Atoms [CHEMISTRY]

• An atom is the basic unit of an element that can enter into a chemical combination. Scientists are very much interested in studying atoms and atomic structures simply because understanding atoms ultimately leads to understanding why certain elements behave or react in a certain manner.
• Atomic Theory of Matter

  In fact, the idea of an atom started as early as 5th century B.C. when Democritus expressed his belief that matter is made up of smaller, indivisible particles he then called ‘atomos’, meaning indivisible. The idea had been ignored for a very long time until John Dalton formulated a precise definition of indivisible building blocks that we now call atoms in 1808. Dalton’s work laid the foundation of the modern era of chemistry
• Atomic Theory of Matter

  Hypothesis 1: All matter consists of indivisible particles called atoms. Dalton’s first hypothesis simply states that atoms are the smallest particles, and it is impossible to divide atoms even further. Of course, nowadays we know that this is not true, as many scientists have proven the existence of particles even smaller than the atom itself. We will discuss more of this in the next sections.
• Atomic Theory of Matter

  Hypothesis 2: All atoms of the same element are identical in terms of size, mass, and chemical properties. The atom of one element is different from the atom of another element. hydrogen atoms are all the same. If you obtain hydrogen atoms from different parts of the world and in outer space, all the atoms will be the same in all respects. In the same way, oxygen atoms are all the same; however, oxygen atoms are different from hydrogen atoms. hydrogen atoms behave similarly regardless of the source, but differently with respect to oxygen atoms
• Atomic Theory of Matter

  Hypothesis 3: Atoms of different elements may combine in fixed proportions to form a compound. To visualize this theory, imagine a water molecule that is chemically written as H2O. This chemical formula implies that ALL molecules of water are composed of two atoms of hydrogen and one atom of oxygen. Varying the proportion of at least one atom in the formula will give rise to a completely different compound.
• Atomic Theory of Matter

  Hypothesis 4: Chemical reactions involve reorganization of the atoms—changes in the way they are bound together. The atoms themselves are not changed in a chemical reaction. This hypothesis is another way of stating the law of conservation of mass. In other words, atoms can neither be created nor destroyed, regardless of the type of chemical reaction it undergoes. Because atoms remain unchanged in a chemical reaction, it follows that the total mass of the reactants is equal to the total mass of the products after the reaction
• Atomic Models and Components of an Atom

  During his time, Dalton’s theory of the atom as the basic unit of an element that can participate in a chemical reaction was widely accepted. Advancement in modern technology, however, led to the discovery that atoms are actually made up of even smaller particles we now know as subatomic particles. These particles include protons, neutrons, and electrons
• Atomic Models and Components of an Atom

  Among the three subatomic particles, the electron was the very first to be discovered. In 1897, Sir Joseph John Thomson discovered the electron and through his cathode ray experiment, was actually able to determine that its charge-to-mass ratio is - 1.76 x 10 8 C/g.
• Atomic Models and Components of an Atom

  Since mass can never be negative, Sir Thomson's findings clearly suggest that electrons are negatively-charged. However, for an atom to be neutral (which is usually the case), there must be an equal number of positive and negative charges. This led Sir Thomson to propose that the atom is a uniform, positive sphere with electrons embedded in it like a raisin, giving rise to his plum pudding model of the atom.
• Atomic Models and Components of an Atom

  Years later, Robert Millikan’s oil drop experiment allowed him to calculate the charge of an electron, which he found out to be - 1.6022 x 10 -19 C. Using his data and Thomson’s ratio, Millikan was able to derive the mass of a single electron, which is 9.10 x 10 -28 g!
• Atomic Models and Components of an Atom

  In Rutherford's experiment, they bombarded a very thin layer of gold with a positive ɑparticle. If Thomson’s model is correct, the diffused positive charge of the atom should have caused the ɑ particle to pass through the foil with very little deflection. Instead, Rutherford observed that the majority of the particles passed through the foil either completely undeflected or with very minimal deflection, while others deflected at a large angle, and in extreme cases, bounced back to the direction where they came from = new atomic model
• Atomic Models and Components of an Atom

  Rutherford inferred that atoms are mostly empty space. This is the reason why most of the ɑ particles passed through the foil either undeflected or slightly deflected only. As for the particles which either largely deflected or bounced back, Rutherford proposed that contrary to Thomson’s model, an atom has its positive charges concentrated in its core, which he called the nucleus.
• Atomic Models and Components of an Atom

  Rutherford: As the positive ɑ particle approaches the positively-charged nucleus, it experiences repulsion due to similar charges, causing the particle to be deflected at such a large angle. Meanwhile, ɑ particles that directly hit the nucleus are deflected back towards the direction where they came from
• Atomic Models and Components of an Atom

  Rutherford’s proposed model, however, left an unsolved problem, specifically the mass ratio of hydrogen and helium. The atomic mass of hydrogen, an element with 1 proton and 1 electron, is 1.008 g/mole. Meanwhile, helium, an element with 2 protons and 2 electrons, has an atomic mass of 4.003. Therefore, the ratio of their atomic masses is about 1:4.
• Atomic Models and Components of an Atom

  Now, if protons and electrons are the only subatomic particles, their mass ratio should be 1:2 (mass of electrons is usually omitted since it is 1840 times lighter than the proton!). In 1932, James Chadwick was able to account for this unexplained mass ratio due to his discovery of neutrons. Succeeding experiments proved that the third subatomic particle is electrically-neutral, hence the name neutron.
• Particle | Mass (g) | Charge (Coulomb) | Charge unit

