Section 4–2:
The Structure of an Atom
Coach Kelsoe
Physical Science
Pages 108–112
Objectives
• Identify three subatomic particles and compare
their properties.
• Distinguish the atomic number of an element
from the mass number of an isotope, and use
these numbers to describe the structure of atoms.
Properties of Subatomic Particles
• By 1920, Rutherford had seen evidence for the
existence of two subatomic particles and had
predicted the existence of a third particle.
• There are three subatomic particles –
– Protons
– Electrons
– Neutrons
Protons
• Rutherford determined that the
amount of positive charge varies
among elements.
• Each nucleus must contain at least one
particle with a positive charge. These
particles are called protons.
• A proton is a positively charged
subatomic particle that is found in the
nucleus of an atom. Each proton is
assigned a charge of 1+.
Electrons
• The particles that Thomson detected
were later named electrons.
• An electron is a negatively charged
subatomic particle that is found in the
space outside the nucleus. Each
electron has a charge of 1-.
Neutrons
• In 1932, the English physicist James
Chadwick designed an experiment to
show that neutrons exist. He concluded
that the particles he produced were
neutral because a charged object did
not deflect their paths.
• A neutron is a neutral subatomic
particle that is found in the nucleus of
an atom. Its mass is almost exactly
equal to that of a proton.
Comparing Subatomic Particles
• Protons, electrons, and neutrons can be
distinguished by mass, charge, and location in an
atom.
• Protons and neutrons have almost the exact same
mass, but electrons are 1/2000 the size of
protons. Although an electron is smaller in mass,
they have an equivalent negative charge to a
proton’s positive charge.
Atomic Number
• The atoms of any given element
always have the same number of
protons. There are always 3
protons in lithium and 4 in
beryllium.
• The atomic number of an
element equals the number of
protons in an atom of that
element.
Atomic Number
• Atoms of different elements have different
numbers of protons. This is what gives atoms
their identity!
• Each positive charge in an atom is balanced by a
negative charge because atoms are neutral. So
the atomic number of an element also equals the
number of electrons in an atom. Lithium has 3
electrons; beryllium has 4.
Mass Number
• The atomic number tells you the number of
protons in the nucleus of an atom, but doesn’t tell
you how many neutrons there are.
• The mass number of an atom is the sum of the
protons and neutrons in the nucleus of that atom.
• For example, an atom of carbon that has 6
protons and 7 neutrons would have a mass
number of 13.
Mass Number
• If you know the atomic number and mass number
of an atom, you can find the number of neutrons
by subtracting.
• Example: 23 (MN of Na) – 11 (AN of Na) = 12
Isotopes
• In Dalton’s atomic theory, he states that all atoms
of a given element are identical. Every atom of a
given element has the same number of protons as
electrons.
• However, atoms of the same element can have
different masses due to the number of neutrons!
Isotopes
• Isotopes are atoms of the same element that
have different numbers of neutrons and different
mass numbers.
• Isotopes of an element have the same atomic
number but different mass numbers because they
have different numbers of neutrons.
Isotopes
• For example, there are three types of oxygen: oxygen-16,
oxygen-17, and oxygen-18. The number at the end
represents the mass number.
• All of these oxygen atoms have 8 protons (its atomic
number is 8), but the number of neutrons will change:
– Oxygen 16  16-8 = 8 neutrons
– Oxygen 17  17-8 = 9 neutrons
– Oxygen 18  18-8 = 10 neutrons
Vocabulary
•
•
•
•
•
•
Proton
Electron
Neutron
Atomic number
Mass number
Isotopes