Holt Chemistry Concept Review Structure of Atoms Answer Key

Atomic structure refers to the structure of an atom comprising a nucleus (heart) in which the protons (positively charged) and neutrons (neutral) are present. The negatively charged particles called electrons revolve effectually the centre of the nucleus.

The history of atomic structure and quantum mechanics dates dorsum to the times of Democritus, the human being who first proposed that matter is equanimous of atoms. The study about the structure of an atom gives a great insight into the entire class of chemical reactions, bonds and their concrete backdrop. The first scientific theory of diminutive construction was proposed by John Dalton in the 1800s.

Construction of Atom – Important Topics

Table of Content

• What is Atomic Structure?
• Diminutive Models
• Dalton'due south Atomic Theory
• Thomson Diminutive Model
• Rutherford Atomic Theory
• Subatomic Particles
• Atomic Structure of Isotopes
• Bohr'due south Atomic Theory
• Dual Nature of Matter
• FAQs

The advances in atomic structure and breakthrough mechanics have led to the discovery of other primal particles. The discovery of subatomic particles has been the base for many other discoveries and inventions.

What is Atomic Structure?

The diminutive structure of an element refers to the constitution of its nucleus and the arrangement of the electrons around it. Primarily, the diminutive structure of matter is fabricated up of protons, electrons and neutrons.

The protons and neutrons make upwards the nucleus of the cantlet, which is surrounded past the electrons belonging to the atom. The atomic number of an element describes the full number of protons in its nucleus.

Neutral atoms have equal numbers of protons and electrons. Notwithstanding, atoms may gain or lose electrons in order to increase their stability and the resulting charged entity is called an ion.

Atoms of different elements take different atomic structures because they contain different numbers of protons and electrons. This is the reason for the unique characteristics of dissimilar elements.

Atomic Models

In the 18th and 19th centuries, many scientists attempted to explain the construction of the atom with the assist of diminutive models. Each of these models had their own claim and demerits and were pivotal to the development of the modern atomic model. The most notable contributions to the field were by the scientists such as John Dalton, J.J. Thomson, Ernest Rutherford and Niels Bohr. Their ideas on the structure of the cantlet are discussed in this subsection.

Dalton's Atomic Theory

The English language pharmacist John Dalton suggested that all matter is made up of atoms, which were indivisible and indestructible. He also stated that all the atoms of an element were exactly the same, but the atoms of different elements differ in size and mass.

Chemical reactions, according to Dalton'south atomic theory, involve a rearrangement of atoms to class products. According to the postulates proposed by Dalton, the atomic structure comprised atoms, the smallest particle responsible for the chemical reactions to occur.

The following are the postulates of his theory:

• Every matter is made up of atoms.
• Atoms are indivisible.
• Specific elements have only one blazon of atoms in them.
• Each atom has its own constant mass that varies from element to element.
• Atoms undergo rearrangement during a chemical reaction.
• Atoms can neither be created nor exist destroyed but can be transformed from 1 form to another.

Dalton's atomic theory successfully explained the Laws of chemical reactions, namely, the Law of conservation of mass, Law of constant properties, Law of multiple proportions and Law of reciprocal proportions.

Demerits of Dalton'due south Diminutive Theory

• The theory was unable to explain the existence of isotopes.
• Nothing almost the structure of atom was accordingly explained.
• After, the scientists discovered particles within the cantlet that proved, the atoms are divisible.

The discovery of particles inside atoms led to a meliorate understanding of chemical species, these particles inside the atoms are chosen subatomic particles. The discovery of diverse subatomic particles is as follows:

Thomson Diminutive Model

The English chemist Sir Joseph John Thomson put forth his model describing the atomic structure in the early on 1900s.

He was later awarded the Nobel prize for the discovery of "electrons". His work is based on an experiment chosen cathode ray experiment. The construction of working of the experiment is every bit follows:

Cathode Ray Experiment

It has a tube made of glass which has two openings, one for the vacuum pump and the other for the inlet through which a gas is pumped in.

The part of the vacuum pump is to maintain "fractional vacuum" inside the glass chamber. A high voltage ability supply is connected using electrodes i.e. cathode and Anode is fitted inside the glass tube.

Observations:

• When a loftier voltage power supply is switched on, there were rays emerging from the cathode towards the anode. This was confirmed by the 'Fluorescent spots' on the ZnS screen used. These rays were called "Cathode Rays".
• When an external electric field is applied, the cathode rays get deflected towards the positive electrode, simply in the absence of electrical field, they travel in a straight line.
• When rotor Blades are placed in the path of the cathode rays, they seem to rotate. This proves that the cathode rays are made upward of particles of a certain mass, and then that they have some energy.

• With all this prove, Thompson ended that cathode rays are made of negatively charged particles called "electrons".
• On applying the electrical and magnetic field upon the cathode rays (electrons), Thomson found the charge to mass ratio (e/one thousand) of electrons. (eastward/yard) for electron: 17588 × x¹¹ e/bg.

