Lesson 2 Structure of an Atom
Objectives- Describe the 3 main subatomic particles; Explain how the atomic mass and atomic
number for an element is determined.
An atom is the smallest part of matter that has the same properties of the matter it came
from. For example, one atom of gold has the same properties(density, melting point etc.) as a bar
of gold does. Atoms are made up of smaller parts but these do not have the same properties as
the material it came from. Every atom has a nucleus or center. Inside the nucleus are protons and
neutrons. Protons are positively charged. Neutrons have no electrical charge and are neutral.
Both of these are the same size. The mass of each proton and neutron is 1amu or atomic mass
unit. Because these are so small, chemist need to come up with a new measurement scale to
describe their mass. Outside of the nucleus is the electron cloud or region where the electrons can
be found. Electrons are extremely small. It takes almost 2000 electrons to equal the mass of 1
proton or neutron. Since their mass is so small, their mass is not considered or 0amu. Electrons
have a negative charge. Most of the atom is empty space. If the nucleus was represented by a
pea on the 50 yard line, the electrons would be found in the end zone of a football field! Usually
the number of protons and electrons equal each other.
The electron cloud is made up of shells or energy levels. Each energy level can hold only
a certain amount of electrons. The farther out the shell is, the more electrons it can hold. The
first shell can hold 2 electrons, the second can hold 8 electrons, the third can hold 18 and the
fourth can hold 32. The electrons can be found anywhere inside a shell. The do not follow paths.
The names of elements can be abbreviated with chemical symbols. Symbols, if written
with one letter, are always capitalized. If there is a second letter, the second is always a lower
case letter. Sometimes the symbol is obvious and is letters from the name of the element. Other
times it is not. The symbol can come from the Latin or Greek name of the element, a famous
person or even where it was discovered.
Each element is assigned an atomic number. The atomic number equals the number of
protons an element has. The atomic number is usually located above the symbol for the element
in the periodic table. If you know the atomic number, then you also know the number of
electrons.
The atomic mass of an atom is the sum of the protons and neutrons. The only two parts
of an atom with any mass are the protons and neutrons. To determine the number of neutrons,
subtract the atomic number from the atomic mass (rounded). Since the atomic mass is both the
protons and neutrons, if we take away the number of protons, all we have left is the number of
neutrons.
Questions
Complete the table.
Subatomic Particles
Name of location
particle
proton
neutron
electrons
Mass
Charge
What is the symbol for potassium- element 19?
What is the atomic number of potassium?
How many protons does potassium have?
How many electrons does potassium have?
What is the chemical symbol for sodium?
What element has 13 protons?
If the protons and electrons are equal, what is the electrical charge on an atom?
What is the atomic mass of phosphorus (atomic number 15)?
How many protons does phosphorus have?
How many electrons does phosphorus have?
How many neutrons does phosphorus have?