Microbiology Demystified

states that an element cannot be decomposed into two or more chemical sub-
stances because the element consists of one kind of atom. The atom is also the
smallest amount of matter that can enter into a chemical reaction. You’ll learn
about chemical reactions later in this chapter.
At the center of every atom is a nucleus. The nucleus does not change spon-
taneously unless it is unstable—making it radioactive—and does not participate
in a chemical reaction. It is for this reason that the nucleus for most atoms is con-
sidered stable.
Moving around the nucleus are electrons. An electron is a negatively charged
particle that follows a path called an orbital. Electrons are the parts of an atom
that enter into a chemical reaction.
The nucleus is made up of protons and neutrons. Aprotonis a positively
charged particle. Aneutronis a particle that does not have a charge; it is called
neutral or uncharged. The number of protons in the nucleus equals the number
of electrons in an electrically stable atom. This makes the atom neutral because
the number of positively charged particles (protons) offsets the number of neg-
atively charged particles (electrons).
An element is identified by its atomic number. The atomic numberis the num-
ber of protons in the nucleus of the atom. The atomic mass(also called the atomic
weight) is slightly less than the sum of the masses of an atom’s neutrons and pro-
tons. The standard for measuring atomic mass is called a dalton, named for John
Dalton. A dalton is also known as an atomic mass unit (amu). For example, a neu-
tron has an atomic mass of 1.088 daltons. A proton has an atomic mass of 1.077
daltons. An electron has an atomic mass of 0.0005 dalton.
Atoms that have the same atomic number are classified as the same chemical
element because these atoms behave the same way. Therefore, a chemical ele-
ment consists of one or more atoms that have the same atomic number.

CHAPTER 2 Chemical Elements of Microorganisms^25

Electron

Electron shells

Proton

Neutron

Orbital

Nucleus

Fig. 2-1. An atom is the smallest particle of an element.