modern atomic therory

Atoms emit energy as light when _____.

*electrons move to a lower energy level* An electron can move to a lower energy level, and the energy it releases may be in the form of light or heat.

Niels Bohr

1913- discovered that electrons move around the nucleus in orbits called electron shells.

isotopes

Atoms of the same element that have different numbers of neutrons

models of the atom

Dalton-He theorized that atoms were tiny, indestructible spheres. Thomson-He theorized that atoms resemble "plum pudding." Rutherford-He theorized that an atom has a dense, positively charged nucleus. Bohr-He theorized that the electrons move in spherical orbits.

Electron Configurations

How are the seats in your classroom arranged? Are they lined up neatly in rows, or are they grouped in clusters? A configuration is an arrangement of objects in a given space.Some configurations are more stable than others, meaning that they are less likely to change. The position of the gymnast on the balance beam in the figure is not very stable because the beam is only 10 centimeters wide. An electron configuration is the arrangement of electrons in the orbitals of an atom. The most stable electron configuration is the one in which the electrons are in orbitals with the lowest possible energies. When all the electrons in an atom have the lowest possible energies, the atom is said to be in its ground state. To better explain orbitals, let's take a look at a sodium atom. Sodium is a soft, silvery-white metal with an atomic number of 11, which means that a sodium atom has 11 electrons.In the ground state, two of the electrons are in the orbital of the first energy level. There are 8 electrons in the second energy level, leaving one electron to occupy the third energy level. If an atom absorbs enough energy, one of its electrons can move to an orbital with a higher energy. This configuration is referred to as an excited state. An excited state is less stable than the ground state. Eventually, the electron that was promoted to a higher energy level loses energy, and the atom returns to the ground state.Helium, neon, argon, krypton, and xenon atoms returning from excited states to the ground state emit the light you see in "neon" lights.

Energy levels

In Bohr's model, electrons move with constant speed in fixed orbits around the nucleus, like planets around a sun. Each electron in an atom has a specific amount of energy. If an atom gains or loses energy, the energy of an electron can change. The possible energies that electrons in an atom can have are called energy levels. To understand energy levels, picture them as steps in a staircase. As you move up or down the staircase, you can measure how your position changes by counting the number of steps you take. You might take one step up, or you might jump two steps down.Whether you are going up or down, you can move only in whole step increments. Just as you cannot stand between steps on a staircase, an electron cannot exist between energy levels. The landing at the bottom of the staircase is like the lowest energy level in an atom. Each step up represents a higher energy level. The distance between two steps represents the difference in energy between two energy levels. To continue the analogy, there would need to be a different staircase for each element because no two elements have the same set of energy levels. An electron in an atom can move from one energy level to another when the atom gains or loses energy. An electron may move up two energy levels if it gains the right amount of energy. An electron in a higher energy level may move down two energy levels if it loses the right amount of energy. The size of the jump between energy levels determines the amount of energy gained or lost.

Electron Cloud Model

Like earlier models, Bohr's model was improved as scientists made further discoveries. Bohr was correct in assigning energy levels to electrons. But he was incorrect in assuming that electrons moved like planets in a solar system. Today, scientists know that electrons move in a less predictable way. Scientists must deal with probability when trying to predict the locations and motions of electrons in atoms. An electron cloud is a visual model of the most likely locations for electrons in an atom. The cloud is denser at those locations where the probability of finding an electron is high.Scientists use the electron cloud model to describe the possible locations of electrons around the nucleus. The propellers of an airplane provide an analogy for an electron cloud. When the propeller of an airplane is at rest, you can count the number of blades. Also, you can see the locations of the blades.When the propeller is moving, the blades spin so fast that you see only a blur. You know that the blades are located somewhere in the blur, but at any specific moment in time you can't be exactly sure where each blade is located.The blurred, moving propellers are similar to an electron cloud.

Science and History

Models of the Atom The development of the scientific ideas on the structure of atoms has passed several key milestones during the last 200 years.