From this ratio, the charge of the electron was found by Mullikin through oil drop experiment. [Charge of e^– = one.6 × 10^-xvi C and Mass of e^– = 9.1093 × 10^-31 kg].

Conclusions:

Based on conclusions from his cathode ray experiment, Thomson described the diminutive structure as a positively charged sphere into which negatively charged electrons were embedded.

It is ordinarily referred to as the "plum pudding model" because it tin can be visualized as a plum pudding dish where the pudding describes the positively charged cantlet and the plum pieces describe the electrons.

Thomson's diminutive construction described atoms equally electrically neutral, i.e. the positive and the negative charges were of equal magnitude.

Limitations of Thomson's Atomic Structure:Thomson's atomic model does not clearly explain the stability of an cantlet. Also, farther discoveries of other subatomic particles, couldn't be placed inside his atomic model.

Rutherford Diminutive Theory

Rutherford, a educatee of J. J. Thomson modified the atomic construction with the discovery of some other subatomic particle chosen "Nucleus". His atomic model is based on the Blastoff ray scattering experiment.

Blastoff Ray Handful Experiment

Construction:

• A very thin gilt foil of 1000 atoms thick is taken.
• Alpha rays (doubly charged Helium Heⁱⁱ⁺) were made to bombard the gold foil.
• Zn Due south screen is placed backside the gold foil.

Observations:

• Most of the rays only went through the gold foil making scintillations (bright spots) in the ZnS screen.
• A few rays got reflected after striking the gilt foil.
• One in 1000 rays got reflected past an bending of 180° (retraced path) after hit the golden foil.

Conclusions:

• Since near rays passed through, Rutherford ended that most of the space inside the atom is empty.
• Few rays got reflected considering of the repulsion of its positive with some other positive charge within the cantlet.
• 1/1000th of rays got strongly deflected because of a very strong positive accuse in the center of the atom. He called this strong positive charge as "nucleus".
• He said most of the accuse and mass of the cantlet resides in the Nucleus

Rutherford'southward Structure of Atom

Based on the higher up observations and conclusions, Rutherford proposed his own atomic structure which is as follows.

• The nucleus is at the center of an atom, where most of the accuse and mass are concentrated.
• Atomic structure is spherical.
• Electrons revolve around the nucleus in a circular orbit, similar to the mode planets orbit the sun.

Limitations of Rutherford Atomic Model

• If electrons have to circumduct around the nucleus, they will spend energy and that too confronting the strong force of allure from the nucleus, a lot of energy will be spent past the electrons and eventually, they will lose all their energy and will autumn into the nucleus so the stability of atom is not explained.
• If electrons continuously circumduct effectually the 'nucleus, the blazon of spectrum expected is a continuous spectrum. But in reality, what we see is a line spectrum.

Subatomic Particles

Protons

• Protons are positively charged subatomic particles. The charge of a proton is 1e, which corresponds to approximately 1.602 × ten^-19
• The mass of a proton is approximately 1.672 × 10^-24
• Protons are over 1800 times heavier than electrons.
• The total number of protons in the atoms of an element is always equal to the atomic number of the chemical element.

Neutrons

• The mass of a neutron is most the same as that of a proton i.e. ane.674×10^-24
• Neutrons are electrically neutral particles and carry no charge.
• Different isotopes of an element have the aforementioned number of protons but vary in the number of neutrons present in their respective nuclei.

Electrons

• The charge of an electron is -1e, which approximates to -1.602 × ten^-nineteen
• The mass of an electron is approximately 9.i × 10^-31.
• Due to the relatively negligible mass of electrons, they are ignored when calculating the mass of an atom.

Atomic Structure of Isotopes

Nucleons are the components of the nucleus of an atom. A nucleon can either be a proton or a neutron. Each element has a unique number of protons in it, which is described by its unique atomic number. However, several atomic structures of an chemical element can exist, which differ in the total number of nucleons.

These variants of elements having a unlike nucleon number (also known every bit the mass number) are called isotopes of the element. Therefore, the isotopes of an chemical element take the same number of protons but differ in the number of neutrons.

The atomic construction of an isotope is described with the help of the chemical symbol of the element, the atomic number of the element, and the mass number of the isotope. For example, at that place exist iii known naturally occurring isotopes of hydrogen, namely, protium, deuterium, and tritium. The atomic structures of these hydrogen isotopes are illustrated below.

The isotopes of an element vary in stability. The half-lives of isotopes also differ. Notwithstanding, they mostly take similar chemic behavior owing to the fact that they hold the same electronic structures.

Atomic Structures of Some Elements

The structure of atom of an element can exist just represented via the total number of protons, electrons, and neutrons nowadays in information technology. The atomic structures of a few elements are illustrated below.

Hydrogen

The well-nigh abundant isotope of hydrogen on the planet Earth is protium. The diminutive number and the mass number of this isotope are 1 and 1, respectively.

