Do Now
Which phrase describes an atom?
1. a negatively charged nucleus surrounded by
positively charged protons
2. a negatively charged nucleus surrounded by
positively charged electrons
3. a positively charged nucleus surrounded by
negatively charged protons
4. a positively charged nucleus surrounded by
negatively charged electrons
Unit 2, Day 2: The Nucleus, Part 1
Williamsburg High School for Architecture and Design
Mr. Quinn and Ms. Tom
9/18/13
Nuclear Stability
● Small elements
have 1:1 protons:
neutrons
● Large elements get
closer to 1:2
protons:neutrons
● WHY?
Forces in the Nucleus
Electrical forces are additive!
Isotopes
● Atoms with the same atomic number and
different mass numbers
○ The difference is in the neutrons
● Caused by fission and fusion
● Nomenclature →
○ Example: 6 protons, 6 neutrons
○ This element is CARBON
○ Ways we can write it:
■ Carbon-12
■ C-12
■ 12C
■ 126C
Average Mass
● Natural abundance: each isotope makes up a
certain percentage of the total amount of
each atom
○ We use this to calculate the average mass
■ Average mass is on the periodic table
○ Example -->
Practice #1
Carbon is 99% C-12 and 1% C-13 (approximately).
What is the average mass of carbon?
Practice #2
Bromine is half Br-79 and half Br-81. What is
the average mass of bromine?
Practice #3
Cl-35 is three times as abundant as Cl-37.
What is the average mass of chlorine?
Independent Work #1
An atom that has 13 protons and 15 neutrons is
an isotope of the element
1. nickel
2. silicon
3. aluminum
4. phosphorus
Independent Work #2
The greatest composition by mass in an atom of
17
O is due to the total mass of its
8
1.
2.
3.
4.
electrons
neutrons
positrons
protons
Independent Work #3
The total number of protons, electrons, and neutrons in each of four different
atoms are shown in the table below.
Which two atoms are isotopes of the same element?
1.
2.
3.
4.
A and D
A and Z
X and D
X and Z
Independent Work #4
The stability of an isotope is based on its
1. number of neutrons, only
2. number of protons, only
3. ratio of neutrons to protons
4. ratio of electrons to protons
Independent Work #5
What is the structure of a krypton-85 atom?
1. 49 electrons, 49 protons, and 85 neutrons
2. 49 electrons, 49 protons, and 49 neutrons
3. 36 electrons, 36 protons, and 85 neutrons
4. 36 electrons, 36 protons, and 49 neutrons