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What Is Atomic Structure?
What Is Atomic Structure?
Atomic structure describes how atoms—the building blocks of all matter—are arranged and what they are made of. Everything around you is made of atoms.
The word atom comes from the Greek word atomos, meaning “uncuttable.” Atoms are the smallest part of an element that still has that element’s properties.
This a Bohr model of Helium
Atoms contain three main particles: protons, neutrons, and electrons.
The protons and neutrons are found in the center of the atom, called the nucleus, while the electrons move around the nucleus in energy levels, also called shells.
The History of Atomic Models
The History of Atomic Models
Democritus
Democritus (around 460 BC)
Democritus was a Greek philosopher who believed that all matter is made of tiny, invisible particles he called atomos, meaning “uncuttable.”
He said that atoms were always moving and that different materials were made of different types of atoms.
Although Democritus’ idea was correct, most people of his time believed Aristotle, who taught that matter was made of four elements: Earth🌎, Air💨, Water💧, and Fire🔥.
John Dalton - 1808
John Dalton - 1808
He said that all matter is composed of atoms which cannot be created or destroyed. All atoms of the same element are identical. For instance, gold atoms make up gold. Oxygen atoms make up oxygen.
Different elements can combine in definite ratios to form compounds. For instance, two hydrogen atoms and one oxygen atom can bond together to form water.
Atoms are rearranged during chemical reactions forming new compounds. He measured how heavy atoms of different elements were compared to each other.
John Dalton
JJ Thomson - 1897
JJ Thomson - 1897
JJ Thomson used a cathode ray tube as well as magnets to discover the electron. Since atoms are electrically neutral, he thought that atoms must have positive and negative charges throughout.
He proposed the plum pudding model. Think of chocolate chip cookie dough. In this model, the plums (or chips) represent negatively charged electrons that can be plucked out of the atom, leaving behind some positively charged pudding (the dough).
Ernest Rutherford - 1911
Ernest Rutherford - 1911
THE ATOM IS MOSTLY OPEN SPACE!
The Nucleus: In 1911, Ernest Rutherford shot alpha particles (positive in charge) through gold foil.
Most of the particles passed through the foil but occasionally, some ricocheted in different directions.
Rutherford concluded that the atom was mostly open space but that it had a dense positive core which we call the "nucleus."
If the nucleus were 2 cm in diameter, the nearest nucleus of an adjacent atom would be 400 meters away.
That means atoms are mostly empty space!
Rutherford-Bohr Model of the Atom
Rutherford-Bohr Model of the Atom
In 1913 Niels Bohr and Ernest Rutherford proposed a model of the atom where electrons travel in shells around the nucleus.
Electrons can move from one orbit to another by emitting or absorbing energy.
Niels Bohr
Ernest Rutherford
James Chadwick
James Chadwick (1932)
James Chadwick (1932)
James Chadwick discovered the neutron, a neutral particle found in the nucleus along with protons.
Neutrons help hold the nucleus together by reducing the repulsion between the positively charged protons. Neutrons are like glue that keep the nucleus together.
Without neutrons, the nucleus would fly apart.
Subatomic Particles
Subatomic Particles
Atomic structure of Helium
Subatomic Particles: Protons, neutrons and electrons are subatomic particles.
Subatomic Particles: Protons, neutrons and electrons are subatomic particles.
In other words, they are particles that are smaller than the atom.
In other words, they are particles that are smaller than the atom.
Atomic structure of Carbon
Protons, found in the nucleus, are positive. Protons determine what atom is.
For example, atoms with 2 protons in the nucleus are Helium atoms. Atoms with 6 protons in the nucleus are Carbon atoms.
Atomic structure of Oxygen
Electrons are negative and fly around the outside of the atom in shells. Electrons bond one atom to another.
They are involved in chemical reactions.
They have hardly any mass. In fact, the mass of 1 proton = the mass of 1837 electrons.
Atomic structure of Aluminum
Neutrons are neutral and hold the nucleus together; they are like glue.
Neutrons are slightly heavier than protons.
Shells: 2, 8, 8
Shells: 2, 8, 8
2 electrons fill the first level, ring, or shell. How many electrons can hydrogen hold? The answer is 2.
8 electrons fill the second level, ring, or shell.
8 fill the third. (Actually, 18 electrons will fill it. However, you will not see this until high school.)
The electrons in the outermost shell are called valence electrons. Valence electrons are important because they determine how an atom bonds with other atoms during chemical reactions.
Hydrogen makes one bond.
Hydrogen makes one bond.
This fluorine atom also makes one bond.
This fluorine atom also makes one bond.
This sulfur atom makes two bonds.
This sulfur atom makes two bonds.
The number of valence electrons is not responsible for identifying atoms because electrons readily come and go. When it comes to bonding, valence electrons are the ones that matter.
Atomic Number, Mass Number, and Neutrons
Atomic Number, Mass Number, and Neutrons
Each element on the periodic table has an atomic number, which tells you how many protons are in its atoms.
The atomic mass (or mass number) is the total number of protons and neutrons in the nucleus.
To find the number of neutrons:
Number of Neutrons = Atomic Mass – Number of Protons
Isotopes
Isotopes
These are hydrogen isotopes. These hydrogen atoms only differ in mass, due to extra neutrons.
Atoms of the same element can have different numbers of neutrons. These versions are called isotopes.
Isotopes have the same atomic number but different atomic masses. Many isotopes are useful in medicine and science.
Technetium-99 is used in medical imaging.
Cobalt-60 is used in cancer treatment.
The Modern Atomic Model
While the Bohr model is useful for showing energy levels, scientists now understand that electrons do not orbit the nucleus in perfect circles.
Instead, they move within regions called electron clouds, where there is a high chance of finding an electron.
This modern quantum model shows that atoms are mostly empty space and that electrons move too fast to have fixed paths.
Summary
Summary
Atoms are the smallest units of matter and make up everything around us. They contain a nucleus made of protons and neutrons, surrounded by electrons in shells or energy levels.
Over time, scientists developed models—from Democritus’ tiny particles to the modern quantum model—that help us understand how atoms behave, bond, and form the matter that makes up our universe.
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