Structure of Atom: Atomic Model, Nucleus, Cathode Ray Experiment

Structure of Atom consists of three basic particles: Electrons, Protons, and Neutrons. Nucleus is center of atom and contains protons. Protons are positively charged, whereas, Neutrons have no charge. In absence of electrical charge, neutron is not repelled by cloud of electrons or by nucleus. Thus, it becomes a useful tool for probing structure of atom. Outermost parts of atom are called Electron Shells and contain negatively charged electrons.

Key Terms: Nucleus, Proton, Neutron, Electron, Radioactivity, Orbit, Atomic Mass, Atomic Number

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What is Atom?

Atom is smallest unit of matter that may be divided without releasing electrically charged particles. It's also smallest unit of matter with chemical element features. Components of Atom are Electrons and compact nucleus of Protons and Neutrons. 

Electric charge of Atom is zero, or ion charge. Protons and neutrons have same mass (approx.), nearly 1.67 × 10-24 grams. Scientists have defined this as one atomic mass unit (amu) or one Dalton.

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Cathode Ray Experiment

J.J. Thompson performed the first cathode ray tube experiment to demonstrate that rays released by an electron cannon are inseparable from latent charge. On the other end of his cathode ray tube, he created a metal cylinder. Two microscopic perforations in metal led to an electrometer that could measure a modest electric charge. He observed that electrometers ceased measuring electric charge after first experiment. 

He then performed a second experiment to determine whether charge carried by cathode rays was negative or positive. He now placed a negatively charged metal plate on one side of cathode rays to allow them to pass through anode and a positively charged metal plate on the other. 

Thompson used fluorescent coated tube instead of electrometer at one end of Cathode Ray Tube, which would illuminate when cathode ray hit it. He discovered that when he used charged metal plates, cathode rays bent away from negative plate and towards positive plate. Cathode rays were found to be negatively charged as a result of this.

Following these investigations, he reasoned that “Inside each atom lies a subatomic particle called a 'corpuscle,' which is 1800 times lighter than the mass of a hydrogen atom (Lightest atom).”

Apparatus of Cathode Ray Experiment

Electrons

• Atom's electrons are negatively charged subatomic particles.
• Electron's mass is assumed negligible, and its charge is -1.
• Electron's symbol is e–.
• Electrons are very small particles.
• They exist outside of the nucleus.

Also read: 

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Thomson’s Model of Atom

J. J. Thomson proposed that atom is shaped like a sphere with radius of around 10-10m with uniformly distributed positive charge. Electrons are encased in this sphere to create stable electrostatic configuration possible. 

Thomson’s Model of Atom

The above illustration resembles a chopped watermelon with electrons representing seeds. As a result, watermelon model, plum pudding model, and raisin pudding model are all names for this model. Assumption that atom's mass is equally distributed across atom is key feature of this model. Atomic model of Thomson was successful in explaining atom's general neutrality. However, findings of subsequent trials contradicted its assertions.

Read More: Thomson's Atomic Model

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Radioactivity

Process through which unstable nucleus of an atom loses energy by emitting particles is known as radioactivity. It accomplishes this by emitting particles such as alpha and beta. This is a natural process. If nucleus of an atom is unbalanced, that is, if number of protons and neutrons differs, atom is unstable.

Radioactivity

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Rutherford Model

Rutherford carried out an experiment in which he bombarded a thin sheet of gold foil with particles and then analyzed track of these particles after they collided with gold foil. In his experiment, Rutherford focused on high-energy particle streams from a radioactive source at a thin sheet of gold (100 nm thickness). 

He wrapped a fluorescent zinc sulphide screen around thin gold foil to investigate deflection generated by particles. Certain observations obtained by Rutherford contradicted Thomson's atomic hypothesis.

Rutherford Model of Atoms

Read: Alpha-Particle scattering and Rutherford's Nuclear Model of Atom

Observations of Rutherford’s Alpha Scattering Experiment

Rutherford's observations lead him to following conclusions:

• Because a large portion of particles fired at the gold sheet passed through it without being deflected, majority of space in one atom is vacant.
• Gold sheet deflected some particles at minor angles, causing positive charge in each atom to be unevenly distributed. In an atom, positive charge is concentrated in a relatively small volume.
• Only a few particles were deflected back, implying that only a few particles had almost 180° deflection angles. As a result, positively charged particles in an atom occupy very little volume in comparison to entire volume of atom.

Rutherford Experiment Observations

Limitations of Rutherford Atomic Model

Even though Rutherford atomic model was founded on experimental findings, it was unable to explain certain phenomena.

• According to Rutherford, electrons orbit nucleus in set trajectories termed orbits. According to Maxwell, accelerating charged particles release electromagnetic radiation, thus an electron rotating around nucleus should do same. This radiation would transmit energy from electron's motion, but at expense of orbital shrinkage. 
• Electrons would eventually collapse in nucleus. According to calculations, an electron would disintegrate in nucleus in less than 10-8 seconds using Rutherford model. As a result, Rutherford model contradicted Maxwell's theory and was unable to explain atom stability.
• One of the flaws of Rutherford model was that he did not address configuration of electrons in an atom, leaving his theory incomplete.
• Even though early atomic models were erroneous and failed to explain certain experimental results, they served as foundation for future quantum mechanics breakthroughs.

Also read: Bohr Model of the Hydrogen Atom

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Neils Bohr Model

Bohr theory was proposed by Niels Bohr atomic structure model was modified by stating that electrons move in fixed orbitals (shells) and not everywhere in between, and that each orbit (shell) has a fixed energy level. Rutherford essentially explained how an atom's nucleus works, and Bohr expanded that model to include electrons and their energy levels.

Also read: Electron Spin

A tiny (positively charged) nucleus is surrounded by negative electrons traveling in orbits around nucleus in Bohr's concept. Bohr discovered that electrons far away from the nucleus had more energy than electrons near nucleus.

Neils Bohr Model

Limitations of Bohr’s Model of an Atom

• Zeeman Effect was not explained by Bohr's atom model (effect of magnetic field on spectra of atoms).
• It also didn't explain how Stark effect works (effect of electric field on the spectra of atoms).
• Heisenberg Uncertainty Principle is broken.
• It was unable to account for spectra observed from bigger atoms.

Read more: Limitations of Bohr’s Atomic Model

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Electron Distribution in Different Orbits

Bohr and Bury recommended distribution:

• Formula 2n², where 'n' is orbit number or energy level index, 1,2,3,...., gives maximum number of electrons present in a shell.
• Maximum number of electrons in each orbit is as follows: first orbit has 2*12=2, second orbit has 2*2Msup>2=8, third orbit has 2*3²=18, fourth orbit has 2*4²=32, and so on.
• From lower to higher energy levels, shells are always filled in a step-by-step manner. Electrons aren't filled in following shell until shells before it have been filled.

Valency

• Valence electrons are electrons that make up an atom's outermost shell.
• Valency of an atom refers to its capacity for combining or proclivity to react and form molecules with atoms of same or different elements.
• Chemical activity is minimal in atoms with a filled outermost shell.
• Their valency (or combining capacity) is 0.

Also read:

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Things to Remember

• Atoms are fundamental building blocks of matter and structural elements.
• Structure of an atom is made up of three basic particles: protons, electrons, and neutrons.
• E. Goldstein invented protons, which are positively charged particles.
• J.J. Thomson discovered electrons, which are negatively charged particles.
• Chadwick invented neutrons, which have no charge.
• Protons and neutrons are found in nucleus, which is located in center of atom.
• Shell/energy level/orbits refer to outer area of atom that holds electrons in orbit around nucleus.
• Subshells are subdivided from these shells.

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