They all have 6 in its outer shell because they are all in the same group in the periodic table. Your VERY VERY welcome (:
A sister chromatid refers to the identical copies formed by the replication of a chromosome, with both copies joined together by a common centromere. In other words, a sister chromatid may also be said to be 'one-half' of the duplicated chromosome. A pair of sister chromatids is called a dyad. A full set of sister chromatids is created during the synthesis (S) phase of interphase, when all the chromosomes in a cell are replicated. The two sister chromatids are separated from each other into two different cells during mitosis or during the second division of meiosis. Compare sister chromatids to homologous chromosomes, which are the two different copies of a chromosome that diploid organisms (like humans) inherit, one from each parent.
The DNA sequences of homologous chromosomes are usually not exactly identical. The nuclei of most human cells contain 46 chromosomes. These 46 chromosomes consist of 23 pairs of homologous chromosomes, or homologs, meaning each of these pairs are alike, but not necessarily identical.