Chapter 1: Periodic table (C8038327)

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1 Electronic structure

An atom consists of protons, neutrons and electrons. Protons and neutrons are called nucleons, as they are found in the nucleus. Nucleons are held together by the strong nuclear force, which is the energy that causes the masses of the constituents to be greater than the combined mass, as mass is converted to energy per [mathjax]E=mc^2[/mathjax]. The neutron is neutrally charged, the proton is positively charged, and the electron is negatively charged. The charge on the proton is equal and opposite to the charge on the electron, and is 1 eV, which is [mathjax]1.6 \times 10^-19 C[/mathjax]. The atom is mostly space, as the nucleus is a very dense area in the center, and the electrons orbit from a very far distance. An atom is only electrically neutral if it has the same number of protons and electrons; otherwise, it is known as an ion.

Frequently asked questions
What is an atom?
The smallest division of an element, such that it is no longer that same element if divided further. It is made of electrons orbiting far away from the nucleus.

What is an element?
A substance containing the same type of atom. Elements can be found on the periodic table.

What's the nucleus?
The dense center in the middle of the atom. It contains neutrons and protons.

What's the difference between electrons, neutrons, and protons?
Electrons are negatively charged, neutrons are neutral, and protons are positively charged.

What is an ion?
When an atom is electrically charged. This occurs when there is an imbalance between protons and electrons, causing a net charge.

An atom is the smallest division of an element, such that it cannot be the same element if further divided. An element is a substance containing any same type of atom. Elements are represented by a chemical symbol per the periodic table, which have an according number of protons, known as the atomic number (Z). Elements may sometimes be expressed with a number, which is its mass number (A), which represents the number of nucleons present. Isotopes are atoms with the same atomic number (thus same number of protons), but different mass numbers, indicating there is a difference in numbers of neutrons. For example, carbon naturally occurs as carbon-12, -13 and -14, and though all have 6 protons, carbon-12 has 6 neutrons, carbon-13 has 7 neutrons, and carbon-14 has 8 neutrons.

Mass number is measured by atomic mass unit (AMU), which is defined as [mathjax]1/12[/mathjax] of the mass of carbon-12, which is [mathjax]1.66 \times 10^-27 kg[/mathjax]. It is notable that [mathjax]1 AMU[/mathjax] is less than the mass of a proton (carbon-12 has 12 protons, and no neutrons, and with electrons too, the constituent mass should be greater than the mass of a proton), because when adjoined, mass is turned into binding energy in accordance with [mathjax]E=mc^2[/mathjax] (from ). Note also, that a neutron is slightly heavier than a proton. Nevertheless, [mathjax]1 AMU[/mathjax] is approximately the mass of a nucleon. Notice that despite the definition of AMU comes from carbon-12, the mass number of carbon enlisted on the periodic table is [mathjax]12.011 AMU[/mathjax] and not [mathjax]12[/mathjax], because it is a weighted average of the mass of its natural isotopes.

Frequently asked questions
On the periodic table, what do the 2 numbers shown mean?
The subscript is the atomic number, and the superscript is the mass number.

What's the difference between the atomic and mass number?
Atomic number describes how many protons are present, and mass number describes how many nucleons (i.e. protons AND neutrons) are present. Thus, the mass number is always greater than the atomic number.

What is mass number measured by? Kilograms?
No. It is measured by 1/12th the mass of carbon-12. That's [mathjax]1.66 \times 10^-27 kg[/mathjax].

Oh, so it's not even measured by the weight of hydrogen?
No. 1/12th the mass of carbon-12. Hydrogen actually weighs 1.008 AMU. It's because when adjoined, due to [mathjax]E=mc^2[/mathjax], mass is converted into binding energy. So that's why when alone, hydrogen is slightly heavier.

But why isn't the mass of carbon 12 AMU? Why is it 12.011?
Because its a weighted average of the mass of its natural isotopes.

Are neutrons and protons the same weight?
Almost. Neutrons are slightly heavier.

What are isotopes?
Atoms of the same element, but of different weight.

Electron configuration is the enlisting of the electrons, from the lowest to highest [energy] orbitals. This method works due to the Aufbau principle, which states that an atom is built up progressively by adding electrons, first filling orbitals with lower energy states, before higher ones. For example, Na is [mathjax]1s^2 2s^2 2p^6 3s^1[/mathjax], and [mathjax]\ce{Na+}[/mathjax] is [mathjax]1s^2 2s^2 2p^6[/mathjax]. Note that [mathjax]\ce{Na+}[/mathjax] has the same configuration as the noble gas [mathjax]Ne[/mathjax], which is unsurprising because noble gases are most stable. Cations lose electrons from the highest principal quantum number. For example, the transition metal [mathjax]Fe[/mathjax] is [mathjax]1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^6[/mathjax], and [mathjax]\ce{Fe^{2+}}[/mathjax] loses from the 4s subshell and not the 3d, making [mathjax]\ce{Fe^{2+}}[/mathjax] therefore [mathjax]1s^2 2s^2 2p^6 3s^2 3p^6 3d^6[/mathjax].

