Atomic Structure and Properties

Atomic Theory and Structure

• Atoms are the smallest unit of matter that retains an element's chemical properties.
• Molecules are formed when atoms combine, interacting as solids, liquids, or gases.
• Water molecules are composed of hydrogen and oxygen atoms.
• Biological processes break down molecules into atoms to reassemble them into more useful molecules.

Subatomic Properties

• Atoms consist of protons, electrons, and neutrons.
• Protons carry a positive charge.
• Electrons carry a negative charge.
• Neutrons carry no charge.
• The nucleus is the atom's center and contains protons and neutrons.
• Electron shells, which contain electrons, form the outermost regions of the atom.

Atomic Mass

• The single most important characteristic of the atom is its atomic number, denoted by the letter Z.
• The atomic number is defined by the number of units of positive charge (protons) in the nucleus.
• The arrangement and number of basic particles give atoms different properties.
• Protons and neutrons have approximately the same mass, about 1.67 x 10^-24 grams.
• Atomic mass unit (amu) or Dalton defines this amount of mass which scientists use.
• Protons are positively charged, while neutrons have no charge, but are similar in mass.
• The number of neutrons contributes significantly to an atom's mass but not its charge.
• Electrons are much smaller in mass than protons, at only 9.11 × 10^-28 grams, or 1/1800 of an atomic mass unit.
• Electrons contribute minimally to an element's overall atomic mass.
• Electrons contribute greatly to the atom's charge, having a negative charge equal to a proton's positive charge.
• Scientists define the charges of electrons and protons as respectively "+1" and "-1.”
• In an uncharged, neutral atom, the number of orbiting electrons equals the number of protons inside the nucleus.
• In neutral atoms, the positive and negative charges cancel each other out leading to an overall net charge of zero.
• A hydrogen atom (H) contains only one proton, one electron, and no neutrons.
• Atomic number and mass number can determine the contents of an element.
• Protons have a charge of +1, a mass of 1 amu, and are located in the nucleus.
• Neutrons have a charge of 0, a mass of 1 amu, and are located in the nucleus.
• Electrons have a charge of -1, a mass of 0 amu, and are located in the orbitals.

The Periodic Table

• This table represents chemical elements by their symbols.
• The table includes these elements
• Hydrogen (H.)
• Helium (He)
• Lithium (Li)
• Elements are arranged in increasing order by:
• Atomic number (number of protons in the nucleus).
• Pattern of electron arrangement around the nucleus (electron configurations).
• Usual chemical properties.
• For elements, the symbol is simply its first letter.
• Hydrogen = H
• Sulfur = S
• Carbon = C
• Element symbols use the first letter plus one other letter of the element's name for other elements.
• The first letter is CAPITALIZED, and the second letter is not.
• Aluminum = Al
• Platinum = Pt
• Cadmium = Cd
• Some elements have symbols that refer to the element's name in Latin.
• Gold = Au
• Lead = Pb
• Copper = Cu
• Dimitri Mendeleev was the first scientist to notice the relationship between the elements.
• Mendeleev arranged the periodic table by atomic mass.
• He predicted the properties of unknown elements from the properties of known elements.
• For example, he predicted properties of aluminum (Al).
• The periodic nature of elements is based on atomic number, not atomic mass.
• Periodic refers to patterns.
• The periodic table has 18 vertical columns called GROUPS, numbered 1 to 18 with a unique configuration.
• The periodic table has seven horizontal rows called PERIODS, numbered from 1 to 7.
• 114 elements are known today.
• 90 elements are naturally occurring and the remaining have been artificially synthesized.
• Man-made elements are often applied to transuranic elements (elements listed after uranium).
• The periodic table lists the atomic number and the element's relative atomic mass, the weighted average for its naturally occurring isotopes on earth.
• Hydrogen's symbol, H, name, and the atomic number of one is in the upper left-hand corner along with its relative atomic mass of 1.01.

Reactivity and Electron Configuration

• The number and spatial distribution of electrons determines chemical reactivity between elements.
• Reacting atoms and chemical bond formation form a molecule or compound if two atoms have complementary electron patterns.
• Organization of elements in the periodic table reflects their number and pattern of electrons, predicting reactivity.

