Atoms, elements, and molecules
Simple modelsof simple atoms demonstrate the three main subatomic particles: the electron, proton, and neutron.
Atomic orbitalsare the regions in space where electrons are most likely to be found. An atom's first two electrons occupy an s orbital, the next six electrons are paired in three p orbitals, and the next 10 in five d orbitals. The orbitals have the shapes illustrated along the bottom of these pages.
Everything in the universe—metals, stars, food, the air we breathe, and we ourselves—is made up of combinations of different chemical elements. There are 109 known chemical elements, such as oxygen, aluminum, carbon, and uranium. The building blocks of these chemical elements are called atoms.
An atom can be divided into three smaller particles: protons, neutrons, and electrons. Protons and neutrons form the nucleus (center) of the atom. The electrons turn around the nucleus. Except for hydrogen, which has a center with a single proton, the nucleus is always composed of protons and neutrons.
These particles can be even further divided into smaller parts. However, this is the proper study of a branch of physics. In general, chemistry deals with what happens between atoms during chemical changes. Physics concerns itself with activity inside an atom.
The number of protons in an atomic nucleus identifies an atom as being an atom of a particular element. All atoms with just one proton in the nucleus are atoms of the element hydrogen; all those with two protons are atoms of helium, and so on. The number of protons is called the atomic number. This is also the number by which the element is identified in the periodic table of the elements (see page 23).
Electricity is the energy that binds together the parts of an atom. A proton in the atomic nucleus is electrically positive. An electron, rotating around that nucleus, is electrically negative. The attraction of these opposite electric charges keeps the electron spinning around the proton. In a neutral atom, the numbers of protons and electrons are equal.
The number of neutrons, which have no electric charge, may vary. This produces different isotopes (forms) of the same element. The number of protons must remain constant. Otherwise, the atom would become an atom of another element. The sum of an atom's protons and neutrons is called the mass number. Thus, different isotopes of the same element have the same atomic numbers, but different mass numbers. In general, the different isotopes of an element behave in a similar manner chemically. However, their physical prop-
erties may vary slightly.
For example, hydrogen usually has a single proton. Rarely, a neutron is also present. This produces the isotope deuterium, otherwise known as "heavy hydrogen." Deuterium has the same atomic number as hydrogen— 1; but its mass number is 2. Hydrogen's mass number is 1. Many of the elements with which we are familiar are made up of mixtures of several isotopes. The metal lead, for example, is a mixture of several different isotopes of the element lead.
Date: 2015-12-11; view: 784