Distinguishing Among Atoms
Section 4.3
_________________ theorized that all atoms of a particular element are the same, and that all
atoms of different elements are different. Let’s investigate these similarities and differences.
Atomic Number
 Atomic number _________________________________ an element
 Hydrogen
 Atomic # is _______
 ________ proton in each hydrogen atom
 Polonium (Po)
 Atomic # is _______
 _______ protons in each Polonium atom
 For an ________________ atom, the number of _________________ (positively charged particles) is equal to
the number of _______________ (negatively charged particles

Ex. Uncharged Hydrogen
_________ e-

Uncharged Polonium
_________ e-
Element
Atomic #
K
19
Protons
Electrons
19
5
16
23
Mass Number
 The mass of an atom is concentrated in its _________________

Because the mass of _________________ are so small, we can assume their mass to be ______.

The mass of an atom depends on __________________________________________________.

Mass Number –


Carbon
 ____ protons
____ neutrons
Mass # =______
An atom’s composition can be determined by the _________ number and __________ number.

Number of Neutrons = ________________________________

Oxygen
 Atomic # = _______
Mass # = ______ Protons = _______
Neutrons = ________
Fill in the chart below (uncharged atoms)
Element
Atomic #
Mass #
Beryllium (Be)
4
9
10
20
Sodium (Na)
Protons
Neutrons
Electrons
23
Atom Composition Notation
 How many protons are in an atom of Lithium?

How many neutrons are in an atom of Lithium?
Naming Elements
 You may name elements using their ____________________________

Ex. Gold with a mass number of 197 can be _________________ or ______________

Uranium (U) with a mass of 238 can be ____________________ or _______________

Write the symbols for Gold-197 and Uranium -238 using the subscript and superscript notation
shown above.

Same __________________ number, different ___________________ number

Isotopes have identical numbers of ______________ and _________________

How does the discovery of isotopes contradict Dalton’s Atomic Theory? Describe below.

Fill in the chart below for the following isotopes of neon. All are uncharged atoms.
Isotopes:
Isotope
Protons
Neutrons
Electrons
Neon-20
Neon-21
Neon-22

How would you write the symbols for the 3 isotopes of Hydrogen? Show as you did above for Uranium
and Gold.
 Hydrogen – 1 (hydrogen)
Hydrogen – 2 (deuterium)
Hydrogen – 3 (tritium)
Atomic Mass
 The mass of atoms can be determined using an instrument called a mass _____________________

The isotope of Carbon-12 was assigned a mass of ________atomic mass units (amu)

Atomic mass unit (amu) – _________________________________________

The mass of a single ___________________ or _____________________ is about 1 amu.

The atomic mass of an element is a ____________________________ of the masses of a naturally
occurring sample of the element.

calculated by:
 The number of __________________________ of the element
 The ________________ of each isotope

The natural ________________________________ of each isotope
 Remember, a mass spec. machine can determine the mass of a particular atom in a sample. It can also tell
how many there are relative to other isotopes in that same sample.
 Given the ____________________ and its __________________________ in a sample, you can determine
the atomic mass of a “mystery element!”
 Isotope mass x fractional abundance = mass of that isotope in the sample. Add up the masses of all the
isotopes to get the atomic mass of the element.
 Remember sig figs.
 Let’s practice…



Isotope 10X
 Mass = 10.012 amu
Isotope 11X
 Mass = 11.009 amu
Calculate the element’s atomic mass
 10X =

11

Element X: atomic mass =
Relative abundance = 19.91% = 0.1991
Relative abundance = 80.09% = 0.8009
X=
KEY UNDERSTANDINGS: CHECK YOUR KNOWLEDGE
 Elements are different from one another because they contain different numbers
of ____________________.
 Elements have different forms called ___________________. The only thing that
differs between these isotopes is the number of __________________.
 Atomic mass is measured in _____________.
 The number of neutrons in an atom can be determined by subtracting the
_____________________ from the ______________________.