Shells and Orbitals

• In 1913, Niels Bohr developed an early model of the atom.
• The Bohr model depicts the atom with a central nucleus containing protons and neutrons.
• In the Bohr model, electrons are in circular electron shells at defined distances from the nucleus, similar to planets orbiting the sun.
• Each electron shell has its own energy level, with shells closest to the nucleus having lower energy.
• Electron configuration is the arrangement of electrons in an atom.
• Principal energy levels or principal quantum shells describe electron arrangement around the nucleus.
• Energy levels or quantum shells are numbered using principal quantum numbers (n).
• The lower a principal quantum number is the closer the shell is to the nucleus.
• Each principal quantum number has a fixed number of electrons it can hold.
• up to 2 electrons fit in n = 1.
• up to 8 electrons fit in n = 2.
• up to 18 electrons fit in n = 3.
• up to 32 electrons fit in n = 4.
• The number of electrons in the outermost shell determines an atom's reactivity, or tendency to form chemical bonds with other atoms.
• Valence shell is the name for the outermost shell, and its electrons are called valence electrons.
• Atoms are most stable and least reactive when their outermost electron shell is full.
• Most of the elements important in biology need eight electrons in their outermost shell to be stable, resulting in the octet rule.
• Some atoms can be stable with an octet even in the 3n shell, which can hold up to 18 electrons.
• Elements are placed in order on the periodic table based on their atomic number, how many protons they have.
• The number of electrons will equal the number of protons in a neutral atom, allowing easy determination of electron number from atomic number.
• An element's periodic table position—column/group and row/period—gives information about how those electrons are arranged.
• The first three rows of the table include the main elements for life, and each row corresponds to the filling of a different electron shell.
• Helium and hydrogen place their electrons in the 1n shell, while.
• Second-row elements like Li start filling the 2n shell.
• Third-row elements like Na continue with the 3n shell.
• An element's column number gives information about its number of valence electrons and reactivity.
• The number of valence electrons is generally the same within a column and increases from left to right within a row.
• Group 1 elements have one valence electron and group 18 elements have eight, except for helium, which has two.
• Group number is a good predictor of how reactive each element will be.
• The Bohr model can explain the reactivity and chemical bonding of elements but is inaccurate in how electrons are distributed around the nucleus.
• Electrons do not orbit the nucleus but rather spend time in electron orbitals around the nucleus.
• It is impossible to know where an electron is at any given moment, however it is possible to determine the volume of space in which it is most likely to be found, around 90% of the time.
• This high-probability region is made up of an orbital, which is able to hold a maximum of two electrons.

Subshells

• Electrons can move in different ways around the nucleus within a SHELL resulting in "paths" called SUBSHELLS.
• Principal quantum shells split into subshells lettered s, p and d. Elements with more than 57 electrons also have an f shell.
• The energy of the electrons in the subshells increase with the order of s < p < d.
• Subshells include one or more atomic orbitals, the specific regions of space where electrons may exist.
• The subshell determines the SHAPE of an orbital.
• Both the SHELL and the kind of SUBSHELL give the ENERGY of an orbital
• Orbitals exist at specific energy levels and only contain electrons can only exist at those levels.
• Each atomic orbital will only contain 2 electrons.
• Orbitals exist as follows:
• s: one orbital (1 x 2 = total of 2 electrons)
• p : three orbitals ( 3 x 2 = total of 6 electrons)
• d : five orbitals (5 x 2 = total of 10 electrons)
• f : seven orbitals (7 x 2 = total of 14 electrons)
• Ground state is the most stable electronic configuration of an atom which has the lowest amount of energy.
• This is achieved by filling the subshells of energy with the lowest energy first (1s).
• The order of the subshells in terms of increasing energy does not follow a regular pattern at n= 3 and higher.
• The maximum number of electrons at each principle quantum number is found by the following formula:
• Electron capacity = 2n^2, where n is the principle quantum number.

Hydrogen

• Hydrogen (chemical symbol H, atomic number 1) is one of the chemical elements.
• Hydrogen is the simplest and lightest element on the periodic table with an atomic weight of 1.00794 amu.
• Hydrogen properties:
• Electrons per shell: 1
• Atomic number: 1
• Electron configuration: 1s¹
• Electronegativity: 2.2
• Atomic mass: 1.00794 ± 0.00001 u
• Discoverer: Henry Cavendish
• Hydrogen has three common isotopes:
• Hydrogen (aka "protium", 1 proton, 0 neutrons).
• Deuterium (1 proton, 1 neutron).
• Tritium (1 proton, 2 neutrons).
• The most common isotope has 1 proton, 1 electron and no neutrons.
• In nature, free hydrogen is a diatomic molecule (H₂).
• A hydrogen molecule has two protons, and 2 electrons, which form a bond between hydrogen atoms.
• The heavier mass isotopes of hydrogen and oxygen form "Heavy" water.
• Ice cubes from heavy water sink to the bottom of a container of ordinary water.
• Its electronic configuration of hydrogen is 1s¹.

Helium

• Helium has two electrons going round the 1s orbital at the same time.
• The maximum number of electrons an s orbital can hold is 2.
• The electronic structure of helium is 1s².
• Both electrons are found in the first shell (orbital) of a helium atom.

Carbon

• Carbon's properties:
• Symbol: C
• Electrons per shell: 2, 4
• Atomic number: 6
• Electron configuration: [He] 2s²2p2
• Discovered: 3750 BC
• Electronegativity: 2.55
• Atomic mass: 12.0107 ± 0.0008 u

Potassium

• The atomic number of Potassium is 19.
• A neutral Potassium element has 19 electrons.
• Its electron configuration is 1s2 2s2 2p6 3s2 3p 6 4s1 or simply put [Ar] 4s1.

Subshells - Orbitals

• The s subshell has 1 orbital and can accommodate a maximum of 2 electrons.
• The p subshell has 3 orbitals.
• Each orbital is able to accommodate a maximum of 2 electrons.
• The p subshell itself is able to accommodate a maximum of 6 electrons.
• The d subshell has 5 orbitals and is only able to accommodate a maximu of 10 electrons.