Atomic Structures in Chemistry

These are my notes on atomic structures.

Science and Math Books


The atom is the smallest unit of an element. It is made up of three different particles. They are:

  • Electrons
  • Protons
  • Neutrons

The combination of these particles are unique for each element. Each atom of the same element has the same combination of protons and electrons. An atom of helium on some faraway planet has the same combination of electrons and protons as one on Earth. 


Each element has a unique combination of protons and electrons in its atom. The combination of electrons and protons in an atom of one element is different from that in an atom of any other element. Since each element has a known number of protons and electrons, you can identify an element if you know these numbers. 



Protons are particles with a positive charge. Electrons are particles with a negative charge. Atoms have a neutral charge if they have the same number of electrons and protons. In any neutral atom, the number of electrons is always equal to the number of protons. Each element has a unique number of electrons and protons in its atoms. We can identify any element if we know either the number of protons or the number of electrons. 


Periodic Table

The periodic table describes the atoms of every element. For every listed element, the number of protons is listed at the top and the atomic weight is listed at the bottom. The number of protons in an element is known as its atomic number. 


Niels Bohr came up with an atomic model that pictured an atom with a nucleus of protons in the center and electrons spinning in an orbit around it. The model helps us remember and picture the details of an atom. The electron always has a negative charge. A proton carries a charge opposite that of an electron, which is positive. 



An electron has very little mass compared to a proton. It takes around 1836 electrons to equal the weight of just one proton. In any atom, most of the weight is through the proton. However, the weight of an atom is not just protons and electrons. The other particle, neutrons, make up the rest. Neutrons are located in the nucleus along with protons. Neutrons can be found in most atoms of elements. 


In the periodic table, each element is represented by a one or two letter symbol. These symbols serve as a shorthand notation for the elements. The shorthand notation for each case represents a neutral atom. The periodic table of elements is made up of several rows and columns. The rows are called periods and the columns are called groups. Groups are often called families because the elements that make up the groups have similar chemical properties. 

The groups have names that identify them.

  • Group VIIIA = Noble gases
  • Group IA = alkali metals
  • Group IIa = alkaline earth metals
  • Group VIIA = halogens


The periodic table can also be divided into three categories. These are:

  • Metals
  • Nonmetals
  • Metalloids

Elements to the left of the stairs on the periodic table are called metals. Metals have certain properties. It is what makes them metals. Metals are:

  • Malleable
  • Ductile

Malleable means they can be beaten into fine sheets like gold. Being ductile means they can be drawn into wires. Metals are also good conductors of heat. They usually have a shiny surface. Most metals are solid at room temperatures. 


Nonmetals are located on the right side of the stairs on the periodic table. These are halogens and noble gases. Their properties are almost opposite that of metals. They are usually brittle and do not conduct electricity or heat. Nonmetals are usually gases at room temperature. 


The last category is called metalloid. They are a hybrid of the other two categories of elements. That means they have properties of both metals and nonmetals. 


Mass and mass Number

When looking at an element on the periodic table, the top number is the atomic number of the element and is also the number of protons in an atom of this element. The atomic number is often written as a subscript preceding an element’s symbol. The symbol and number \(_7 N\) indicate nitrogen with atomic number 7. 


Almost all of the mass of an atom is attributed to the nucleus. The nucleus is made up mostly of protons and neutrons. Mass is a measure of the amount of matter. The mass of an object determines its weight. Weight is the effect of gravity on mass. 


Adding together the number of protons and neutrons in the nucleus of an atom results in what is known as the mass number of the atom. The mass number is simply the number of protons added to the number of neutrons in an atom.  By convention, the mass number is often written as a superscript in front of the element symbol. 


The quantum atomic model of an atom uses the term shells to divide up an atom. Shells are numbered 1-7. Subshells are part of each shell. They are labeled s, p, d, and f for each shell. 

  • S subshell has a size of one
  • P subshell has a size of three
  • D subshell has a size of five
  • F subshell has a size of seven

Each compartment in a subshell is called an orbital. 


Electrons prefer the lower shells and the smaller subshells. Electrons will fill shells and subshells in this order:

  • 1s
  • 2s
  • 2p
  • 3s
  • 3p
  • 4s
  • 3d
  • 4p
  • 5s
  • 4d
  • 5p
  • 6s
  • 4f
  • 5d
  • 6p
  • 7s
  • 5f
  • 6d

This is called the electron configuration of an atom. We can also identify an atom if given its electron configuration. Remember, only one electron will occupy an orbital in a given subshell until all the orbitals in that subshell have one electron in them. Electrons occupy as many orbitals as possible in the same subshell before pairing with another electron. This is called the principle of maximum multiplicity. 


Each group of elements in the periodic table has similar subshells with similar numbers of electrons in the outermost shell. The outermost shell consists of the subshells that are filled last. This situation serves to explain the similar chemical properties of elements within the same groups.