Atomic Structure

mass, charge, chemical symbol, and location in the atom of electrons

0 amu, -1, in the electron shells/orbitals

mass, charge, chemical symbol, and location in the atom of neutrons

1 amu(a little heavier cause it's an electron + a proton), 0, in the nucleus

mass, charge, chemical symbol, and location in the atom of protons

1 amu, +1, in the nucleus

Dalton's theory can be summarized in a few points

All chemical elements are composed of small, indivisible particles called atoms and that each unique element is composed of a single type of atom. Atoms of a single type of element are identical in terms of properties like mass, but the atoms of one element are different from the atoms of all other elements. During chemical reactions, atoms are combined, separated, or rearranged into different ratios John Dalton suggested that elements are made of small, indivisible particles called atoms. Each element is made up of particles that are identical to each other, but different from every other element. Compounds are created when elements of two or more different types of atoms are combined in consistent ratios. In 1803 He deduced that all elements are composed of atoms. Atoms are indivisible and indestructible particles. Atoms of the same element are exactly alike and atoms of different elements are different. He realized that Compounds are formed by the joining of atoms of two or more elements.

The quantum model of the atom

Although the Bohr model of the atom is still used to introduce the concepts of quantization and energy levels, a more complex model of the atom called the Quantum Model is the most widely accepted atomic model today. In 1927, Erwin Schrödinger proposed a mathematical equation describing the behavior of the electron in the hydrogen atom. This equation describes the likelihood of finding the electron in a certain position relative to the nucleus of the hydrogen atom. In the quantum model of the atom, electrons occupy orbitals, which are volumes of space in defined shapes and sizes around the nucleus. These orbitals are sometimes called "electron clouds". Each orbital of an atom has a different shape and volume, based on the probability functions. These probabilities have different shapes for higher energy orbitals. In 1926, Schrödinger explained the nature of electrons in an atom by stating that the exact location of an electron cannot be stated; therefore, it is more accurate to view the electrons in regions called electron clouds; electron clouds are places where the electrons are likely to be found. His model is called the Wave Model. According to the theory of wave mechanics, electrons do not move about an atom in a definite path, like the planets around the sun. In fact, it is impossible to determine the exact location of an electron. The probable location of an electron is based on how much energy the electron has. According to the modern atomic model, at atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make an atom neutral.

What was missing from J.J. Thompson's model?

At the time, this was the most complete picture of an atom, and scientists were confident with this model. However, this model was not able to explain and predict future observations of the atom, which led to this model ultimately being replaced by other theories.

Summary (read)

Atomic theories that were based on experimentation began when John Dalton recognized that atoms combined in the exact same ratio, and theorized that a single element is all made up of small particles that could not be split apart. Following Dalton's work, electrons were identified as sub-atomic particles, which contradicted the previous theories. Based on results from experiments with cathode rays, J.J. Thompson suggested a model where negatively charged electrons were suspended in a positively charged matrix. This model is known as the plum pudding model. In 1909, Ernest Rutherford performed the gold-foil experiment, which proved that the positive charge in an atom is contained in one very small volume, which is now called the nucleus. This model, however, was not able to explain how electrons were arranged around the nucleus. In order to explain how electrons were arranged, and explain the emission spectra that elements produced, Neils Bohr proposed the concept of quantization, where electrons were confined to very specific energy levels, and could absorb or emit exact quantities of energy to move between energy levels. Unfortunately, his mathematical equations could only explain single electron systems (H, He+, Li2+, etc.). To expand the idea of quantization to multi-electron systems, Erwin Schrödinger developed new mathematical formulas that describe the statistical probability of each electron's location. These statistical models provide shapes of orbitals where electrons of varying energies are most likely found.

Which of the following advances to atomic theory were made using the gold foil experiment?

Atoms are mostly empty space. The positive charge in each atom is contained in a small, dense volume.

Advances in atomic theory from Bohr's model

Bohr was the first scientist to propose quantization, which had led to the development of a whole field of study today: quantum mechanics. In this theory, since electrons exist in very specific energy levels, this solved the problem of electrons in motion losing energy and spiraling into the nucleus.