Degenerate orbitals are quantum states that are all at the same energy level. For example, for some first row transition metal, the 4s and 3d subshells are degenerate, meaning empty 3d orbitals may be filled before filling the 4s orbital, because both are at the same energy level, and are thus as equally probable of being filled.

“Although all Christians are baptized and indwell in the Holy Spirit (Romans 8:9-10), not all Christians enjoy the fullness of the Spirit, known as ‘filled’ with the Holy Spirit. In 1 Corinthians 3:1, Paul distinguishes between ‘Spiritual men’, and those who are like the natural man who lives under flesh. Paul confirms in 3:10-15 that these people enter Heaven, but their work is burned up as worthless like combusted wood, hay or straw.”

“To be filled means to be controlled and empowered by the Holy Spirit, analogous to saying someone is ‘filled’ with rage or jealousy, or even drunk with wine, which dominates, motivates, determines behavior, and has them under control,” Mandy commented, “essentially, it is to be fully surrendered, committed, and yielded to God as living sacrifices, both body and mind (Romans 12:1-2).”

“This is not just a one-time altar call decision, but a lifetime surrender,” Blaire remarked, “The problem with a ‘living surrender’ is that it tends to crawl off the altar, so it must be continuously re-presented every day. This is called ‘walking in the Spirit’ (Galatians 5:16,25).”

Though it may be true that the Holy Spirit can walk you into failure and hardship, you will be strengthen as you go through, as you are where God wants you to be. You know that it is your 'Best Life Now', because it's what God wants to do, and it's perfect, and you will experience the goodness of willing and joyful submission.

“You just lost half your audience quoting Joel Osteen ,” Blaire giggled.

“In the absence of being sold out, we are reliant on self-effort, which Jesus actually notes as impossible (John 15:4-5), and what is actually left, is legalism, which is self-defeating,” Mandy noted.

Frequently asked questions
What is electron configuration?
It's a listing of the electrons from lowest to highest energy orbitals. It works because of the Aufbau principle.

What's the Aufbau principle?
Atoms are built up progressively, filling orbitals with lower energy states, before higher [energy states].

Why is the 4s filled before the 3d subshell?
Because 4s has a lower energy state than 3d.

Ions can have the same electron configuration as another element?
Yes, because you're considering electrons. Not protons. So for example, Na+ has the same electron configuration as Ne. That's because Na+ loses an electron, thus becoming Ne.

What subshell do electrons from ions come off from?
The highest principle quantum number. So even though 3d may be a higher energy state than 4s, it comes off the 4s, because "4" is a higher principle quantum number than "3".

What are degenerate orbitals?
Quantum states at the same energy level. For example, [high energy] 3d orbitals may be filled before filling the [lower energy] 4s orbitals.

Quantum numbers are a set of four discrete numbers assigned to each electron. They are unique, and describe the location and energy of the electron. The four discrete numbers include:

  • Principal quantum number, which designates the shell, representing the energy level. The value can be any positive integer (1, 2, 3, ...). The quantum number of the last added electron, is the respective element’s period number, with the exception of transition metal, which is one number less than the period. For example, the principal quantum number of the last electron added for hydrogen is 1, for sodium is 3, and for iron is 3 (despite it is in the 4th period)
  • Azimuthal quantum number, which designates the subshell. The value can be 0 to 1 less than the principal quantum number, or alternatively, s, p, d, f, g, ...
  • Magnetic quantum number, which designates the orbitals within the subshell. The value can range from the negative value of the azimuthal quantum number, to the positive value of the azimuthal quantum number. For example, for the d subshell (value 2), the magnetic quantum numbers can be either -2, -1, 0, 1, or 2. This infers there are 5 orbitals in the d subshell. For the others, these are 1 orbital in the s subshell, 3 orbitals in the p subshell, 7 orbitals in the f subshell, and 9 orbitals in the g subshell. As each orbital can inhabit 2 electrons, the electrons in the respective subshells are 2 electrons in the s subshell, 6 electrons in the p subshell, 10 electrons in the d subshell, 14 electrons in the f subshell, and 18 electrons in the g subshell. This explains why the shells () have 2 electrons in the s subshell, 8 electrons in the p subshell (2+6), 18 electrons in the d subshell (2+6+10), 32 electrons in the f subshell (2+6+10+14), and 50 electrons in the g subshell (2+6+10+14+18)
  • Spin quantum number, which designates the spin of the electron. As two spins are possible in each orbital, the values can be either [mathjax]-\dfrac{1}{2}[/mathjax] or [mathjax]\dfrac{1}{2}[/mathjax]

No two electrons can have the same four quantum numbers, known as the Pauli exclusion principle.

Heisenberg’s uncertainty principle is that both the position and momentum of an electron cannot be simultaneously known. More that is known about one, will reduce the precision of the other, and vice versa.

Hund’s rule is that electrons will add to empty orbitals (despite partially filled orbitals with the same energy are available), before adding to partially filled orbitals at the same energy level.

Frequently asked questions
What are quantum numbers?
They uniquely describe each electron. They are a set of 4 numbers. You can think of them like a GPS coordinate.

