Structure of Atom: Structure, Model & Examples

Structure of atom consists of three basic particles, namely Electrons, Protons, and Neutrons. The nucleus is the centre of the atom and contains protons. Protons are positively charged, whereas neutrons have no charge. 

• The structure of atom is also known as atomic structure.
• It is a building block of matter.
• When an atom is combined with another atom, it will form molecules.
• The size of an atom is around 100 picometres.
• The outermost parts of an atom are called Electron Shells .
• Electron shells contain negatively charged electrons.
• In the absence of an electrical charge, the neutron is not repelled by a cloud of electrons or by the nucleus. 
• Neutron becomes a useful tool for probing the structure of atom.

Read More: Hund’s Rule of Maximum Multiplicity

Key Terms: Structure of atom, Nucleus, Proton, Neutron, Electron, Radioactivity, Orbit, Atomic Mass, Atomic Number, Electron Shell, Atom, Element

What is Atom?

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

• The electric charge of Atom is zero, or ion charge. 
• Protons and neutrons have the same mass (approx.), nearly 1.67 × 10^-24 grams. 
• Scientists have defined this as one atomic mass unit (amu) or one Dalton.
• One atom combines with another atom with the help of a chemical bond.
• A neutral atom has an equal number of protons and electrons.
• The atomic number is denoted by the total number of protons in the nucleus.

Structure of Atom

Cathode Ray Experiment

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J.J. Thompson performed the first cathode ray tube experiment to demonstrate that rays released by an electron cannon are inseparable from latent charge. He created a metal cylinder on the other end of his cathode ray tube. 

• Two microscopic perforations in metal led to an electrometer that could measure a modest electric charge. 
• He observed that electrometers ceased electric charge after the first experiment. 
• Later, a second experiment was carried out to determine whether the charge was negative or positive. 
• Place a negatively charged metal plate on one side of the cathode rays.
• Allow charge to pass through the anode and a positively charged metal plate.
• He used a fluorescent-coated tube instead of an electrometer at one end of the Cathode Ray Tube.
• It would illuminate when a cathode ray hit it. 
• He discovered that when he used charged metal plates, cathode rays bent away towards the positive plate. 
• Cathode rays were found to be negatively charged.

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

Read More: Phase Changes

Thomson’s Model of Atom

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J. J. Thomson proposed first in a series of atomic models in 1898. He proposed that an atom is shaped like a sphere with a radius of around 10-10m with uniformly distributed positive charge. Electrons are encased in this sphere to create the most stable electrostatic configuration possible.

• The illustration below resembles a chopped watermelon with electrons representing seeds. 
• Watermelon, plum pudding, and raisin pudding models are all names for this model. 
• The atomic model of Thomson successfully explained the atom's general neutrality. 
• However, the findings of subsequent trials contradicted its assertions.
• Electrons are embedded in negatively charged sub-particles.
• The size of the atom is extremely small.

Thomson’s Model of Atom

Read More: Thomson's Atomic Model

Radioactivity

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

• Radioactivity was discovered by Henry Becquerel.
• In the experiment, uranium is placed in a drawer wrapped in black paper.
• It was contained inside a photographic plate.
• When plates were opened, it was determined that they were exposed to radioactivity.
• It is divided into alpha, beta and gamma decay.

Radioactivity

Rutherford Model

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Rutherford conducted an experiment in which he bombarded a thin sheet of gold foil with particles. He later analyzed the 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). 

• Rutherford model wrapped a fluorescent zinc sulphide screen around the thin gold foil.
• He investigated deflection generated by particles. 
• Certain observations obtained by Rutherford contradicted Thomson's atomic hypothesis.
• He proposed that a negative charge atom surrounds the nucleus of an atom.
• Electrons revolve around the nucleus in a circular path.

Rutherford Model of Atoms

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

Observations of Rutherford’s Alpha Scattering Experiment

Rutherford's observations led him to the following conclusions:

• A large portion of particles fired at the gold sheet passed through it without being deflected.
• Majority of space in one atom is vacant.
• The gold sheet deflected some particles at minor angles.
• Positive charge in each atom is unevenly distributed. 
• In an atom, positive charge is concentrated in a relatively small volume.
• Only a few particles were deflected back.
• It implies that only a few particles had almost 180° deflection angles. 
• Positively charged particles occupy little volume in comparison to the entire volume of an atom.

Rutherford Experiment Observations

Limitations of Rutherford Atomic Model

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

• According to Rutherford, electrons orbit the nucleus in set trajectories, termed as orbits. 
• According to Maxwell, accelerating charged particles release electromagnetic radiation. 
• This radiation would transmit energy from the electron's motion but at the expense of orbital shrinkage. 
• Electrons would eventually collapse in the nucleus. 
• According to calculations, an electron would disintegrate in the nucleus in less than 10-8 seconds. 
• As a result, Rutherford's model contradicted Maxwell's theory and was unable to explain atom stability.
• One of the flaws of Rutherford's model was that he did not address the configuration of electrons in an atom, leaving his theory incomplete.

The video below explains this:

Rutherford's Alpha Ray Scattering Experiment Detailed Video Explanation:

Read More: Bohr Model of the Hydrogen Atom

Neils Bohr Model

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In 1915, Bohr proposed a theory, which was a modified version of Rutherford's Model of an atom. It stated that electrons move in fixed orbitals (shells) and not everywhere in between. Bohr's model also added that each orbit (shell) has a fixed energy level. 

• Rutherford essentially explained how an atom's nucleus works.
• Bohr expanded that model to include electrons and their energy levels.
• In Bohr's concept, a tiny (positively charged) nucleus is surrounded by negative electrons travelling in orbits around the nucleus. 
• He discovered that electrons away from the nucleus had more energy than electrons near the nucleus.

Read More: Electron Spin

Neils Bohr Model

Limitations of Bohr’s Model of an Atom

The limitation of Bohr’s model of an atom are as follows:

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

Read More: Limitations of Bohr’s Atomic Model

Electron Distribution in Different Orbits

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Bohr and Bury recommended distribution for electron distribution are as follows:

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

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Valency

Valence electrons are electrons that make up an atom's outermost shell. It refers to its capacity for combining or proclivity to react and form molecules with atoms of the same or different elements.

• Chemical activity is minimal in atoms with a filled outermost shell.
• Their valency (or combining capacity) is 0.

Read More: Nucleus of an Atom and Atomic Mass

Things to Remember

• Structure of atom is a fundamental building block of structural elements.
• It 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 the nucleus, located in the atom's centre.
• Shell/energy level/orbits refer to the outer area of the atom that holds electrons in orbit around the nucleus.
• Subshells are subdivided from these shells.

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