Construction of Hydrogen atom: This implies that it contains one proton, one electron, and no neutrons ( total number of neutrons = mass number – diminutive number)

Carbon

Carbon has two stable isotopes – 12C and 13C. Of these isotopes, 12C has an abundance of 98.9%. Information technology contains 6 protons, 6 electrons, and 6 neutrons.

Construction of Carbon atom: The electrons are distributed into two shells and the outermost crush (valence crush) has iv electrons. The tetravalency of carbon enables it to grade a variety of chemical bonds with various elements.

Oxygen

There be three stable isotopes of oxygen – 18O, 17O, and 16O. Nevertheless, oxygen-xvi is the near arable isotope.

Structure of Oxygen atom: Since the atomic number of this isotope is viii and the mass number is sixteen, it consists of 8 protons and eight neutrons. 6 out of the 8 electrons in an oxygen atom lie in the valence shell.

Bohr's Atomic Theory

Neils Bohr put forth his model of the atom in the year 1915. This is the about widely used atomic model to describe the atomic structure of an element which is based on Planck'south theory of quantization.

Postulates:

• The electrons inside atoms are placed in discrete orbits called "stationery orbits".
• The energy levels of these shells tin be represented via quantum numbers.
• Electrons can jump to higher levels by absorbing energy and move to lower energy levels by losing or emitting its energy.
• Equally longs as, an electron stays in its own stationery, there will exist no absorption or emission of energy.
• Electrons revolve around the nucleus in these stationery orbits only.
• The energy of the stationary orbits is quantized.

Limitations of Bohr's Atomic Theory:

• Bohr's diminutive structure works only for unmarried electron species such every bit H, He+, Li2+, Be3+, ….
• When the emission spectrum of hydrogen was observed under a more than authentic spectrometer, each line spectrum was seen to exist a combination of no of smaller discrete lines.
• Both Stark and Zeeman effects couldn't be explain using Bohr's theory.

Heisenberg's uncertainty principle: Heisenberg stated that no two conjugate physical quantities can exist measured simultaneously with 100% accurateness. These will always be some mistake or uncertainty in the measurement.

Drawback: Position and momentum are two such conjugate quantities that were measured accurately past Bohr (theoretically).

Stark result: Phenomenon of deflection of electrons in the presence of an electric field.

Zeeman effect: Phenomenon of deflection of electrons in the presence of a magnetic field.

Dual Nature of Affair

The electrons which were treated to be particles, the evidence of photoelectric effect shows they likewise accept wave nature. This was proved past Thomas young with the help of his double slit experiment.

De-Broglie concluded that since nature is symmetrical, so should exist calorie-free or any other matter moving ridge.

Quantum Numbers

• Principal Breakthrough number (northward): It denotes the orbital number or beat number of electron.
• Azimuthal Quantum numbers ( l ): It denotes the orbital (sub-orbit) of the electron.
• Magnetic Quantum number: It denotes the number of energy states in each orbit.
• Spin Quantum number(southward): It denotes the direction of spin, Southward = -½ = Anticlockwise and ½ = Clockwise.

Electronic Configuration of an Atom

The electrons have to be filled in the due south, p, d, f in accord with the following dominion.

ane. Aufbau's principle: The filling of electrons should take place in accordance with the ascending order of energy of orbitals:

• Lower free energy orbital should be filled first and higher energy levels.
• The energy of orbital α(p + fifty) value information technology ii orbitals have same (n + l) value, East α due north
• Ascending order of energy 1s, 2s, 2p, 3s, 3p, 4s, 3d, . . .

2. Pauli's exclusion principle: No 2 electrons tin can have all the four breakthrough numbers to be the aforementioned or if 2 electrons have to be placed in an energy land they should be placed with reverse spies.

3. Hund'southward rule of maximum multiplicity: In the case of filling degenerate (same free energy) orbitals, all the degenerate orbitals accept to exist singly filled first and then just pairing has to happen.

Frequently Asked Questions On Atomic Structure

What are subatomic particles?

Subatomic particles are the particles that found an atom. Generally, this term refers to protons, electrons, and neutrons.

How do the atomic structures of isotopes vary?

They vary in terms of the full number of neutrons present in the nucleus of the atom, which is described past their nucleon numbers.

What are the shortcomings of Bohr's atomic model?

Co-ordinate to this diminutive model, the structure of an atom offers poor spectral predictions for larger atoms. Information technology also failed to explain the Zeeman result. It could only successfully explicate the hydrogen spectrum.

How can the full number of neutrons in the nucleus of a given isotope be adamant?

The mass number of an isotope is given by the sum of the total number of protons and neutrons in it. The diminutive number describes the total number of protons in the nucleus. Therefore, the number of neutrons can be determined by subtracting the atomic number from the mass number.

Atomic Construction Solved Problems and Solutions

Atomic Structure – Of import Questions

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