Crossing over happens in Meiosis 1 only. In Prophase 1 a cells chromatin (chromatin = chromosomes that have not condensed yet) condense and pair up forming homologous chromosomes (paired = XX (2 chromosomes together)). When this happens segments/ alleles of the chromosomes pairing up swap over. This creates genetic diversity as each chromosome is different, it has parts from its pair. This leaves every chromosome unique and individual.
I hope this helps, sorry some of the vocab is rather technical. By the way I would suggest watching the 'Crash Course: Meiosis' on YouTube, this really helped me when I was learning this topic. :D
Crossing over can be expressed in three points: 1. Crossing over is when two chromosomes exchange their genetic materials in a process called synapsis. 2. It occurs in meiosis 1 during prophase 1. 3. One example: Your chromosomes may end up crossing over with wrong chromosomes due to mutation in one of the chromosome or both.
This process by which an unstable atomic nucleus loses energy by releasing radiation is called radioactive decay. The thing that makes this decay process so valuable for determining the age of an object is that each radioactive isotope decays at its own fixed rate, which is expressed in terms of its half-life.