CHEM1305 Chapter 4 – Atoms and Elements
Fall 2021/September 10, 2021
4.1 Experiencing Atoms at Tiburon
- Atom – the smallest identifiable unit of an element.
- Many atoms exist not as free particles but as groups of atoms bound together to form
molecules. All matter is ultimately made of atoms. Because of this, atoms are at the foundation
of our sensations.
- Atoms are incredibly small. A single pebble from the shoreline contains more atoms than you
could ever count. If every atom within a small pebble were the size of the pebble itself, the
pebble would be larger than Mount Everest.
- The exact number of naturally occurring elements is controversial because some elements
previously considered only synthetic may actually occur in nature in very small quantities.
4.2 Indivisible: The Atomic Theory – Recognize that all matter is composed of
atoms.
Leucippus 5th century B.C. and Democritus (460-370 B.C.) were Greek philosophers and theorized that
matter was ultimately composed of small, indivisible particles. Democritus suggested that if you divided
matter into smaller and smaller pieces, you would eventually end up with tiny indestructible particles
called atomos, or atoms. The word atomos means “indivisible.”
Dalton’s Atomic Theory – John Dalton formalized a theory of atoms that gained broad acceptance.
1. Each element is composed of tiny indestructible particles called atoms.
2. All atoms of a given element have the same mass and other properties that distinguish them
from the atoms of other elements.
3. Atoms combine in simple, whole-number ratios to form compounds.
4.3 The Nuclear Atom – Explain how the elements of Thomson and Rutherford led
to the development of the nuclear theory of the atom.
J.J. Thompson (1856-1940) – discovered the electron. Found that they are negatively charged, that they
are much smaller and lighter than atoms and that hey are uniformly present in many different kinds of
substances.
Plum Pudding Model of the Atom – Electric charge is more fully defined in Section 4.4. It is an inherent
property of electrons that causes them to interact with other charged particles.
Rutherford – created a new model to explain his results. Concluded that matter must not be as uniform
as it appears. It contains large regions of empty space dotted with small regions of very dense matter.
The mass and positive charge of an atom must all be concentrated in a space much smaller than the size
of the atom itself. He developed the nuclear theory of the atom – a theory stating that most of the
atom’s mass and all of its positive charge are contained in a small, dense nucleus. Most of the volume of
the atom is empty space occupied by negatively charged electrons.
, Figure 4.5 – Discovery of the Atomic Nucleus
1. Most of the atom’s mass and all of its positive charge are contained in a small core called the
nucleus.
2. Most of the volume of the atom is empty space through which the tiny, negatively charged
electrons are dispersed.
3. There are as many negatively charged electrons outside the nucleus as there are positively
charged particles (protons) inside the nucleus, so that the atom is electrically neutral.
Protons – a positively charged nuclear particle; mass is approximately 1 amu.
Neutron – a nuclear particle with no electrical charge and nearly the same mass as a proton.
Fall 2021/September 10, 2021
4.1 Experiencing Atoms at Tiburon
- Atom – the smallest identifiable unit of an element.
- Many atoms exist not as free particles but as groups of atoms bound together to form
molecules. All matter is ultimately made of atoms. Because of this, atoms are at the foundation
of our sensations.
- Atoms are incredibly small. A single pebble from the shoreline contains more atoms than you
could ever count. If every atom within a small pebble were the size of the pebble itself, the
pebble would be larger than Mount Everest.
- The exact number of naturally occurring elements is controversial because some elements
previously considered only synthetic may actually occur in nature in very small quantities.
4.2 Indivisible: The Atomic Theory – Recognize that all matter is composed of
atoms.
Leucippus 5th century B.C. and Democritus (460-370 B.C.) were Greek philosophers and theorized that
matter was ultimately composed of small, indivisible particles. Democritus suggested that if you divided
matter into smaller and smaller pieces, you would eventually end up with tiny indestructible particles
called atomos, or atoms. The word atomos means “indivisible.”
Dalton’s Atomic Theory – John Dalton formalized a theory of atoms that gained broad acceptance.
1. Each element is composed of tiny indestructible particles called atoms.
2. All atoms of a given element have the same mass and other properties that distinguish them
from the atoms of other elements.
3. Atoms combine in simple, whole-number ratios to form compounds.
4.3 The Nuclear Atom – Explain how the elements of Thomson and Rutherford led
to the development of the nuclear theory of the atom.
J.J. Thompson (1856-1940) – discovered the electron. Found that they are negatively charged, that they
are much smaller and lighter than atoms and that hey are uniformly present in many different kinds of
substances.
Plum Pudding Model of the Atom – Electric charge is more fully defined in Section 4.4. It is an inherent
property of electrons that causes them to interact with other charged particles.
Rutherford – created a new model to explain his results. Concluded that matter must not be as uniform
as it appears. It contains large regions of empty space dotted with small regions of very dense matter.
The mass and positive charge of an atom must all be concentrated in a space much smaller than the size
of the atom itself. He developed the nuclear theory of the atom – a theory stating that most of the
atom’s mass and all of its positive charge are contained in a small, dense nucleus. Most of the volume of
the atom is empty space occupied by negatively charged electrons.
, Figure 4.5 – Discovery of the Atomic Nucleus
1. Most of the atom’s mass and all of its positive charge are contained in a small core called the
nucleus.
2. Most of the volume of the atom is empty space through which the tiny, negatively charged
electrons are dispersed.
3. There are as many negatively charged electrons outside the nucleus as there are positively
charged particles (protons) inside the nucleus, so that the atom is electrically neutral.
Protons – a positively charged nuclear particle; mass is approximately 1 amu.
Neutron – a nuclear particle with no electrical charge and nearly the same mass as a proton.
