Development

The first modern atomic theory was developed by John Dalton and first presented in 1808. Dalton used the term atom (first used by Democritus) to describe the tiny, indivisible particles of an element. Dalton also thought that atoms of an element are the same and atoms of different elements are different. In 1897, J. J. Thompson discovered the existence of the first subatomic particle, the electron, by using magnetic and electric fields. In 1909, Robert Millikan measured the charge on the electron in his oil drop experiment (electron charge = –1.6022 × 10–19 coulombs), and from that he calculated the mass of the electron. Thompson developed an atomic model, the raisin pudding model, which described the atom as being a diffuse positively charged sphere with electrons scattered throughout.

Ernest Rutherford, in 1910, was investigating atomic structure by shooting positively charged alpha particles at a thin gold foil. Most of the particles passed through with no deflection, a few were slightly deflected, and every once in a while an alpha particle was deflected back towards the alpha source. Rutherford concluded from this scattering experiment that the atom was mostly empty space where the electrons were, and that there was a dense core of positive charge at the center of the atom that contained most of the atom's mass. He called that dense core the nucleus.

Subatomic Particles

Our modern theory of the atom describes it as an electrically neutral sphere with a tiny nucleus at the center, which holds the positively charged protons and the neutral neutrons. The negatively charged electrons move around the nucleus in complex paths, all of which comprise the electron cloud. Table 5.1 summarizes the properties of the three fundamental subatomic particles:

Many teachers and books omit the charges on the symbols for the proton and neutron.

The amu (atomic mass unit) is commonly used for the mass of subatomic particles and atoms. An amu is the mass of a carbon-12 atom, which contains 6 protons and 6 neutrons (C-12).

Since the atom itself is neutral, the number of electrons must equal the number of protons. However, the number of neutrons in an atom may vary. Atoms of the same element (same number of protons) that have differing numbers of neutrons are called isotopes. A specific isotope of an element can be represented by the following symbolization:

X represents the element symbol taken from the periodic table. Z is the atomic number of the element, the number of protons in the nucleus. A is the mass number, the sum of the protons and neutrons. By subtracting the atomic number (p) from the mass number (p + n), the number of neutrons may be determined. For example, (U-238) contains 92 protons, 92 electrons, and (238 – 92) 146 neutrons.

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