Structure of atom is an important concept to understand in chemistry, since it lays the foundation for everything else that will be studied as part of the syllabus for chemistry. Plus, once the concept is understood properly, which shouldn’t be so hard, it’s very easy to score well on this subject in the entrance exams, and there’s also a high weightage for this topic, so scoring well in this subject ensures a better rank.
The Structure of an Atom
The word ‘atom’ has always been used to represent a particle that cannot be further cut into smaller particles. However, in the modern scientific terminology, the term ‘atom’ comprises of several subatomic particles. The particles constituting an atom are the electron, the proton, and the neutron. Let’s understand these particles in detail.
The electron is by far the smallest out of these three particles. At 9.11 x 10-31 kg, carrying a negative electrical charge, the electron is too small in size to be measured using the current techniques. Usually, the electrons are bound to the positively charged nucleus due to the attraction created from the opposite electric charges. If the electrons carried by an atom are more or fewer than its atomic number then the atom becomes respectively negatively or positively charged as a whole. A charged atom is known as an ion.
The protons carry a positive charge and at 1.6726 x 10-27 kg, have a mass 1836 times that of the electron. The atomic number represents the number of protons in an atom.
At 1.6929 x 10-27 kg, neutrons are the heaviest of the three constituent particles of an atom. The neutrons carry no electrical charge and have a free mass of 1839 times the mass of the electron. Neutrons and protons have comparable dimensions although the surface of these particles is not clearly defined.
The electrons are truly elementary particles with no internal structure. However, both protons and neutrons are composite particles composed of elementary particles called quarks. There are two types of quarks in atoms, each having a fractional electric charge. Protons are composed of two up quarks (each with charge +2/3) and one down quark (with a charge -1/3). Neutrons consist of one up quark and two down quarks.
The quarks are bound together by a strong force mediated by gluons, which is a member of the family of gauge bosons (elementary particles that mediate physical forces). The nuclear force binds the protons and the neutrons together in the nucleus. The nuclear force is the residuum of the strong force that has somewhat different-range properties.
All the protons and neutrons that are held together in an atom make up a tiny atomic nucleus. They are collectively called nucleons. The nucleons are bound together by a short-ranged attractive potential known as the residual strong force. The radius of the nucleus is much smaller than the radius of the atom.
What is an atom? What are atoms made of?
An atom is the smallest constituent of matter that has properties of the chemical element itself. Every form of matter, be it solid, liquid, or gas, contains atoms either neutral or charged. Every atom has a basic nucleus at the center, consisting of a certain number of protons and neutrons, depending on the element, and a certain number of electrons revolving the nucleus in fixed orbits, at fixed energy levels, with a certain quantum spin value. Most of the mass of the atom comes from the protons and neutrons themselves, whereas electrons are almost 1/1837th times the weight of a proton or neutron. Protons and neutrons are both composed of other particles called quarks and gluons. Protons contain two ‘up’ quarks and one ‘down’ quark while neutrons contain one ‘up’ quark and two ‘down’ quarks. The gluons are responsible for binding the quarks to one another. The atomic number is the number of protons (equal to the number of electrons in a neutral atom) in the atom and the atomic mass number is the sum of the number of protons and neutrons in the atom. An atom may gain a positive or negative charge by either losing or gaining electrons respectively. Atoms may attach themselves to each other (of the same type or different type) to form molecules of different compounds, to form matter.
ISOTOPES: DIFFERENT TYPES OF ATOMS
Isotopes are atoms in a chemical element having different numbers of neutrons than protons and electrons. The atoms in a particular element have the same number of protons and electrons but can carry varying numbers of neutrons.
Let’s understand the above with the example of the Hydrogen atom.
Hydrogen’s atomic number is 1 i.e. its nucleus contains 1 proton. It also has one electron. Therefore, the Hydrogen atom is neutral since it contains the same number of protons and electrons (as the positive and negative charges cancel each other out).
However, approximately, one hydrogen atom out of 6000 contains a neutron in its nucleus. These atoms are still hydrogen because they have one proton and one electron; they simply have a neutron that most hydrogen atoms do not carry. Hence, these atoms are called Isotopes.
There’s also an isotope of hydrogen that contains two neutrons. It’s called Tritium, it doesn’t occur naturally on earth, but it can easily be created.
Early Theories about Structure of Atom:
John Dalton first found out the existence of the atom as a fundamental particle, Robert Brown discovered that atoms exhibit a particular random zig-zag motion, JJ Thompson proved the existence of the electron by measuring the mass of cathode rays, Ernest Rutherford discovered the nucleus through the gold foil experiment, following which James Chadwick discovered the neutron. These were some of the earliest speculations about the structure of atom.
