# Atoms Section 2 What Is in an Atom? Atoms Section 2 What Is in an Atom? What is the difference between protons, neutrons, and electrons? The three main subatomic particles are distinguished by mass, charge, and location in the atom. Atoms

Section 2 What Is in an Atom?, continued Each element has a unique number of protons. Unreacted atoms have no overall charge. Because there is an equal number of protons and electrons, the charges cancel out. The electric force holds the atom together. Positive protons are attracted to negative electrons by the electric force. This force holds the atom together.

Atoms Section 2 Atomic Number and Mass Number What do atoms of an element have in common with other atoms of the same element? Atoms of each element have the same number of protons, but they can have different numbers of neutrons.

Atoms Section 2 Atomic Number and Mass Number, continued The atomic number equals the number of protons.The mass number equals the total number of subatomic particles in the nucleus. atomic number: the number of protons in the nucleus of an atom mass number: the sum of the numbers of protons

and neutrons in the nucleus of an atom Atoms Section 2 Atomic Number and Mass Number, continued Atoms Section 2

Isotopes Why do isotopes of the same element have different atomic masses? Isotopes of an element vary in mass because their numbers of neutrons differ. Atoms Isotopes, continued Section 2

Atoms Section 2 Isotopes, continued Some isotopes are more common than others. radioisotopes: unstable isotopes that emit radiation and decay into other isotopes The number of neutrons can be calculated. number of neutrons = mass number atomic number

Atoms Section 2 Atomic Masses What unit is used to express atomic mass? Because working with such tiny masses is difficult, atomic masses are usually expressed in unified atomic mass units. unified atomic mass unit: a unit of mass that describes the mass of an atom or molecule; it is

exactly 1/12 the mass of a carbon atom with mass number 12 (symbol, u) Atoms Section 2 Atomic Masses, continued Average atomic mass is a weighted average. Isotope abundance determines the average atomic mass.

Example: Chlorine-35 is more abundant than chlorine-37, so chlorines average atomic mass (35.453 u) is closer to 35 than to 37. Atoms Section 2 Atomic Masses, continued

The mole is useful for counting small particles. 1 mol = 602, 213, 670, 000, 000, 000, 000, 000 particles mole: the SI base unit used to measure the amount of a substance whose number of particles is the same as the number of atoms of carbon in exactly 12 g of carbon-12 (abbreviation, mol) This number, usually written as 6.022 1023, is called Avogadros number.