Evidence supporting the Bohr model

Bohr's model of quantization was able to explain the emission spectra that are observed when heated elements emit light. Bohr proposed that, while electrons are not allowed to exist between shells, they are able to move between shells by gaining or losing specific quantities of energy. This quantity of energy is equal to the energy difference between the two energy levels. When an electron moves between energy levels it is either absorbing or releasing a specific wavelength of light. Energy is absorbed when an electron moves to a higher energy level. This is called excitation. Energy is released when an electron moves to a lower energy level. This is called emission. Each line on an emission spectrum correlates with a very specific amount of energy that is released. This specific amount of energy supports the quantization theory proposed by Bohr.

The Bohr model

By the end of the 1800s and into the early 1900s we knew of the existence of electrons (thanks to J.J. Thompson) and the nucleus (thanks to Rutherford). The model of atomic structure at this time was the planetary model, in which electrons orbit the nucleus of an atom just like planets orbit the sun. While Rutherford's model was based on scientific evidence, it still could not explain all scientific observations and questions. For example: If electrons with a negative charge orbit around a positively charged nucleus, why don't the electrons crash into the nucleus? How exactly do electrons behave? Another experimental observation that remained unexplained was the line spectra produced when light is emitted by different elements. It was experimentally shown that elements, when heated to a high temperature, emitted wavelengths of light that corresponded to very specific wavelengths. Each element produced its own unique line spectrum. This phenomenon is still observed today. Hot hydrogen gas sends wavelengths of light through a prism. When the light splits apart, there are only 4 very specific wavelengths of light, purple, blueish-purple, blue-green, and red. What caused this phenomenon? Why did the emission spectrum of hydrogen (and other elements) indicate only specific energies of light? In 1913, the Danish scientist Niels Bohr proposed an improvement. In his model, he placed each electron in a specific energy level. This explained why the electrons did not spiral toward and collide with the nucleus.

What was missing from Dalton's model?

Dalton described atoms as though they were indivisible, and the smallest building blocks of matter. Later experiments showed that there were particles (such as electrons) that were smaller than the atoms that Dalton proposed. We now know that atoms are composed of smaller sub-atomic particles: protons, neutrons and electrons.

Put the following atomic models in order of their development, with the earliest model on the left.

Dalton's atomic theory, J.J. Thompson's plum pudding model, Rutherford's planetary model, Bohr's atomic model, Schrödinger's quantum mechanical model. This is our current model.

Which subatomic particle is J.J. Thompson credited with confirming?

Electron

Which of the following descriptions best match the most current model of atomic theory?

Electron Cloud Model

Ernest Rutherford's theory of the atom

Ernest Rutherford, who was a student of J.J. Thomson, performed the gold foil experiment, which disproved the plum pudding model in 1909. In 1908, the English physicist Ernest Rutherford Rutherford's experiment Involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick).

Dalton's atomic theory

In 1804 John Dalton proposed his theory of the atom, drawing from Antoine Lavoisier's work on the law of conservation of mass. Dalton's theory was the first atomic theory that was based on scientific evidence. The English Chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms.

Bohr model of the atom

In 1913, Niels Bohr developed a theory of quantization. What does "quantization" mean? You can think of quantization as "set values". Bohr proposed that, at the atomic level, energy was quantized, meaning that atoms had energy levels that correlated with very specific quantities of energy. For example, as shown in Figure 6, the electrons in hydrogen atoms only gave off light corresponding to the four shown wavelengths, nothing in between. He started with Rutherford's planetary model of hydrogen and said that electrons could only orbit the nucleus in a fixed set of circular shells, which have very precise and specific energies. These shells are positioned at a specific distance from the nucleus. As long as the electrons are in their specific shell, they had a constant energy, and thus could not lose energy and crash into the nucleus of the atom. As more electrons are added to an atom, they must go in increasingly higher energy shells. Electrons must have the exact energy of these shells, and cannot appear in between shells. The restriction of electron energies to only certain values (the energies of each shell), is known as quantization. According to Bohr's atomic model, electrons move in definite orbits around the nucleus, much like planets circle the sun. These orbits, or energy levels, are located at certain distances from the nucleus. He discovered that electrons can jump from a path on one level to a path on another level.

