Ch 1 : Where are the electrons ?

Chapter 1: Where are the Electrons ?

Atomic structure

Let's start with a review of some key aspects of atomic structure:

atoms have a central nucleus at their core and are surrounded by clouds of electrons, e^- in orbitals
the nucleus is comprised of protons, p⁺ and neutrons, n⁰
- neutrons and protons are almost the same mass (relative masses 1.000 and 0.998 respectively)
- neutrons have no charge
- protons are positively charged, +1 unit*
electrons are much smaller (only about 1/1840 the mass of a neutron) and negatively charged, -1 unit*

* the charge on a proton is 1.602 x 10^-19 C

He atom structure

the total number of protons and neutrons in the nucleus is given by the mass number, A
the number of protons in the nucleus of an atom is given by a unique atomic number, Z which defines the atom type (i.e. H, He, Li, Be, B etc.)
since atoms are electronically neutral, an atom must have the same number of electrons surrounding the nucleus as there are protons in the nucleus, so Z also defines the number of electrons in a neutral atom.
Isotopes are variants of chemical elements with the same atomic number, Z, but different mass numbers, A.
This means that isotopes have different numbers of neutrons in the nucleus and hence different atomic weights.
Isotopes occur naturally e.g. carbon is found in nature as three naturally occuring isotopes: ¹²C (about 99%), ¹³C (about 1%) and ¹⁴C (less than 10^-10%).

A
Symbol Z

4.00
He
2

The periodic table of elements is arranged based on increasing atomic number, starting with hydrogen at the top left. As we traverse the periodic table, from the top left, going left to right, then down to the next row, at each step we are adding another proton to the nucleus and therefore another electron. Note that when getting the mass number from a periodic table, it usually gives the relative atomic mass, a weighted average of the isotopic masses based on a normal, natural sample.

Could you determine how many protons, neutrons or electrons a specific atom or ion has from it's atomic symbol or the periodic table?

In introductory organic chemistry most of the organic compounds we will be interested in are based on only a small group of elements: H, C, N, O, and the halogens Cl, Br and I. We will also occasionaly encounter a few other atoms such as Li, Na, Mg, Al, B, P, S and F. In more advanced organic chemistry, when you look at more advanced synthetic methods, one needs to be familiar with many more elements.

Try the questions about atoms.