Atomic Structure

What is an atom? An atom is an electrically neutral, spherical entity. It is comprised of a positively charged nucleus surrounded by one or more negatively charged electrons.

Parts of the Atom The atom consists of 3 subatomic particles: Proton Electron Neutron

Particles in an Atom Subatomic Particle ChargeMass (amu)Location in Atom Proton Inside nucleus Electron Outside nucleus Neutron Inside nucleus

Atomic Number The atomic number is located above the symbol of the element on the periodic table. It is represented by Z. The number of protons in the atoms of an element is the same as its atomic number.

Atoms are Electrically Neutral Since atoms are neutral in charge, the number of protons must equal the number of electrons. Therefore, the number of electrons is determined by the atomic number also.

Atomic Mass vs. Mass Number The atomic mass is located below the symbol of an element on the periodic table. It is represented as A. The mass number is the atomic mass rounded to the nearest whole number. Mass number = # of protons + # of neutrons

Sample Problems How many protons, electrons and neutrons are in the atoms of the following elements? Ca C Fe

Symbolic Representations Atomic information is often relayed in the following format:

Let’s Practice Please write the atomic symbol, atomic number and mass number in atomic symbol form in your notebook for the following elements: Silicon Mercury Chlorine Lithium

What is an isotope? Isotopes are atoms of the same element that have a different number of neutrons. To distinguish between different isotopes of an element, the atomic symbol is used.

Isotope Examples State the number of protons and neutrons in the following isotopes of Neon:

Isotopes It is important to remember that elements are made up of many isotopes that occur in nature. Even though their mass numbers are different, their chemical behavior is not!!

Why are protons, neutrons and electrons important? Protons determine an element’s identity. Neutrons determine an element’s mass. Electrons determine an element’s chemical reactivity!!!

So, where are the electrons? The electrons reside outside of the nucleus in discrete energy levels, called shells. The electron arrangement gives the number of electrons that can occupy each shell, or energy level.

Filling Electron Shells You can use the periodic table as a guide when filling electron energy levels. The number of elements in a row determine how many electrons can be placed in that energy level. For example, in row one, there are two elements. In energy level one, there are two electrons.

Electron Energy Levels For example, let’s consider how many energy levels carbon has. So, the number of electrons possible in the first four energy levels are as follows: level 1 = 2 electrons level 2 = 8 electrons level 3 = 8 electrons level 4 = 18 electrons