What are these 4 quantum numbers?
They designate the shell, subshell, orbital, and spin of the electron.

What is the shell?
It is a positive integer (1, 2, 3, ...), and is the last added electron's periodic number. That's except transition metals, which is 1 number less than its period.

What is the subshell?
It is designated as s, p, d, g.

What is the orbital?
It ranges from the minus to plus value of the subshell number.

What is the spin quantum number?
It is either -1/2 or 1/2.

What is Heisenberg's uncertainty principle?
That you can't know both the position and momentum of an electron. You either know one, or the other; but not both.

What is Hund's rule?
Electrons will add to an empty orbital, rather than a partially filled orbital, if the orbitals are at the same energy level.

Quantization is the principle that electromagnetic energy is absorbed and emitted in discrete (cf. continuum) packets, known as photons. The energy of the photon is given by [mathjax]E=h.f[/mathjax], where [mathjax]h[/mathjax] is Planck’s constant, [mathjax]6.6\times 10^-34 J.s[/mathjax].

Photoelectric effect demonstrates the particle nature of light. As established , electrons only exist at discrete energy levels away from the nucleus, such that energy levels further away have higher energies. Thus, photons are emitted when an electron drops energy levels, and photons are absorbed to increase energy levels. Emitted photons have a frequency that is proportional to the drop in energy level, and absorbed photons require a frequency related to the increase in energy level, or will reflect off the electron and not cause any change, in accordance with the all or nothing principle. The photoelectric effect works by shining a light on a metal, such that below a certain frequency, the light will not cause current. If light were a wave, increasing the intensity of the frequency should cause current, but it doesn’t, because light is [simultaneously] a photon particle. As a particle, it collides with one-on-one with electrons, so increasing intensity will increase the number of photons, but not increase the frequency [and hence energy] required to absorb the photon [and excite the electron for release, thereby causing current].

Frequently asked questions
What is quantization?
Electromagnetic energy isn't absorbed and emitted on a continuum. It occurs via discrete packets, known as photons.

Why isn't it continuous?
Photons are emitted when electrons drop energy levels, or are absorbed when electrons increase energy levels. Because the energy levels are discrete, the photons emitted/absorbed are also discrete.

What's the photoelectric effect?
An experiment which proves the particle nature of light. It works by shining a light on a metal.

How exactly does it show light is a particle?
If light were a wave, there should always be current [no matter how small]. However, in the experiment, below a certain frequency, the light will not cause a current. That's because there is insufficient energy to cause the electron to increase energy levels. So nothing happens.

2 Chemical properties

Periodic table is separated into rows known as periods, and columns known as groups. Elements within a group are chemically similar. Groups are numbered from 1-18, with group 1 known as alkali metals, group 2 known as alkaline earth metals, group 17 known as halogens, and group 18 known as the noble gases.

Frequently asked questions
What is the periodic table?
The presentation of all known elements, into periods and groups.

What's the difference between a period and group?
Elements in the same period are in the same row. Elements in the same group are in the same column.

What's the significance of a group?
Groups are numbered 1-18. Elements in the same group (i.e. column) are chemically similar.

Hence, certain groups have special names. Important to know are the first 2 (alkali metals, and alkaline earth metals), and the last 2 (halogens, and noble gases).

Notable periodic trends include:

  • Electronegativity, which is the ability of an atom to attract electrons that is in a covalent bond (discussed )
  • Electron affinity, which is the energy released to add electrons to a lone atom. Creating bonds releases energy
  • Energy of ionization, which is the energy required to remove electrons from a lone atom. Breaking bonds requires energy. Note also that ionization energy increases as more electrons are removed, particularly if these electrons come from an inner shell
  • Atomic radius, which relates to the size of the atom
  • Metallic character, which relates to conductivity of heat and electricity, ability to be stretched without breaking known as ductility, and ability to be compressed without breaking known as malleability

These trends alter according to the mnemonic that anything which begins with “e” increases to the right and going up. [Conversely, if going left and down, these characters decrease.] In contrast, anything which doesn’t begin with an “e” trend in the opposite direction.

Frequently asked questions
What are periodic trends?
Trends that either decrease or increase, based upon whether they are more towards the upper right [or the lower left].

The ones which increase towards the upper right, include electronegativity, electron affinity, and energy of ionization.

The ones which increase towards the lower left, include atomic radius, and metallic character.

What is electronegativity?
Electronegativity is the ability of an atom to attract electrons [into a covalent bond].

How does electron affinity differ from electronegativity? How about energy of ionization?
They're all very similar.

Electron affinity is the ability of an atom to add electrons [to a lone atom].

Energy of ionization is the ability of an atom to remove electrons [from a lone atom].

What is atomic radius?
It describes the size of an atom.

What is metallic character?
It describes being more like a metal. So that is its conductivity of heat and electricity, ductility, and malleability.

What's the difference between ductility and malleability?
Ductility is the ability to stretch without breaking. Malleability is the ability to compress without breaking.




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Chapter 1: Periodic table - General chemistry - Pre-med science - MR. SHUM'S CLASSROOM