Structure of atoms10

Structure of Atoms

The History and Models of Atoms

Studying Atoms

• Scientists have been studying atoms since the 1800s.

• If atoms are too small to be seen, how do we study them?

• Scientists use models to study atoms.• Sometimes scientists can tell what things look

like by studying how they act.– Example: Wind

Models

• In science, a model is an idea, a picture, or an object that is built to explain how something else looks or works.

• The model might not look exactly like the object it is built to describe, but it helps people understand the way the object acts.

History of the Atom

• Democritus, a Greek philosopher, first used the term atom (atomos) around 400 BC.

• John Dalton showed that materials were made of atoms in 1803.

• In the late 1800s we discovered that the atom has many smaller parts.

• In the late 1800s, English physicist J. J. Thomson discovered the electron.

ThomsonDalton

More History of the Atom

• In 1901 Ernest Rutherford found discovered the nucleus, or center of the atom.

• In 1917 Niels Bohr, working as Ernest Rutherford’s student, discovered how electrons orbit the nucleus.– His model of the atom is the current base for the

modern atomic model.

Atomic Mass Unit

• Subatomic Particles are so small that a special unit was devised to measure them.

• This is the atomic mass unit (amu).

• Atoms are almost entirely empty space.• If the nucleus were the size of a basketball,

the first electrons would be somewhere near Springfield.

Model of an AtomThe central part of the atom is called the nucleus.

The nucleus contains positively charged protons that weigh 1 atomic mass unit.

The nucleus also contains neutrons that have no charge and weigh 1 atomic mass unit.

Electrons are extremely small units of an atom that have a negative charge and move around the atom in orbits.

Protons and electrons stay together because they attract each other.

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Model of an Atom

Neutron

Electron

Proton

Nucleus

• The nucleus is where the protons and neutrons are located within the atom.

Atomic Number

• The atomic number of an atom is the number of protons it has.

• If an atom has 6 protons, it is carbon.– It does not matter what else it has, if it has

6 protons then it is carbon.

Atomic Number

Practice Number of Protons

Electron Cloud

• The electron cloud is the region around the nucleus occupied by electrons.

• Atoms are much, much denser near the nucleus.

Electrons

• Electrons are also arranged in shells.• The first shell can hold a maximum of 2

electrons. All other shells can hold up to 8 electrons.

Valence Electrons

• The outermost shell of an atom is called the valence shell.

Protons and Electrons

• In an ATOM the number of electrons should always equal the number of protons.

Isotopes

• An Isotope is when an atom has a different number of protons and neutrons.

• Most elements have many different isotopes. • This makes the nucleus unstable or radioactive

in many cases.

Writing Isotopes

• To describe an atom, write the name of the atom and then write the total number of protons and neutrons that are found in it’s nucleus.

• The total number of protons and neutrons in the nucleus is called the Mass Number.

Isotopes and Mass Number

Practice IsotopesName Atomic # Protons Neutrons Mass Number Full Name

Carbon 6 8

Hydrogen 1 0

Lithium 3 2

Aluminum-26

8 17

Fluorine 20

12 13

Sulfur 34

2 2

Abbreviating Mass Number

• To abbreviate mass number, write the symbol of the atom and then to the upper left write the mass number and under that, write the atomic number.