Atomic Orbitals

The electron cloud represents all the orbitals in an atom. An orbital is a region of space around the nucleus where an electron is likely to be found. To understand the concept of an orbital, imagine a map of the world.Suppose you mark the places you've visited. The places you visit the most—such as your grandparent's town, family vacation spots, and the area near your home—would have the highest concentration of dots. The places you visit the least would have the lowest concentration of dots. The dots on your map are a model of your "orbital." They describe your most likely locations. There are some locations in your orbital that you may not ever visit. These locations may not be represented by a dot on your map.Despite such omissions, the dots on your map are a good model of how you usually behave in your orbital. An electron cloud is a good approximation of how electrons behave in their orbitals. The level in which an electron has the least energy—the lowest energy level—has only one orbital. Higher energy levels have more than one orbital. The table shows the number of orbitals in the first four energy levels of an atom.Notice that the maximum number of electrons in an energy level is twice the number of orbitals. Each orbital can contain two electrons at most.

Evidence for Energy Levels

The movement of electrons between energy levels explains the light you see when fireworks explode. Heat produced by the explosion causes some electrons to move to higher energy levels. When those electrons move back to lower energy levels, they emit energy. Light is a form of energy.Because no two elements have the same set of energy levels, different elements emit different colors of light. This is why fireworks often display a dramatic array of colors. Scientists can measure the energy gained when electrons absorb energy and move to a higher energy level. They can measure the energy released when the electron returns to a lower energy level.For example, when an electron moves to a lower energy level, it may release that energy as light.

Bohr's Model of the Atom

You may have seen diagrams of an atom that look like a solar system with planets revolving around a sun. These diagrams are based on a model of the atom that was developed by Niels Bohr (1885-1962), a Danish physicist who worked for a while with Rutherford.Bohr agreed with Rutherford's model of a nucleus surrounded by a large volume of space. But Bohr's model did something that Rutherford's model did not do. It focused on the electrons. A description of the arrangement of electrons in an atom is the centerpiece of the modern atomic model.

oribtal

a region of space around the nucleus where an electron is likely to be found

atomic orbital

a region of space in which there is a high probability of finding an electron

ground state

a state in which all the electrons in an atom have the lowest possible energies

Atoms release energy when __________________.

electrons move to a lower energy level

Carbon has an atomic number of 6. How many electrons are in a carbon atom's second energy level?

four Two electrons are contained in the first energy level, leaving 4 electrons to occupy the second energy level.

An atom in the ground state ___________________.

is more stable than it would be in an excited state An atom is in the ground state when all of its electrons have the lowest possible energies. This gives the atom greater stability.

Where did Niels Bohr think electrons could be found within an atom?

moving in spherical orbits at fixed distances from the nucleus In Bohr's model, electrons move with constant speed in fixed orbits around the nucleus, like planets around a sun. He suggested that electrons could jump between orbits, releasing or absorbing specific amounts of energy as they did so.

Bohr's Model of the Atom In this section, we have learned the following: Bohr proposed that electrons orbit a nucleus of an atom at fixed levels of energy. An electron in an atom can move from one energy level to another when the atom gains or loses energy. Scientists have measured the amounts of energy absorbed and released when electrons move to different energy levels. Elements emit distinct colors of light when electrons in their atoms move to lower energy levels. Electron Cloud Model In this section, we have learned the following: Scientists found that electrons do not revolve around nuclei in predictable, fixed orbits. They use the electron cloud model to describe the possible locations of electrons around the nucleus. The electron cloud surrounding the nucleus is denser at those locations where the probability of finding an electron is high. Orbitals and Configurations In this section, we have learned the following: An electron cloud is a good approximation of how electrons behave in their orbitals. An orbital is a region of space around the nucleus where an electron is likely to be found. The ground state of an atom has the most stable electron configuration. In the ground state, electrons are in orbitals with the lowest possible energies. When an atom absorbs a certain amount of energy, one or more of its electrons can move to an orbital with higher energy. This configuration is the excited state.

reveiw homie

electron configuration

the arrangement of electrons in the orbitals of an atom

Which subatomic particle did Bohr's model focus on?

the electron A description of the arrangement of electrons in an atom is the centerpiece of the modern atomic model.

How is the electron cloud model different from the model proposed by Bohr?

the precise location of electrons at a specific time is unknown

What determines the amount of energy gained or lost when an electron moves between energy levels?

the size of the jump between energy levels

How many electrons can be in each orbital?

two Each orbital can hold two electrons. For example, if there are four orbitals, the maximum number of electrons is eight.