  • Proton | 1.67262 x 10^-24 | 1.6022 x 10^-19 | +1
  • Neutron | 1.67493 x 10^-24 | 0 | 0
  • Electron | 9.10938 x 10^-28 | -1.6022 x 10^-19 | -1
• Properties of an Atom

  how come a lot of substances vary significantly from one another? The answer is because of the difference in the properties of the atom, which can be ultimately attributed to the number of protons and neutrons, as well as the number and distribution of electrons in the space around the nucleus.
• Properties of an Atom

  • atomic number
  • mass number
  • atomic mass
• Properties of an Atom

  The first atomic property that will be covered in this review is the atomic number (designated as Z). In the modern periodic table, an atomic number is usually written on the upper left side corner of each element block. This number represents the number of protons in the atom’s nucleus. It also gives us an idea about the reactivity of the atoms. For instance, all atoms with an atomic number that is at least 84 are radioactive.
• Properties of an Atom

  If the number of neutrons is added to the atomic number, it becomes the mass number (no. of protons + no. of neutrons). The mass number is usually designated as A and is not written in the modern periodic table of elements.
• Properties of an Atom

  Another property is the atomic mass, which is written below the element name in the periodic table. Due to huge differences between the mass of an electron, and proton and neutron, atomic masses are mostly attributed to the mass of protons and neutrons only (mass of electrons is almost negligible when compared to these two).
• Quantum Mechanics

  If the nucleus lies in the center of the atom, then where do we find the electrons? There are two prevailing models which tried to answer this question. The first one is Neils Bohr’s planetary model, wherein he proposed that electrons revolve around a positive nucleus in a predetermined orbit, just like how the planets in the solar system revolve around the sun.
• Quantum Mechanics

  Later on, it was found out that Bohr’s model is lacking in some respects. Then in 1926, Erwin Schrödinger developed his famous equation, which gave birth to the quantum mechanical model. Contrary to Bohr’s planetary model, this model explains that electrons are most probably found in a three-dimensional space around the nucleus, which is known as the orbital. Schrödinger was able to formulate this using complicated mathematical techniques which require advanced knowledge of calculus to solve
• Quantum Numbers

  it is worth noting that solutions to Schrodinger’s equation are known as quantum numbers, and to completely describe a certain electron in an atom, the four quantum numbers must be specified

  • Principal: $n$
  • Angular: $l$
  • Magnetic: $m_l$
  • Spin: $m_s$
• Quantum Numbers

  The principal quantum number refers to the main energy levels (or shells) of an orbital. The first circle corresponds to n = 1, and the electrons occupying the n = 1 principal QN can be found anywhere within the space enclosed by the said energy level. This space enclosed by n = 1 is what we call the orbital. The principal quantum number can take values from 1 to ∞, and the higher the value of n, the higher is the energy of the orbital, and the farther the electron from the nucleus | Size of the atom. Obviously, the higher the n value, the larger the atom is.

  
• Quantum Numbers

  Azimuthal quantum number, also known as the angular momentum quantum number, pertains to the energy sublevels or subshells of the orbital. This quantum number has something to do with the shape of the orbital, and can only take integral values between 0 to n-1. Different shapes of orbitals are assigned different letters and ℓ values as shown below.
• Letter Designation | Shape | l value

  • s (sharp) | spherical | 0
  • p(principal) | dumb-bell | 1
  • d (diffuse) | not easily visualized | 2
  • f (fundamental) | not easily visualized | 3
  • G | ... | 4
• Quantum Numbers

  Magnetic Quantum Number

  Whereas ℓ has something to do with the shape of the orbital, mℓ tells us about the orientation of orbitals in space. The magnetic quantum number takes the values between —ℓ to +ℓ, including zero. Therefore, for an s orbital, the only possible value of mℓ is 0, which implies that there is only 1 s orbital. Meanwhile, a p orbital can have an mℓ value of —1, 0, and +1, which implies that there are three types of p orbitals, each of which assumes a different orientation in space.
• Quantum Number

  Lastly, the spin quantum number pertains to the electron spin, which can only be either clockwise or counterclockwise. As a result, there are only two possible values of $m_s$, and these are +½ and —½. We can use these quantum numbers to assign electronic configurations on each electron in a multi-electron system. However, some rules/principles must be followed in assigning electronic configurations
• Electron Configuration

  The electron configuration is the distribution of electrons among the various orbitals in an atom, molecule, or ion.

  
• Electron Configuration

  To be able to write electron configuration correctly, it is important that you know three things:

  • the number of electrons present in a certain species
  • the number of electrons each orbital can occupy
  • the correct ordering of the orbitals.
• Electron Configuration

  As for the number of electrons that each orbital can occupy, just remember that s orbitals can occupy a maximum of 2 electrons, p orbitals can accommodate 6 electrons, d orbitals can take 10 electrons, and f orbitals can have a maximum of 14 electrons

  
• The Aufbau Principle

  The (n + ℓ) rule answers the dilemma established from the Aufbau principle. When drawing the orbital diagram, we must adhere to Hund’s rule of multiplicity, which states that every orbital in a subshell is singly occupied with one electron before any orbital is doubly occupied, and all electrons in singly occupied orbitals have the same spin. This is a direct consequence of Pauli's exclusion principle, which states that no two electrons can have the same set of four quantum numbers