# modern atomic theory

## energy levels

Basic to Bohrs model is the idea of energy levels. Energy levels are areas located at fixed distances from the nucleus of the atom. They are the only places where electrons can be found. Energy levels are a little like rungs on a ladder. You can stand on one rung or another but not between the rungs. The same goes for electrons. They can occupy one energy level or another but not the space between energy levels. The model of an atom in Figure 5.15 has six energy levels. The level with the least energy is the one closest to the nucleus. As you go farther from the nucleus, the levels have more and more energy. Electrons can jump from one energy level to another. If an atom absorbs energy, some of its electrons can jump to a higher energy level. If electrons jump to a lower energy level, the atom emits, or gives off, energy. You can see an animation at this happening at the URL below.

## energy levels in action

Bohrs idea of energy levels is still useful today. It helps explain how matter behaves. For example, when chemicals in fireworks explode, their atoms absorb energy. Some of their electrons jump to a higher energy level. When the electrons move back to their original energy level, they give off the energy as light. Different chemicals have different arrangements of electrons, so they give off light of different colors. This explains the blue- and purple- colored fireworks in Figure 5.16.

## bohrs model of the atom

Bohrs research focused on electrons. In 1913, he discovered evidence that the orbits of electrons are located at fixed distances from the nucleus. Remember, Rutherford thought that electrons orbit the nucleus at random. Figure 5.14 shows Bohrs model of the atom.

## orbitals

Some regions of the electron cloud are denser than others. The denser regions are areas where electrons are most likely to be. These regions are called orbitals. Each orbital has a maximum of just two electrons. Different energy levels in the cloud have different numbers of orbitals. Therefore, different energy levels have different maximum numbers of electrons. Table 5.1 lists the number of orbitals and electrons for the first four energy levels. Energy levels farther from the nucleus have more orbitals. Therefore, these levels can hold more electrons. Energy Level Number of Orbitals 1 2 3 4 1 4 9 16 Max. No. of Electrons (@ 2 per orbital) 2 8 18 32 Figure 5.18 shows the arrangement of electrons in an atom of magnesium as an example. The most stable arrange- ment of electrons occurs when electrons fill the orbitals at the lowest energy levels first before more are added at higher levels. You can learn more about orbitals and their electrons at the URL below:

## electron cloud and orbitals

In the 1920s, physicists discovered that electrons do not travel in fixed paths. In fact, they found that electrons only have a certain chance of being in any particular place. They could only describe where electrons are with mathematical formulas. Thats because electrons have wave-like properties as well as properties of particles of matter. It is the "wave nature" of electrons that lets them exist only at certain distances from the nucleus. The negative electrons are attracted to the positive nucleus. However, because the electrons behave like waves, they bend around the nucleus instead of falling toward it. Electrons exist only where the wave is stable. These are the orbitals. They do not exist where the wave is not stable. These are the places between orbitals.

## electron cloud model

Today, these ideas about electrons are represented by the electron cloud model. The electron cloud is an area around the nucleus where electrons are likely to be. Figure 5.17 shows an electron cloud model for a helium atom.

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

Which statement about energy levels is false?

``````a. They are located at fixed distances from the nucleus of the atom.

b. They are the only places where electrons can be found.

c. They have more energy when they are farther from the nucleus.

-->  d. They all have the same number of electrons.
``````

What are orbitals?

``````-->  a. regions in the electron cloud where electrons are most likely to be

b. fixed paths in which electrons orbit the nucleus

c. places where electron waves are unstable

d. none of the above
``````

An electron emits energy when it jumps from

``````a. a proton to a neutron.

b. an electron cloud to an orbital.

c. an orbital to the atomic nucleus.

-->  d. a higher energy level to a lower energy level.
``````

How many electrons can there be in energy level 1?

``````a. 0

b. 1

-->  c. 2

d. 3
``````

Bohrs model of the atom differs from Rutherfords model in the

``````a. placement of the nucleus.

b. charge of the nucleus.

c. number of electrons.

-->  d. location of electrons.
``````

Bohrs research focused on

``````-->  a. electrons.

b. neutrons.

c. protons.

d. none of the above
``````

Which statement about energy levels is false?

``````a. They are located at fixed distances from the nucleus.

b. They are the only places where electrons can be found.

c. They have more energy when they are farther from the nucleus.

-->  d. There are only two of them.
``````

Fireworks give off light energy when their electrons

``````a. flow to different atoms.

c. produce electric current.

d. change from matter to energy.
``````

The focus of Bohrs research was the nucleus.

``````a. true

-->  b. false
``````

Energy levels farther from the nucleus have

``````a. less energy.

b. more orbitals.

c. a greater maximum number of electrons.

-->  d. two of the above
``````

Bohr rejected modern atomic theory.

``````a. true

-->  b. false
``````

How many orbitals are there at energy level 3?

``````a. 1

b. 4

-->  c. 9

d. 16
``````

Electrons bend around the nucleus instead of falling toward it because electrons behave like

``````a. protons.

b. orbitals.

c. clouds.

-->  d. waves.
``````

There is a maximum of two energy levels in an atom.

``````a. true

-->  b. false
``````

Where would you not be likely to find electrons in an atom?

``````a. inside the nucleus

b. attached to the nucleus

c. between energy levels

-->  d. all of the above
``````

Electrons fall toward the nucleus because they behave like waves.

``````a. true

-->  b. false
``````

Energy levels are located between the orbitals of atoms.

``````a. true

-->  b. false
``````

Energy level 1 has the most energy.

``````a. true

-->  b. false
``````

Electrons can move from one energy level to another.

``````-->  a. true

b. false
``````

Scientists can now determine the exact location of any given electron.

``````a. true

-->  b. false
``````

Electrons are attracted to the nucleus because of the strong force.

``````a. true

-->  b. false
``````

Some regions of the electron cloud are denser than others.

``````-->  a. true

b. false
``````

There is a maximum of two orbitals per energy level.

``````a. true

-->  b. false
``````

Fireworks give off light when their electrons split in two.

``````a. true

-->  b. false
``````

Since the 1920s, physicists have known that electrons travel in fixed paths.

``````a. true

-->  b. false
``````

Wavelike particles in the atom exist only where the wave is stable.

``````-->  a. true

b. false
``````

All energy levels have the same maximum number of electrons.

``````a. true

-->  b. false
``````

number of orbitals in the first energy level

``````a. electron cloud

b. energy level

c. Rutherford

d. Bohr

e. electron

f. two

-->  g. one
``````

area surrounding the nucleus of an atom where electrons are likely to be

``````-->  a. electron cloud

b. energy level

c. Rutherford

d. Bohr

e. electron

f. two

g. one
``````

scientist who thought that electrons orbit the nucleus like planets orbit the sun

``````a. electron cloud

b. energy level

-->  c. Rutherford

d. Bohr

e. electron

f. two

g. one
``````

maximum number of electrons per orbital

``````a. electron cloud

b. energy level

c. Rutherford

d. Bohr

e. electron

-->  f. two

g. one
``````

area located at a fixed distance from the nucleus of an atom where electrons can orbit the nucleus

``````a. electron cloud

-->  b. energy level

c. Rutherford

d. Bohr

e. electron

f. two

g. one
``````

wavelike particles that move around the nucleus of an atom

``````a. electron cloud

b. energy level

c. Rutherford

d. Bohr

-->  e. electron

f. two

g. one
``````

scientist who discovered energy levels

``````a. electron cloud

b. energy level

c. Rutherford

-->  d. Bohr

e. electron

f. two

g. one
``````

## diagram questions

No diagram questions associated with this lesson