Dalton’s Atomic Theory:
Postulates of Dalton’s atomic theory:
- All matter is made of very tiny particles called atoms.
- Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
- Atoms of a given element are identical in mass and chemical properties.
- Atoms of different elements have different masses and chemical properties.
- Atoms combine in the ratio of small whole numbers to form compounds.
- The relative number and kinds of atoms are constant in a given compound.
Bohr’s Model of Structure of Atom:
Neils Bohr proposed the Bohr’s model of structure of atom, wherein he assumed the electrons to orbit around the nucleus in a fixed set of orbits, and could change orbits by absorbing or emitting a photon in the form of electromagnetic radiation. Following are the postulates of the Bohr model:-
1. The nucleus in the atom is orbited by electrons.
2. Electrons can only orbit stably, without radiating, in certain orbits at certain discrete sets of distances from the nucleus. These orbits have definite energies and are also called energy shells or levels. In these orbits, the electron’s acceleration does not result in radiation and energy loss as required by classical electromagnetics.
3. Electrons can only gain and lose energy by jumping from one allowed orbit to another, absorbing or emitting electromagnetic radiation with a frequency ν determined by the energy difference of the levels according to the Planck relation:
ΔE = E2 − E1 = hν
where h is Planck’s constant.
The frequency of the radiation emitted at an orbit of period T is as it would be in classical mechanics; it is the reciprocal of the classical orbit period.
L = nh/2π = nℏ
where n = 1, 2, 3, … is called the principal quantum number, and ħ = h/2π. The lowest value of n is 1. Once an electron is in this lowest orbit, it can get no closer to the proton. Starting from the angular momentum quantum rule, Bohr was able to calculate the energies of the allowed orbits of the hydrogen atom and other hydrogen-like atoms and ions.
The Rydberg formula is seen in Bohr’s model of structure of atom by describing the energies of quantum jumps or transitions between one orbital energy level to the next. It gives the numerical value of the already-known and measured Rydberg’s constant, albeit in terms of more fundamental constants of nature, including the charge of the electron and Planck’s constant.
E = Ei − Ef = RE ( 1/(nf^2) − 1/(ni^2))
where nf is the final energy level, and ni is the initial energy level. Bohr derived the Rydberg constant, as well as the concurring of Bohr’s formula with experimentally observed spectral lines of the Lyman (nf =1), Balmer (nf =2), and Paschen (nf =3) series.
Failure of Bohr’s Model:
Some of the shortcomings of Bohr’s model is as follows:-
1. It failed to explain much of the larger atom’s spectra. It makes speculations about the K-alpha and L-alpha X-ray emission spectra for larger atoms. Emission spectra for atoms with a single outer-shell electron may also be speculated. These techniques actually make use of Bohr’s Newtonian energy-potential viewpoint of the atom.
2. It fails to explain the Zeeman effect – the changes in spectral lines due to external magnetic fields, also due to complicated quantum principles interacting with magnetic fields and electron spin.
3. The presence of fine and hyperfine structures in spectral lines, which are because of a variety of relativistic and subtle effects, as well as electron spin complications.
Properties of atoms and molecules:
One of the most important characteristic of an atom is its atomic number (usually denoted by the letter Z), which is defined as the number of units of positive charge (protons) in the nucleus. It is the number of protons in the nucleus that determines the chemical properties of an atom.
Atomic mass of an atom is the sum of the mass of protons and neutrons. The mass number is denoted by A.
Normally an atom is electrically neutral. But it can gain or lose electrons. Those atoms that gain or lose electrons are called ions. They have an electric charge. Atoms that lose electrons become positive ions and the ones that gain electrons become negative ions.
In some atoms, the nucleus can change naturally. Such an atom is radioactive. In nature, there are some elements that are radioactive, like uranium or radium. In labs, scientists can produce radioactivity by bombarding atoms with smaller particles.
Relative atomic mass:
Relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to 1/12th the mass of one carbon-12 atom.
Formula unit mass:
The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound.
The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. It is therefore the relative mass of molecule expressed in atomic mass units (u).
This was a basic introduction into the structure of atom, its properties and a preliminary insight into Bohr’s atomic model. Understanding the basic concepts will allow you to solve numericals relating to this subject and this is imperative as most questions from this subject will be of the numerical type. Understanding the concept well will help you score much-needed points in this subject easily.