Introduction

In the previous section, we’ve introduced the basics of atoms and how they are composed of subatomic particles including protons, neutrons, and electrons. In this section we’ll go over some common elements and how they can form isotopes and ions depending on their number of neutrons and electrons.

Elements

An element is a pure substance that consists of only one type of atom and cannot be broken down into simpler substances by chemical means. Each element is defined by the number of protons in its nucleus which is unique to that element. Elements are represented by one or two letter symbols, where the first letter is always capitalized and the second, if present is lowercase.

The first few elements are Hydrogen (H), Helium (He), Lithium (Li), Beryllium (Be), Boron (B), Carbon (C), Nitrogen (N), Oxygen (O), Fluorine (F), and Neon (Ne). Each of these elements has distinguishable chemical characteristics, some of which are crucial in our every day lives. For example, many important components of the human body are composed of carbon atoms, the air we breathe is composed of oxygen and nitrogen, and the water we drink is composed of oxygen and hydrogen.

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Atomic Number

The number of protons of an element determines its atomic number(Z), which is used to organize the elements on the periodic table. It is written in the lower left hand corner of the element’s symbol. The mass number (A) represents the number of nucleons (protons plus neutrons) in an element and it is written in the upper left hand corner of the element. The general form to represent an atom, X, is:

$$ ^{A} _{Z}X $$

To illustrate this, let’s consider the first few elements on the periodic table. Hydrogen is the simplest element with only one proton. It is denoted by the symbol H written as:

$$ ^{1} _{1}H $$

Here, the atomic number is 1 and there are no neutrons so the mass number is also 1. Note that $_2H$ is not possible, as an atom with 2 protons is helium, not hydrogen.

The second element, Helium (He) has 2 protons and 2 neutrons in the nucleus. Its atomic number is 2 and its mass number is 4 written as:

$$ ^{4} _{2}He $$

Lithium (Li), the third element has 3 protons and typically has 4 neutrons in the nucleus making its atomic number 3 and its mass number 7.

$$ ^{7} _{3}Li $$

Below is a table of the first 10 elements.

<aside> 💡 Table 1.3.1: The Atomic Number of the First Ten Elements

To reiterate, the most important thing to understand is that the atomic number is the number of protons. If the atomic number is 1, there’s 1 proton, and the atom is hydrogen. If the atomic number is 5, there are 5 protons, and the atom is Boron.