Atoms, elements, and molecules

Helium

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.

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: 1218