James Chadwick

In 1932 Discovered the neutron as a particle in the nucleus Realized that the atomic mass of most elements was double the number of protons. This explained why the mass of an atom could not be only from the protons.

J.J. Thompson's model of the atom

In the late 1800s, several people proposed ideas for subatomic particles and their properties. These were some of the first ideas stating that atoms could be broken down further into even smaller particles. In 1897 J.J. Thompson confirmed the existence of electrons by noticing that when a negative ray of particles in a cathode ray tube was exposed to charged plates, it veered towards the positive plate and was deflected by the negative plate. This indicates that the ray was composed of negative charges. A negative charge was created at one end of the tube using electricity, and an invisible beam of "cathode rays" traveled to the positive anode. The anode had a small hole, letting a beam of cathode rays through to travel the length of the tube. At the other end of the cathode ray tube, a fluorescent dye coating would detect the rays hitting it. In 1897, the English scientist J.J. Thomson provided the first hint that an atom is made of even smaller particles. He is now known for discovering Electrons. Thomson studied the passage of an electric current through a gas. As the current passed through the gas, it gave off rays of negatively charged particles. This surprised Thomson, because the atoms of the gas were uncharged. Where had the negative charges come from? Thomson concluded that the negative charges came from within the atom. A particle smaller than an atom had to exist. The atom was divisible! Thomson called the negatively charged "corpuscles," today known as electrons. Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom. But he could never find them.

What type of particle did Rutherford aim at gold foil in order to study the atoms in the foil?

Positively charged alpha particles

What is the term for energy levels occurring in discrete packets of energy?

Quantization

The gold foil experiment

Rutherford hypothesized that if atoms were structured according to J.J. Thompson's plum pudding model, then in an experiment where a beam of positively charged alpha particles was directed at a piece of gold foil, the alpha particles would pass straight through, without significant deflection. Rutherford reasoned that any very small deflections would be the result of the positively charged alpha particles interacting with the negatively charged electrons, and was hoping to learn about the arrangement of the electrons within the atom. Alpha particles passing almost straight through the plum pudding model. Electrons cause a few alpha particles to bend their path in small angles. However, this is not what Rutherford observed. When running the experiment, Rutherford noticed that while most of the alpha particles were passing straight through the gold foil, a small number of particles were deflected at significant angles, some even higher than 90∘. Based on this observation, Rutherford proposed that there was something very small and dense in the centre of the atom that contained all of the positive charge for each atom. We now understand this to be the nucleus of the atom. The nucleus is very small, but every so often, an alpha particle would hit it or pass very close to it, which would cause the large deflections Rutherford observed. Additionally, since all of the positive charge was in one small area, and electrons were already known to be very small, Rutherford concluded that the majority of the atom was actually empty space. In this experiment, positively charged alpha particles were shot through a thin strip of gold foil. Rutherford found that most of the particles went straight through the foil, however, some particles were scattered. This implies that the atom is composed mostly of empty space, allowing the majority of the particles to pass straight through. Rutherford suggested that the small number of scattered particles was a result of collisions occurring at a dense, positively charged core in the center of the atom, which we now understand to be the nucleus. This experiment showed that the atom is mostly composed of empty space, with a small nucleus of positive charge in the center. Rutherford had determined that all of the positive charge in an atom was condensed into one small space at the centre of the atom; however, he wasn't entirely sure how the negative electrons were arranged around the nucleus. His model was vague on this point, and listed several possibilities. Most of the positively charged "bullets" passed right through the gold atoms in the sheet of gold foil without changing course at all. Some of the positively charged "bullets," however, did bounce away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges. his could only mean that the gold atoms in the sheet were mostly open space. Atoms were not a pudding filled with a positively charged material. Rutherford concluded that an atom had a small, dense, positively charged center that repelled his positively charged "bullets." He called the center of the atom the "nucleus" The nucleus is tiny compared to the atom as a whole. Rutherford reasoned that all of an atom's positively charged particles were contained in the nucleus. The negatively charged particles were scattered outside the nucleus around the atom's edge. Atomic model has massive nucleus with electrons in circle around it like a solar system.

