Final answer:
The universe formed the lightest elements hydrogen (H), helium (He), and a small amount of lithium (Li) during its formation some 14 billion years ago. These were the only elements formed in appreciable amounts during the initial hot, dense state of the universe. Heavier elements were later synthesized within stars.
Step-by-step explanation:
Elements Formed During the Formation of the Universe
During the formation of the universe about 14 billion years ago, the conditions were right for the birth of the first elements. According to the big bang theory, the universe began in a hot, dense state and cooled as it expanded. This cooling allowed for the first atomic nuclei to form. When the universe was about 3 minutes old, it had cooled to approximately 900 million Kelvin, a temperature sufficiently cool for protons and neutrons to combine, which led to nucleosynthesis, the process by which the nuclei of elements are created.
The lightest elements formed in this primordial fireball include the most abundant element, hydrogen (H), followed by helium (He), which makes up about 24% of the universe's baryonic (normal) matter. Beyond these, only a trace amount of the third-lightest element, lithium (Li), was produced. These nucleosynthesis processes lasted for only a short period before the expanding universe cooled further, making it impossible for heavier elements to form.
Later on, as the first stars and galaxies began to coalesce from clouds of these primordial gases, the heavier elements were formed in the cores of stars through the process of stellar fusion. As stars age and eventually explode as supernovae, they spread these heavier elements into the surrounding space, seeding future generations of stars and planets with materials like carbon, oxygen, and iron—elements essential for life as we understand it. The discovery that stars are the source of most of the chemical richness of the universe was a fundamental milestone in astronomy.