UNIT # 3 Introduction Objectives 3.1 Structure of the Atom 3.2 3.3 Arrangement of Electrons 3.4 3.5 Atomic Mass Problems 1 | 2 | 3

# ATOMIC STRUCTURE 3.1 - Structure of the Atom

3.1.3 - The number of neutrons in an atom

In most cases atoms of a given element have different mass. The general symbol to describe an atom of an element has the following format.

 A = mass number E = symbol of the element Z = atomic number or Sometimes Z is omitted because if the symbol of the element is known, including the atomic number becomes redundant.

In order to determine the number of neutrons present in the nucleus of an atom of an element, we must know the mass number of the element.

Since the number of protons = atomic number, it follows that

The number of neutrons in the nucleus of an atom is

Take a look at 35Cl and 40Ca and see if you can figure out how I arrived at:

• # of protons,
• # of neutrons,
• # of electrons, and
• mass #, the mass number.

# of protons # of neutrons # of electrons mass #

17 17 35
20 40-20 = 20 20 40

Let's try figuring out the number of fundamental particles for their ions, 35Cl-, and 40Ca2+.

# of protons # of neutrons # of electrons mass #

17 18 35
20 40-20 = 20 18 40

Points to note:

• The only change in the number of subatomic particles between the atom and its ion is in the number of electrons. The number of protons of a neutral atom and its ion is the same.
• 40Ca and 40Ca2+ both have 20 protons.
• 40Ca has 20 electrons. 40Ca2+ has 18 electrons.
• The +2 charge on 40Ca2+ indicates that 40Ca2+ has 2 less electrons than 40Ca.
• 35Cl- and 40Ca2+ have the same number of electrons. They are isoelectronic.

Section 4.4
General Arrangement of Electrons, Protons, and Neutrons. Atomic Number ..p83

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