What was missing from Rutherford's model?

Rutherford realized that the negatively charged electrons must be moving, because otherwise they would simply be pulled into the nucleus; however, according to classical physics, if the electrons were moving, they would be releasing energy, and ultimately still spiral into the nucleus. Rutherford was not able to explain how the atom could be stable, however chose to overlook this problem when he proposed his theory. While the gold foil experiment revealed new insights into atomic structure, it also led to even more questions such as how are electrons arranged, and why they did not collapse into the nucleus.

Advances in atomic theory from Rutherford's model

Rutherford was able to experimentally prove that atoms contained nuclei which held all of the positive charges in the atom. He reasoned that since electrons were known to be very small, and he calculated the nucleus of the atom to be very small, that atoms must be mostly empty space.

Ancient Theories: Where does the history of atomic theory start?

The ancient Greeks were one of the earliest known Western societies to develop a theory of what matter is made of. Around 400 BCE philosophers Democritus and Leucippus proposed the idea that all of matter is composed of tiny, indivisible particles called atomos, the Greek word for indivisible. While the ancient Greeks developed their theories of the atom in the Western world, philosophers in India around the same time also developed similar ideas. Some of the ancient Indian ideas of the concept of atoms may have even been developed prior to the Greeks. Ancient ideas provided the groundwork for all future study of atoms. In fact, the word atom is derived from the ancient Greek word atomos. The main aspect lacking from this idea, is proof. The ancient philosophers did not have the tools and techniques to develop evidence to support their theories. The first evidence-based theory was developed about 2000 years after the Greek philosophers first proposed the concept of atomos.

Which of the following properties are not described by Dalton's theory?

The charge of electrons within the atom, The identity of subatomic particles within an element.

Democritus

This is the Greek philosopher Democritus who began the search for a description of matter more than 2400 years ago (around 460 BC). He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided? His theory was that Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. This piece would be indivisible. He named the smallest piece of matter "atomos," meaning "not to be cut." To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes. Atoms were infinite in number, always moving and capable of joining together. This theory was ignored and forgotten for more than 2000 years.

What was missing from Bohr's model?

This model is able to explain a single electron interacting with a nucleus (for example, a neutral hydrogen atom, or a He+ ion). It is also able to explain most of the features in the emission spectrum of hydrogen. However, the mathematical equations used in the Bohr model are not able to model the observed properties of multi-electron atoms, such as their spectra.

Advances in atomic theory from J.J. Thompson's model

This model was the first to confirm the idea that atoms were composed of smaller subatomic particles. Experimentalists of the time realized that these subatomic particles possessed charges, but that overall, atoms were neutral.

Advances in atomic theory from Dalton's model

This was the first atomic model to be based on experimental evidence, and provided the starting point for all future atomic theories. Dalton recognized that elements were made of small particles (atoms), and that in chemical compounds, elements were always present in the exact same ratio of atoms.

-ions

anions

only use the word _______ if it is neutral

atom ion if not

isotopes of an element have the same _____________________ but different

atomic number (protons), mass number (neutrons).

Be able to determine the atomic number, mass number and charge on an atom and write its complete chemical symbol including atomic number, mass number and charge.

atomic number = # of protons and bottom left, mass number = neutrons + protons, charge = top right

+ions

cations

atoms become ions by

gaining and losing electrons, not by changing protons.

the percent abundance of an isotope is

how often a specific isotope occurs in nature.

Ions

individual atoms or groups of atoms(polyatomic ion) that have a charge

If given the isotope notation (ex. sodium-23 or Na-23), know what the number after the name or symbol represents and be able to determine the protons, neutrons and electrons.

mass number, protons = on pt, neutron = mass - protons, electrons = protons

what are protons and neutrons called

nucleons

Thompson's idea lead to the

plum pudding model, in which negatively charged electrons (the plums) were scattered about a positive charge (the pudding). This can also be thought of as the chocolate chip cookie model, where the negatively charged electrons are like the chocolate chips, scattered through the positively charged cookie. He proposed a model of the atom that is sometimes called the "Plum Pudding" model. Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding.